网上Glide源码解析的资料比较多，但鱼龙混杂，so我决定还是自己跟一遍源码吧，看源码真的是一件爆炸的事，好几次想放弃，但最后还是坚持下来了，唉。首先先来捋一遍Glide的具体用法，从具体用法出发我们再来分析Glide内部都是怎么来实现这些方法的。

这是我从网上找来的Glide的主要功能示意图，这张图一目了然的显示了Glide能实现的功能。

主要的功能列表示意图

先来看Glide最基础的图片加载用法：

Glide.with(context).load(url).into(imageView);

这里的load()方法就是指定要加载的图片资源，可以是网络图片，也可以是本地图片，资源图片，二进制流，Uri对象等等。当然我们也可以在加载图片的基础上做很多事，比如设置缺省图，加载失败的时候显示的图，设置图片尺寸，加载动画等。。。就像这样：

Glide
.with(context)
.load(imageUrl)
.placeholder(R.mipmap.ic_launcher)  //占位图
.error(R.mipmap.ic_launcher)        //加载错误显示的图
.animate(R.anim.anim)               //加载动画
.override(88, 88)                   //图片尺寸
.into(imageView);

这里都是采用多态形式来实现的。
Glide还可以指定要加载的图片格式。什么意思呢？就比如我这里有一个gif图片的图片链接地址，那我什么都不指定，用Glide加载，加载显示出来的就是gif图，但是如果我加了一个asBitmap()方法，指定加载的必须是静态图片，那么Glide就会只加载gif图的第一帧。
ok，接下来看Glide的图片缓存：
设置磁盘缓存

Glide.with(this).load(url).diskCacheStrategy(DiskCacheStrategy.ALL).into(imageView);

对应的缓存参数如下：
缓存参数说明
DiskCacheStrategy.NONE：不缓存任何图片，即禁用磁盘缓存
DiskCacheStrategy.ALL ：缓存原始图片 & 转换后的图片（默认）
DiskCacheStrategy.SOURCE：只缓存原始图片（原来的全分辨率的图像，即不缓存转换后的图片）
DiskCacheStrategy.RESULT：只缓存转换后的图片（即最终的图像：降低分辨率后 / 或者转换后，不缓存原始图片

ok，基本用法讲解完毕，之后就从用法出发，开始分析Glide的源码。先来看with()方法，with方法传了一个Context进去，其实很简单，就是绑定Glide生命周期的意思，来看源码：

 public static RequestManager with(Context context) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(context);
    }


    public static RequestManager with(Activity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }


    public static RequestManager with(FragmentActivity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }

with()方法有很多重构方法，其实也就是参数不同而已，实现里可以看到都是跟RequestManagerRetriever这个类有关，RequestManagerRetriever.get()方法从代码里可以看到是为了得到它的单例，那就来看看RequestManagerRetriever这个类，先看下面这三个我抽取出来的方法：

public RequestManager get(Context context) {
        if (context == null) {
            throw new IllegalArgumentException("You cannot start a load on a null Context");
        } else if (Util.isOnMainThread() && !(context instanceof Application)) {
            if (context instanceof FragmentActivity) {
                return get((FragmentActivity) context);
            } else if (context instanceof Activity) {
                return get((Activity) context);
            } else if (context instanceof ContextWrapper) {
                return get(((ContextWrapper) context).getBaseContext());
            }
        }

        return getApplicationManager(context);
    }


public RequestManager get(FragmentActivity activity) {
        if (Util.isOnBackgroundThread()) {
            return get(activity.getApplicationContext());
        } else {
            assertNotDestroyed(activity);
            FragmentManager fm = activity.getSupportFragmentManager();
            return supportFragmentGet(activity, fm);
        }
    }

private RequestManager getApplicationManager(Context context) {
        if (applicationManager == null) {
            synchronized (this) {
                if (applicationManager == null) {
                    applicationManager = new RequestManager(context.getApplicationContext(),
                            new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());
                }
            }
        }

        return applicationManager;
    }

第一个方法中context传进来以后把FragmentActivity，Activity，ContextWrapper都分流出去，除了这三种情况之外（其实也就是application context）都return getApplicationManager(context)也就是上面代码中的第三个方法。从上述代码可以看出，当传入的context是application或者在子线程中，那么最后都会调用getApplicationManager，在getApplicationManager(Context context)方法中，创建了一个RequestManager对象，因为Application的生命周期是跟着app的生命周期的，所以当传入的context为application的时候Glide没有进行任何关于生命周期的判断。先来看另外一种也就是当传入Activity这些的时候：

RequestManager supportFragmentGet(Context context, FragmentManager fm) {
        SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);
        RequestManager requestManager = current.getRequestManager();
        if (requestManager == null) {
            requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
            current.setRequestManager(requestManager);
        }
        return requestManager;
    }

@TargetApi(Build.VERSION_CODES.HONEYCOMB)
    RequestManager fragmentGet(Context context, android.app.FragmentManager fm) {
        RequestManagerFragment current = getRequestManagerFragment(fm);
        RequestManager requestManager = current.getRequestManager();
        if (requestManager == null) {
            requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
            current.setRequestManager(requestManager);
        }
        return requestManager;
    }

RequestManager supportFragmentGet(Context context, FragmentManager fm) {
        SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);
        RequestManager requestManager = current.getRequestManager();
        if (requestManager == null) {
            requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
            current.setRequestManager(requestManager);
        }
        return requestManager;
    }

我们发现到最后也是创建了一个RequestManager对象，从后面两个方法可以看出，不管你context传入的是Activity还是FragmentActivitiy，都会在Activity的基础上创建一个Fragment，这是为什么呢？因为Glide无法监听到Activity的生命周期，所以往Activity中添加一个Fragment来监听Fragment的生命周期，因为如果Activity消亡了，Fragment当然也就消亡了，具体代码看下面，不过传入的参数跟之前Application的不同，这里的SupportRequestManagerFragment的current.getLifecycle()方法我们进去会发现这其实是一个继承了Fragment的类，也就是往Activity里添加的Fragment，lifecycle实现了各种生命周期的方法：

@Override
    public void onStart() {
        super.onStart();
        lifecycle.onStart();
    }

    @Override
    public void onStop() {
        super.onStop();
        lifecycle.onStop();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        lifecycle.onDestroy();
    }

ok那现在回到RequestManager这个类，实现了LifecycleListener接口：

public interface LifecycleListener {

    /**
     * Callback for when {@link android.app.Fragment#onStart()}} or {@link android.app.Activity#onStart()} is called.
     */
    void onStart();

    /**
     * Callback for when {@link android.app.Fragment#onStop()}} or {@link android.app.Activity#onStop()}} is called.
     */
    void onStop();

    /**
     * Callback for when {@link android.app.Fragment#onDestroy()}} or {@link android.app.Activity#onDestroy()} is
     * called.
     */
    void onDestroy();
}

在RequestManager类中分别在这三个生命周期方法中实现了各自的操作，操作代码全都在RequestTracker这个类里，这个类主要负责跟踪，取消，重启在进程中的或者已经完成了或者已经失败了的请求，看RequestTracker可知其实就是在Glide内部维护了一个Request请求的list：

public class RequestTracker {
    private final Set<Request> requests = Collections.newSetFromMap(new WeakHashMap<Request, Boolean>());
    @SuppressWarnings("MismatchedQueryAndUpdateOfCollection")
    private final List<Request> pendingRequests = new ArrayList<Request>();

    private boolean isPaused;
    public void runRequest(Request request) {
        requests.add(request);
        if (!isPaused) {
            request.begin();
        } else {
            pendingRequests.add(request);
        }
    }

    void addRequest(Request request) {
        requests.add(request);
    }

    public void removeRequest(Request request) {
        requests.remove(request);
        pendingRequests.remove(request);
    }

    public boolean isPaused() {
        return isPaused;
    }

    public void pauseRequests() {
        isPaused = true;
        for (Request request : Util.getSnapshot(requests)) {
            if (request.isRunning()) {
                request.pause();
                pendingRequests.add(request);
            }
        }
    }

    public void resumeRequests() {
        isPaused = false;
        for (Request request : Util.getSnapshot(requests)) {
            if (!request.isComplete() && !request.isCancelled() && !request.isRunning()) {
                request.begin();
            }
        }
        pendingRequests.clear();
    }

    public void clearRequests() {
        for (Request request : Util.getSnapshot(requests)) {
            request.clear();
        }
        pendingRequests.clear();
    }

    public void restartRequests() {
        for (Request request : Util.getSnapshot(requests)) {
            if (!request.isComplete() && !request.isCancelled()) {
                // Ensure the request will be restarted in onResume.
                request.pause();
                if (!isPaused) {
                    request.begin();
                } else {
                    pendingRequests.add(request);
                }
            }
        }
    }
}

其实给Glide添加生命周期的意义很简单，当你在一个Activity里去加载图片，当图片还没加载出来的时候用户把Activity消亡了，那图片加载的request是不是得取消，其实就是这么一个道理。

ok，接下来来看load()方法，因为with(context)方法返回的是RequestManager，所以load()方法自然也就在RequestManager类里，可以看到：

public DrawableTypeRequest<String> load(String string) {
        return (DrawableTypeRequest<String>) fromString().load(string);
    }

方法实现非常简单，先来看fromString()，点到fromString()方法发现其实就是调用了loadGeneric(String.class)方法，ok那就来看loadGeneric()方法：

private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {
        ModelLoader<T, InputStream> streamModelLoader = Glide.buildStreamModelLoader(modelClass, context);
        ModelLoader<T, ParcelFileDescriptor> fileDescriptorModelLoader =
                Glide.buildFileDescriptorModelLoader(modelClass, context);
        if (modelClass != null && streamModelLoader == null && fileDescriptorModelLoader == null) {
            throw new IllegalArgumentException("Unknown type " + modelClass + ". You must provide a Model of a type for"
                    + " which there is a registered ModelLoader, if you are using a custom model, you must first call"
                    + " Glide#register with a ModelLoaderFactory for your custom model class");
        }

        return optionsApplier.apply(
                new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
                        glide, requestTracker, lifecycle, optionsApplier));
    }

在这个方法里其实就是根据传进来的class类来创建不同的ModelLoader类，然后再把这些参数都穿进去创造一个DrawableTypeRequest对象返回回去，因为这里我参考的是load(url)加载网络图片链接，所以传进去的就是String，这个ModelLoader是干嘛的呢，接着往下看，Glide.buildStreamModelLoader(modelClass, context):

/**
     * A method to build a {@link ModelLoader} for the given model that produces {@link InputStream}s using a registered
     * factory.
     *
     * @see #buildModelLoader(Class, Class, android.content.Context)
     */
    public static <T> ModelLoader<T, InputStream> buildStreamModelLoader(Class<T> modelClass, Context context) {
        return buildModelLoader(modelClass, InputStream.class, context);
    }

//省略一大段代码

public static <T, Y> ModelLoader<T, Y> buildModelLoader(Class<T> modelClass, Class<Y> resourceClass,
            Context context) {
         if (modelClass == null) {
            if (Log.isLoggable(TAG, Log.DEBUG)) {
                Log.d(TAG, "Unable to load null model, setting placeholder only");
            }
            return null;
        }
        return Glide.get(context).getLoaderFactory().buildModelLoader(modelClass, resourceClass);
    }

创建一个ModelLoader对象有什么用呢？其实就是为了提供inputStream输入流，可以看到该方法最后return了一个buildModelLoader(modelClass, resourceClass)的对象，接下来就进入到这个方法，这个方法在GenericLoaderFactory类里面，GenericLoaderFactory这个类其实比较短，所以全部简略的全部贴出来：

public class GenericLoaderFactory {
    private final Map<Class/*T*/, Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/>> modelClassToResourceFactories =
            new HashMap<Class, Map<Class, ModelLoaderFactory>>();
    private final Map<Class/*T*/, Map<Class/*Y*/, ModelLoader/*T, Y*/>> cachedModelLoaders =
            new HashMap<Class, Map<Class, ModelLoader>>();

    private static final ModelLoader NULL_MODEL_LOADER = new ModelLoader() {
        @Override
        public DataFetcher getResourceFetcher(Object model, int width, int height) {
            throw new NoSuchMethodError("This should never be called!");
        }

        @Override
        public String toString() {
            return "NULL_MODEL_LOADER";
        }
    };

    private final Context context;

    public GenericLoaderFactory(Context context) {
       this.context = context.getApplicationContext();
    }


    public synchronized <T, Y> ModelLoaderFactory<T, Y> unregister(Class<T> modelClass, Class<Y> resourceClass) {
        cachedModelLoaders.clear();

        ModelLoaderFactory/*T, Y*/ result = null;
        Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> resourceToFactories = modelClassToResourceFactories.get(modelClass);
        if (resourceToFactories != null) {
            result = resourceToFactories.remove(resourceClass);
        }
        return result;
    }

    public synchronized <T, Y> ModelLoaderFactory<T, Y> register(Class<T> modelClass, Class<Y> resourceClass,
            ModelLoaderFactory<T, Y> factory) {
        cachedModelLoaders.clear();

        Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> resourceToFactories = modelClassToResourceFactories.get(modelClass);
        if (resourceToFactories == null) {
            resourceToFactories = new HashMap<Class/*Y*/, ModelLoaderFactory/*T, Y*/>();
            modelClassToResourceFactories.put(modelClass, resourceToFactories);
        }

        ModelLoaderFactory/*T, Y*/ previous = resourceToFactories.put(resourceClass, factory);

        if (previous != null) {
            // This factory may be being used by another model. We don't want to say it has been removed unless we
            // know it has been removed for all models.
            for (Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> factories : modelClassToResourceFactories.values()) {
                if (factories.containsValue(previous)) {
                    previous = null;
                    break;
                }
            }
        }

        return previous;
    }

    @Deprecated
    public synchronized <T, Y> ModelLoader<T, Y> buildModelLoader(Class<T> modelClass, Class<Y> resourceClass,
            Context context) {
        return buildModelLoader(modelClass, resourceClass);
    }

    public synchronized <T, Y> ModelLoader<T, Y> buildModelLoader(Class<T> modelClass, Class<Y> resourceClass) {
        ModelLoader<T, Y> result = getCachedLoader(modelClass, resourceClass);
        if (result != null) {
            // We've already tried to create a model loader and can't with the currently registered set of factories,
            // but we can't use null to demonstrate that failure because model loaders that haven't been requested
            // yet will be null in the cache. To avoid this, we use a special signal model loader.
            if (NULL_MODEL_LOADER.equals(result)) {
                return null;
            } else {
                return result;
            }
        }

        final ModelLoaderFactory<T, Y> factory = getFactory(modelClass, resourceClass);
        if (factory != null) {
            result = factory.build(context, this);
            cacheModelLoader(modelClass, resourceClass, result);
        } else {
            // We can't generate a model loader for the given arguments with the currently registered set of factories.
            cacheNullLoader(modelClass, resourceClass);
        }
        return result;
    }

    private <T, Y> void cacheNullLoader(Class<T> modelClass, Class<Y> resourceClass) {
        cacheModelLoader(modelClass, resourceClass, NULL_MODEL_LOADER);
    }

    private <T, Y> void cacheModelLoader(Class<T> modelClass, Class<Y> resourceClass, ModelLoader<T, Y> modelLoader) {
        Map<Class/*Y*/, ModelLoader/*T, Y*/> resourceToLoaders = cachedModelLoaders.get(modelClass);
        if (resourceToLoaders == null) {
            resourceToLoaders = new HashMap<Class/*Y*/, ModelLoader/*T, Y*/>();
            cachedModelLoaders.put(modelClass, resourceToLoaders);
        }
        resourceToLoaders.put(resourceClass, modelLoader);
    }

    private <T, Y> ModelLoader<T, Y> getCachedLoader(Class<T> modelClass, Class<Y> resourceClass) {
        Map<Class/*Y*/, ModelLoader/*T, Y*/> resourceToLoaders = cachedModelLoaders.get(modelClass);
        ModelLoader/*T, Y*/ result = null;
        if (resourceToLoaders != null) {
            result = resourceToLoaders.get(resourceClass);
        }

        return result;
    }

    private <T, Y> ModelLoaderFactory<T, Y> getFactory(Class<T> modelClass, Class<Y> resourceClass) {
        Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> resourceToFactories = modelClassToResourceFactories.get(modelClass);
        ModelLoaderFactory/*T, Y*/ result = null;
        if (resourceToFactories != null) {
            result = resourceToFactories.get(resourceClass);
        }

        if (result == null) {
            for (Class<? super T> registeredModelClass : modelClassToResourceFactories.keySet()) {
                if (registeredModelClass.isAssignableFrom(modelClass)) {
                    Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> currentResourceToFactories =
                            modelClassToResourceFactories.get(registeredModelClass);
                    if (currentResourceToFactories != null) {
                        result = currentResourceToFactories.get(resourceClass);
                        if (result != null) {
                            break;
                        }
                    }
                }
            }
        }

        return result;
    }
}

重点来看buildModelLoader方法，这里的T跟Y范型类在我的心路历程里其实就是String跟InputStream。我们可以看到其实我们是通过getFactory(modelClass, resourceClass)这个方法来创建了一个ModelLoaderFactory对象的，看getFactory方法可以看到我们其实就是从一个modelClassToResourceFactories的map获取一个ModelLoaderFactory对象，而且是要已经注册过了的，至于怎么判断是否注册过，就要回到Glide的构造方法：

Glide(Engine engine, MemoryCache memoryCache, BitmapPool bitmapPool, Context context, DecodeFormat decodeFormat) {
        this.engine = engine;
        this.bitmapPool = bitmapPool;
        this.memoryCache = memoryCache;
        this.decodeFormat = decodeFormat;
        loaderFactory = new GenericLoaderFactory(context);
        mainHandler = new Handler(Looper.getMainLooper());
        bitmapPreFiller = new BitmapPreFiller(memoryCache, bitmapPool, decodeFormat);

        dataLoadProviderRegistry = new DataLoadProviderRegistry();

        StreamBitmapDataLoadProvider streamBitmapLoadProvider =
                new StreamBitmapDataLoadProvider(bitmapPool, decodeFormat);
        dataLoadProviderRegistry.register(InputStream.class, Bitmap.class, streamBitmapLoadProvider);

        FileDescriptorBitmapDataLoadProvider fileDescriptorLoadProvider =
                new FileDescriptorBitmapDataLoadProvider(bitmapPool, decodeFormat);
        dataLoadProviderRegistry.register(ParcelFileDescriptor.class, Bitmap.class, fileDescriptorLoadProvider);

        ImageVideoDataLoadProvider imageVideoDataLoadProvider =
                new ImageVideoDataLoadProvider(streamBitmapLoadProvider, fileDescriptorLoadProvider);
        dataLoadProviderRegistry.register(ImageVideoWrapper.class, Bitmap.class, imageVideoDataLoadProvider);

        GifDrawableLoadProvider gifDrawableLoadProvider =
                new GifDrawableLoadProvider(context, bitmapPool);
        dataLoadProviderRegistry.register(InputStream.class, GifDrawable.class, gifDrawableLoadProvider);

        dataLoadProviderRegistry.register(ImageVideoWrapper.class, GifBitmapWrapper.class,
                new ImageVideoGifDrawableLoadProvider(imageVideoDataLoadProvider, gifDrawableLoadProvider, bitmapPool));

        dataLoadProviderRegistry.register(InputStream.class, File.class, new StreamFileDataLoadProvider());

        register(File.class, ParcelFileDescriptor.class, new FileDescriptorFileLoader.Factory());
        register(File.class, InputStream.class, new StreamFileLoader.Factory());
        register(int.class, ParcelFileDescriptor.class, new FileDescriptorResourceLoader.Factory());
        register(int.class, InputStream.class, new StreamResourceLoader.Factory());
        register(Integer.class, ParcelFileDescriptor.class, new FileDescriptorResourceLoader.Factory());
        register(Integer.class, InputStream.class, new StreamResourceLoader.Factory());
        register(String.class, ParcelFileDescriptor.class, new FileDescriptorStringLoader.Factory());
        register(String.class, InputStream.class, new StreamStringLoader.Factory());
        register(Uri.class, ParcelFileDescriptor.class, new FileDescriptorUriLoader.Factory());
        register(Uri.class, InputStream.class, new StreamUriLoader.Factory());
        register(URL.class, InputStream.class, new StreamUrlLoader.Factory());
        register(GlideUrl.class, InputStream.class, new HttpUrlGlideUrlLoader.Factory());
        register(byte[].class, InputStream.class, new StreamByteArrayLoader.Factory());

        transcoderRegistry.register(Bitmap.class, GlideBitmapDrawable.class,
                new GlideBitmapDrawableTranscoder(context.getResources(), bitmapPool));
        transcoderRegistry.register(GifBitmapWrapper.class, GlideDrawable.class,
                new GifBitmapWrapperDrawableTranscoder(
                        new GlideBitmapDrawableTranscoder(context.getResources(), bitmapPool)));

        bitmapCenterCrop = new CenterCrop(bitmapPool);
        drawableCenterCrop = new GifBitmapWrapperTransformation(bitmapPool, bitmapCenterCrop);

        bitmapFitCenter = new FitCenter(bitmapPool);
        drawableFitCenter = new GifBitmapWrapperTransformation(bitmapPool, bitmapFitCenter);
    }

这里注册了FileDescriptorFileLoader、StreamFileLoader等好几个factory，因为我们这里传入的是String跟InputStream所以我们得到的factory其实是StreamStringLoader，后面的那个fileDescriptorModeLoader方法跟这里类似，所以这里就不详述了。 ok，到这里我们就回去，经过这么一串分析之后就生成了一个DrawableTypeRequest对象，那么DrawableTypeRequest究竟是什么呢。。接下来就来look下这个，代码比较少：

public class DrawableTypeRequest<ModelType> extends DrawableRequestBuilder<ModelType> implements DownloadOptions {
    private final ModelLoader<ModelType, InputStream> streamModelLoader;
    private final ModelLoader<ModelType, ParcelFileDescriptor> fileDescriptorModelLoader;
    private final RequestManager.OptionsApplier optionsApplier;

    private static <A, Z, R> FixedLoadProvider<A, ImageVideoWrapper, Z, R> buildProvider(Glide glide,
            ModelLoader<A, InputStream> streamModelLoader,
            ModelLoader<A, ParcelFileDescriptor> fileDescriptorModelLoader, Class<Z> resourceClass,
            Class<R> transcodedClass,
            ResourceTranscoder<Z, R> transcoder) {
        if (streamModelLoader == null && fileDescriptorModelLoader == null) {
            return null;
        }

        if (transcoder == null) {
            transcoder = glide.buildTranscoder(resourceClass, transcodedClass);
        }
        DataLoadProvider<ImageVideoWrapper, Z> dataLoadProvider = glide.buildDataProvider(ImageVideoWrapper.class,
                resourceClass);
        ImageVideoModelLoader<A> modelLoader = new ImageVideoModelLoader<A>(streamModelLoader,
                fileDescriptorModelLoader);
        return new FixedLoadProvider<A, ImageVideoWrapper, Z, R>(modelLoader, transcoder, dataLoadProvider);
    }

    DrawableTypeRequest(Class<ModelType> modelClass, ModelLoader<ModelType, InputStream> streamModelLoader,
            ModelLoader<ModelType, ParcelFileDescriptor> fileDescriptorModelLoader, Context context, Glide glide,
            RequestTracker requestTracker, Lifecycle lifecycle, RequestManager.OptionsApplier optionsApplier) {
        super(context, modelClass,
                buildProvider(glide, streamModelLoader, fileDescriptorModelLoader, GifBitmapWrapper.class,
                        GlideDrawable.class, null),
                glide, requestTracker, lifecycle);
        this.streamModelLoader = streamModelLoader;
        this.fileDescriptorModelLoader = fileDescriptorModelLoader;
        this.optionsApplier = optionsApplier;
    }

    public BitmapTypeRequest<ModelType> asBitmap() {
        return optionsApplier.apply(new BitmapTypeRequest<ModelType>(this, streamModelLoader,
                fileDescriptorModelLoader, optionsApplier));
    }

    public GifTypeRequest<ModelType> asGif() {
        return optionsApplier.apply(new GifTypeRequest<ModelType>(this, streamModelLoader, optionsApplier));
    }

    /**
     * {@inheritDoc}
     */
    public <Y extends Target<File>> Y downloadOnly(Y target) {
        return getDownloadOnlyRequest().downloadOnly(target);
    }

    /**
     * {@inheritDoc}
     */
    public FutureTarget<File> downloadOnly(int width, int height) {
        return getDownloadOnlyRequest().downloadOnly(width, height);
    }

    private GenericTranscodeRequest<ModelType, InputStream, File> getDownloadOnlyRequest() {
        return optionsApplier.apply(new GenericTranscodeRequest<ModelType, InputStream, File>(File.class, this,
                streamModelLoader, InputStream.class, File.class, optionsApplier));
    }
}

眼神好的朋友们是不是已经发现了asBitmap()跟asGif()两个我们之前说用法上的方法了。经过了一层一层底层方法的穿来穿去之后，突然发现这两个方法是不是很舒服。其实很简单，就是分别创建了一个BitmapTypeRequest和GifTypeRequest，根据不同的request然后底层再去加载不同的图片格式。 ok，接下去我们再回到开头，fromString()方法之后返回一个DrawableTypeRequest对象，然后就是调用这个对象的load(string)方法，我们看到load方法其实是跳到了父类DrawableRequestBuilder里，那就来看看它的父类：

//省略一部分代码
    public DrawableRequestBuilder<ModelType> centerCrop() {
        return transform(glide.getDrawableCenterCrop());
    }

    @SuppressWarnings("unchecked")
    public DrawableRequestBuilder<ModelType> fitCenter() {
        return transform(glide.getDrawableFitCenter());
    }

    public DrawableRequestBuilder<ModelType> bitmapTransform(Transformation<Bitmap>... bitmapTransformations) {
        GifBitmapWrapperTransformation[] transformations =
                new GifBitmapWrapperTransformation[bitmapTransformations.length];
        for (int i = 0; i < bitmapTransformations.length; i++) {
            transformations[i] = new GifBitmapWrapperTransformation(glide.getBitmapPool(), bitmapTransformations[i]);
        }
        return transform(transformations);
    }



    /**
     * {@inheritDoc}
     *
     * @see #bitmapTransform(com.bumptech.glide.load.Transformation[])
     * @see #centerCrop()
     * @see #fitCenter()
     */
    @Override
    public DrawableRequestBuilder<ModelType> transform(Transformation<GifBitmapWrapper>... transformation) {
        super.transform(transformation);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> transcoder(
            ResourceTranscoder<GifBitmapWrapper, GlideDrawable> transcoder) {
        super.transcoder(transcoder);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    public final DrawableRequestBuilder<ModelType> crossFade() {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>());
        return this;
    }

    /**
     * {@inheritDoc}
     */
    public DrawableRequestBuilder<ModelType> crossFade(int duration) {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>(duration));
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Deprecated
    public DrawableRequestBuilder<ModelType> crossFade(Animation animation, int duration) {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>(animation, duration));
        return this;
    }

    /**
     * {@inheritDoc}
     */
    public DrawableRequestBuilder<ModelType> crossFade(int animationId, int duration) {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>(context, animationId,
                duration));
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> dontAnimate() {
        super.dontAnimate();
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> animate(ViewPropertyAnimation.Animator animator) {
        super.animate(animator);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> animate(int animationId) {
        super.animate(animationId);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Deprecated
    @SuppressWarnings("deprecation")
    @Override
    public DrawableRequestBuilder<ModelType> animate(Animation animation) {
        super.animate(animation);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> placeholder(int resourceId) {
        super.placeholder(resourceId);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> placeholder(Drawable drawable) {
        super.placeholder(drawable);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> fallback(Drawable drawable) {
        super.fallback(drawable);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> fallback(int resourceId) {
        super.fallback(resourceId);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> error(int resourceId) {
        super.error(resourceId);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> error(Drawable drawable) {
        super.error(drawable);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> listener(
            RequestListener<? super ModelType, GlideDrawable> requestListener) {
        super.listener(requestListener);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> diskCacheStrategy(DiskCacheStrategy strategy) {
        super.diskCacheStrategy(strategy);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> skipMemoryCache(boolean skip) {
        super.skipMemoryCache(skip);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> override(int width, int height) {
        super.override(width, height);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> sourceEncoder(Encoder<ImageVideoWrapper> sourceEncoder) {
        super.sourceEncoder(sourceEncoder);
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public DrawableRequestBuilder<ModelType> dontTransform() {
        super.dontTransform();
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> signature(Key signature) {
        super.signature(signature);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> load(ModelType model) {
        super.load(model);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> clone() {
        return (DrawableRequestBuilder<ModelType>) super.clone();
    }

    @Override
    public Target<GlideDrawable> into(ImageView view) {
        return super.into(view);
    }

    @Override
    void applyFitCenter() {
        fitCenter();
    }

    @Override
    void applyCenterCrop() {
        centerCrop();
    }

有没有觉得很亲切，之前用法里我们用到的很多api方法我们终于看到了，当然我们也看到了into()方法，load方法我们先来看看它父类的实现：

public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> load(ModelType model) {
        this.model = model;
        isModelSet = true;
        return this;
    }

其实就是做了一个状态的保存而已。 ok，那接下来我们就开始分析into()方法，into()方法我逻辑理了n遍，实在太复杂了我的天，稍微注意力不集中一下就晕了。。ok，废话不多说，接着来，into()。

首先，我们定位到DrawableTypeRequest的父类GenericRequestBuilder里的into()方法：

public Target<TranscodeType> into(ImageView view) {
        Util.assertMainThread();
        if (view == null) {
            throw new IllegalArgumentException("You must pass in a non null View");
        }

        if (!isTransformationSet && view.getScaleType() != null) {
            switch (view.getScaleType()) {
                case CENTER_CROP:
                    applyCenterCrop();
                    break;
                case FIT_CENTER:
                case FIT_START:
                case FIT_END:
                    applyFitCenter();
                    break;
                //$CASES-OMITTED$
                default:
                    // Do nothing.
            }
        }

        return into(glide.buildImageViewTarget(view, transcodeClass));
    }

前面一大堆讲的都是图片变化的问题，暂时先放到一边，先来看图片加载的过程，也就是最后一句话，先来看buildImageViewTarget(view, transcoderClass)方法：

<R> Target<R> buildImageViewTarget(ImageView imageView, Class<R> transcodedClass) {
        return imageViewTargetFactory.buildTarget(imageView, transcodedClass);
    }

之前的分析中可以看出transcodedClass也就是TranscodeType的一个实体类，而TranscodeType也就是GlideDrawable类，知道了这个以后那就接着往下看：

public class ImageViewTargetFactory {

    @SuppressWarnings("unchecked")
    public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
        if (GlideDrawable.class.isAssignableFrom(clazz)) {
            return (Target<Z>) new GlideDrawableImageViewTarget(view);
        } else if (Bitmap.class.equals(clazz)) {
            return (Target<Z>) new BitmapImageViewTarget(view);
        } else if (Drawable.class.isAssignableFrom(clazz)) {
            return (Target<Z>) new DrawableImageViewTarget(view);
        } else {
            throw new IllegalArgumentException("Unhandled class: " + clazz
                    + ", try .as*(Class).transcode(ResourceTranscoder)");
        }
    }
}

这里我们看到了三个类GlideDrawableImageViewTarget、BitmapImageViewTarget、DrawableImageViewTarget。在这里，我们也就创建了一个GlideDrawableImageViewTarget对象了。ok，我们知道了这个以后现在回过头去看之前的into(Y target)方法：

public <Y extends Target<TranscodeType>> Y into(Y target) {
        Util.assertMainThread();
        if (target == null) {
            throw new IllegalArgumentException("You must pass in a non null Target");
        }
        if (!isModelSet) {
            throw new IllegalArgumentException("You must first set a model (try #load())");
        }

        Request previous = target.getRequest();

        if (previous != null) {
            previous.clear();
            requestTracker.removeRequest(previous);
            previous.recycle();
        }

        Request request = buildRequest(target);
        target.setRequest(request);
        lifecycle.addListener(target);
        requestTracker.runRequest(request);

        return target;
    }

ok，到这里我们终于发现发出加载图片请求的类了，也就是Request，Request类的具体子类的作用就是根据图片路径来加载图片到imageView上。这里Request对象是通过buildRequest(target)方法来创建的，ok，接下来看这个：

 private Request buildRequest(Target<TranscodeType> target) {
        if (priority == null) {
            priority = Priority.NORMAL;
        }
        return buildRequestRecursive(target, null);
    }

    private Request buildRequestRecursive(Target<TranscodeType> target, ThumbnailRequestCoordinator parentCoordinator) {
        if (thumbnailRequestBuilder != null) {
            if (isThumbnailBuilt) {
                throw new IllegalStateException("You cannot use a request as both the main request and a thumbnail, "
                        + "consider using clone() on the request(s) passed to thumbnail()");
            }
            // Recursive case: contains a potentially recursive thumbnail request builder.
            if (thumbnailRequestBuilder.animationFactory.equals(NoAnimation.getFactory())) {
                thumbnailRequestBuilder.animationFactory = animationFactory;
            }

            if (thumbnailRequestBuilder.priority == null) {
                thumbnailRequestBuilder.priority = getThumbnailPriority();
            }

            if (Util.isValidDimensions(overrideWidth, overrideHeight)
                    && !Util.isValidDimensions(thumbnailRequestBuilder.overrideWidth,
                            thumbnailRequestBuilder.overrideHeight)) {
              thumbnailRequestBuilder.override(overrideWidth, overrideHeight);
            }

            ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
            Request fullRequest = obtainRequest(target, sizeMultiplier, priority, coordinator);
            // Guard against infinite recursion.
            isThumbnailBuilt = true;
            // Recursively generate thumbnail requests.
            Request thumbRequest = thumbnailRequestBuilder.buildRequestRecursive(target, coordinator);
            isThumbnailBuilt = false;
            coordinator.setRequests(fullRequest, thumbRequest);
            return coordinator;
        } else if (thumbSizeMultiplier != null) {
            // Base case: thumbnail multiplier generates a thumbnail request, but cannot recurse.
            ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
            Request fullRequest = obtainRequest(target, sizeMultiplier, priority, coordinator);
            Request thumbnailRequest = obtainRequest(target, thumbSizeMultiplier, getThumbnailPriority(), coordinator);
            coordinator.setRequests(fullRequest, thumbnailRequest);
            return coordinator;
        } else {
            // Base case: no thumbnail.
            return obtainRequest(target, sizeMultiplier, priority, parentCoordinator);
        }
    }

private Request obtainRequest(Target<TranscodeType> target, float sizeMultiplier, Priority priority,
            RequestCoordinator requestCoordinator) {
        return GenericRequest.obtain(
                loadProvider,
                model,
                signature,
                context,
                priority,
                target,
                sizeMultiplier,
                placeholderDrawable,
                placeholderId,
                errorPlaceholder,
                errorId,
                fallbackDrawable,
                fallbackResource,
                requestListener,
                requestCoordinator,
                glide.getEngine(),
                transformation,
                transcodeClass,
                isCacheable,
                animationFactory,
                overrideWidth,
                overrideHeight,
                diskCacheStrategy);
    }

buildRequest方法里调用了buildRequestRecursive方法，这个里面代码不少，我们先把图片变化的都忽略，就会发现主要的还是botainRequest()方法，然后看到return了一个GenericRequest.obtain(n个参数)的方法，这么多参数，乍看一下你会觉得很熟悉，placehoderId，errorId等，这些不都是我们最开始讲Glide的时候用的一些api方法莫，是不是有一种胜利在望的感觉。。ok，那就接着往下看obtain()方法，进入了GenericRequest类：

public static <A, T, Z, R> GenericRequest<A, T, Z, R> obtain(
            LoadProvider<A, T, Z, R> loadProvider,
            A model,
            Key signature,
            Context context,
            Priority priority,
            Target<R> target,
            float sizeMultiplier,
            Drawable placeholderDrawable,
            int placeholderResourceId,
            Drawable errorDrawable,
            int errorResourceId,
            Drawable fallbackDrawable,
            int fallbackResourceId,
            RequestListener<? super A, R> requestListener,
            RequestCoordinator requestCoordinator,
            Engine engine,
            Transformation<Z> transformation,
            Class<R> transcodeClass,
            boolean isMemoryCacheable,
            GlideAnimationFactory<R> animationFactory,
            int overrideWidth,
            int overrideHeight,
            DiskCacheStrategy diskCacheStrategy) {
        @SuppressWarnings("unchecked")
        GenericRequest<A, T, Z, R> request = (GenericRequest<A, T, Z, R>) REQUEST_POOL.poll();
        if (request == null) {
            request = new GenericRequest<A, T, Z, R>();
        }
        request.init(loadProvider,
                model,
                signature,
                context,
                priority,
                target,
                sizeMultiplier,
                placeholderDrawable,
                placeholderResourceId,
                errorDrawable,
                errorResourceId,
                fallbackDrawable,
                fallbackResourceId,
                requestListener,
                requestCoordinator,
                engine,
                transformation,
                transcodeClass,
                isMemoryCacheable,
                animationFactory,
                overrideWidth,
                overrideHeight,
                diskCacheStrategy);
        return request;
    }

乍看一下头已经晕了。。。放松一下，仔细看，我们发现传进来这么多参数以后，方法里先去创建了一个GenericRequest的实体类，然后调用了init方法，进去看，会发现全是一个赋值的一个过程，ok，既然是这样子，我们已经知道了request创建的整个过程了莫，这条路就差不多走到底了，差不多该回去了，那就再回到into方法。

Request request = buildRequest(target);
        target.setRequest(request);
        lifecycle.addListener(target);
        requestTracker.runRequest(request);

首先先把request绑定给了GlideDrawableImageViewTarget对象，为了就是不会被Glide之外的调用，然后我们看到，runRequest(request)方法，直觉告诉我们request对象创建出来之后执行代码应该就在这里了，那我们就进去看看，果然。。

public void runRequest(Request request) {
        requests.add(request);
        if (!isPaused) {
            request.begin();
        } else {
            pendingRequests.add(request);
        }
    }

这个方法在RequestTracker类里，这个类是不是很熟悉，之前在讲with()方法的生命周期里提到过。这个方法里做了一个很简单的逻辑判断，如果这个时候不是暂停状态的话，request.begin()方法去执行request，否则就把该request存放到一个放着没有完成或者可能会重新去运行一遍的request的list里面。接着一步一步下去，进begin()方法。因为之前我们request的创建过程中一步一步下来到最后是return了一个GenericRequest.obtain(n个参数)，所以begin()方法就应该在GenericRequest类里面，ok，定位到begin()方法：

    @Override
    public void begin() {
        startTime = LogTime.getLogTime();
        if (model == null) {
            onException(null);
            return;
        }

        status = Status.WAITING_FOR_SIZE;
        if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
            onSizeReady(overrideWidth, overrideHeight);
        } else {
            target.getSize(this);
        }

        if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
            target.onLoadStarted(getPlaceholderDrawable());
        }
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished run method in " + LogTime.getElapsedMillis(startTime));
        }
    }

ok，这个beging()方法我们慢慢来分析，首先我们先来看model是什么？我们往前看，翻到GenericRequestBuilder的构造函数，然后创建它对象的过程发现model其实在我们这种情况下就是图片的链接地址，也就是load里传过来的东西，因为这里我们分析的是加载网络请求图片，所以也就是图片的链接。ok，知道了model是什么以后我们再来看这个方法，如果model是空的，那么就调用onException(null)方法，我们来看这个方法

@Override
    public void onException(Exception e) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "load failed", e);
        }

        status = Status.FAILED;
        //TODO: what if this is a thumbnail request?
        if (requestListener == null || !requestListener.onException(e, model, target, isFirstReadyResource())) {
            setErrorPlaceholder(e);
        }
    }

哦？这里有个备注：如果这里是缩略图请求怎么办？我思考了半天也是不解这里为什么会有个问号备注，当然这不是重点。这里把图片加载的状态设置了failed，然后调用了setErrorPlacehoder(e)方法，看方法名字就知道这是在设置错误缺省图。

private void setErrorPlaceholder(Exception e) {
        if (!canNotifyStatusChanged()) {
            return;
        }

        Drawable error = model == null ? getFallbackDrawable() : null;
        if (error == null) {
          error = getErrorDrawable();
        }
        if (error == null) {
            error = getPlaceholderDrawable();
        }
        target.onLoadFailed(e, error);
    }

这里我们发现还有一个canNotifyStatusChanged这么一个变量，有一个监听状态是否改变的过程确保了安全性和正确性。再来看这两个方法获取了placeholder跟error的图片：

private Drawable getErrorDrawable() {
        if (errorDrawable == null && errorResourceId > 0) {
            errorDrawable = context.getResources().getDrawable(errorResourceId);
        }
        return errorDrawable;
    }

private Drawable getPlaceholderDrawable() {
        if (placeholderDrawable == null && placeholderResourceId > 0) {
            placeholderDrawable = context.getResources().getDrawable(placeholderResourceId);
        }
        return placeholderDrawable;
    }

接下来来看target.onLoadFailed(e, error)这个方法，要找这个方法又得回到最初的地方，因为target其实也就是GenericRequestBuilder里的into方法里glide.buildImageView()，所以onLoadFailed方法也就应该在ImageViewTarget类里面。

    @Override
    public void onLoadFailed(Exception e, Drawable errorDrawable) {
        view.setImageDrawable(errorDrawable);
    }

很easy，给view设置图片，当然这里我们还看到了设置缺省图:

    @Override
    public void onLoadStarted(Drawable placeholder) {
        view.setImageDrawable(placeholder);
    }

ok，看完错误缺省图的加载过程之后我们再回到GenericRequest类的begin()方法，看源码就是一个走迷宫的过程，一条路走到底了，然后再回头继续走下一条路，注意力稍微有一点不集中，就懵了。。。来看if跟else，if(Util.isValidDimensions(overrideWidth, overrideHeight))看了下工具类里这个方法，也就是说如果宽度和高度都是指定的话，也就是固定的话，就调用onSizeReady(width, height)方法，不然的话就调用target.getSize(this)方法，ok，我们先来看getSize()方法，跟之前的onLoadFailed一样，target就是ImageViewTarget，但是在里面找不到getSize()方法，所以应该在他的父类里，也就是ViewTarget类里找到了这个方法：

@Override
    public void getSize(SizeReadyCallback cb) {
        sizeDeterminer.getSize(cb);
    }

private static class SizeDeterminer {

private void notifyCbs(int width, int height) {
            for (SizeReadyCallback cb : cbs) {
                cb.onSizeReady(width, height);
            }
            cbs.clear();
        }
        //省略代码。。。。。。
        public void getSize(SizeReadyCallback cb) {
            int currentWidth = getViewWidthOrParam();
            int currentHeight = getViewHeightOrParam();
            if (isSizeValid(currentWidth) && isSizeValid(currentHeight)) {
                cb.onSizeReady(currentWidth, currentHeight);
            } else {
                // We want to notify callbacks in the order they were added and we only expect one or two callbacks to
                // be added a time, so a List is a reasonable choice.
                if (!cbs.contains(cb)) {
                    cbs.add(cb);
                }
                if (layoutListener == null) {
                    final ViewTreeObserver observer = view.getViewTreeObserver();
                    layoutListener = new SizeDeterminerLayoutListener(this);
                    observer.addOnPreDrawListener(layoutListener);
                }
            }
        }

       //省略一大段代码
        }
    }

我们可以看到，这里其实根据imageView的width，height做了一系列的计算之后得出了图片应该显示的多宽跟多高。计算结束之后，我们可以看到在SizeDeterminer里面最后的最后还是调用了onSizeReady()方法。那我们就来看看这个方法里到底干了什么：

 @Override
    public void onSizeReady(int width, int height) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
        if (status != Status.WAITING_FOR_SIZE) {
            return;
        }
        status = Status.RUNNING;

        width = Math.round(sizeMultiplier * width);
        height = Math.round(sizeMultiplier * height);

        ModelLoader<A, T> modelLoader = loadProvider.getModelLoader();
        final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);

        if (dataFetcher == null) {
            onException(new Exception("Failed to load model: \'" + model + "\'"));
            return;
        }
        ResourceTranscoder<Z, R> transcoder = loadProvider.getTranscoder();
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
        }
        loadedFromMemoryCache = true;
        loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
                priority, isMemoryCacheable, diskCacheStrategy, this);
        loadedFromMemoryCache = resource != null;
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
    }

ok我们在这里又看到了ModelLoader，是不是很惊喜是不是很惊讶，这里通过loadProvider创建了一个ModelLoader，这个loadProvider又是什么东西呢？这我们就得回去回去看了。让我们回到load()方法，还记得我们load()方法里在buildModelLoader()的时候通过getFactory()创建了一个ModelLoaderFactory嘛，我们在那里生成返回了一个DrawableTypeRequest对象，在创建DrawableTypeRequest的时候new出了一个FixedLoadProvider类，而这个loadProvider其实就是FixedLoadProvider类的一个封装类ChildLoadProvider的一个实体对象。我们可以看看GenericRequestBuilder类的构造函数：

GenericRequestBuilder(Context context, Class<ModelType> modelClass,
            LoadProvider<ModelType, DataType, ResourceType, TranscodeType> loadProvider,
            Class<TranscodeType> transcodeClass, Glide glide, RequestTracker requestTracker, Lifecycle lifecycle) {
        this.context = context;
        this.modelClass = modelClass;
        this.transcodeClass = transcodeClass;
        this.glide = glide;
        this.requestTracker = requestTracker;
        this.lifecycle = lifecycle;
        this.loadProvider = loadProvider != null
                ? new ChildLoadProvider<ModelType, DataType, ResourceType, TranscodeType>(loadProvider) : null;

        if (context == null) {
            throw new NullPointerException("Context can't be null");
        }
        if (modelClass != null && loadProvider == null) {
            throw new NullPointerException("LoadProvider must not be null");
        }
    }

ok，之后我们再回到onSizeReady()方法，之后调用了modelLoader.getResourceFetcher(model, width, height)方法，这个又是干嘛的呢？进去看看，getResourceFetcher()方法在哪里呢。。我们看到之前DrawableTypeRequest的构造函数里其实把loadGeneric()方法中创建的两个modelLoader对象都封装到了ImageVideoModelLoader这个类里面并new出了一个对象，所以getResourceFetcher()应该在这个里面：

@Override
    public DataFetcher<ImageVideoWrapper> getResourceFetcher(A model, int width, int height) {
        DataFetcher<InputStream> streamFetcher = null;
        if (streamLoader != null) {
            streamFetcher = streamLoader.getResourceFetcher(model, width, height);
        }
        DataFetcher<ParcelFileDescriptor> fileDescriptorFetcher = null;
        if (fileDescriptorLoader != null) {
            fileDescriptorFetcher = fileDescriptorLoader.getResourceFetcher(model, width, height);
        }

        if (streamFetcher != null || fileDescriptorFetcher != null) {
            return new ImageVideoFetcher(streamFetcher, fileDescriptorFetcher);
        } else {
            return null;
        }
    }

因为我们现在是第一次调用Glide加载图片，所以生成一个HttpUrlFetch对象。这里绕来绕去其实最后getResourceFetch()方法最后还是返回一个ImageVideoFetcher对象。然后我们再回去onSizeReady()方法，然后就是把这些new出来的东西全部传到engine.load()方法里面去。也就是这句话：

loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
                priority, isMemoryCacheable, diskCacheStrategy, this);

那接下去就来看看Engine呗，Engine这个类看注释主要是负责加载以及管理活跃的和已经缓存了的资源。我们先暂时不看缓存相关的东西，等会讲缓存的东西的时候再翻回来看，我们现在先看加载的整个流程。ok，那我们来看Engine这个类里的load方法：

public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,
            DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,
            Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
        Util.assertMainThread();
        long startTime = LogTime.getLogTime();

        final String id = fetcher.getId();
        EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
                loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
                transcoder, loadProvider.getSourceEncoder());

        EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
        if (cached != null) {
            cb.onResourceReady(cached);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Loaded resource from cache", startTime, key);
            }
            return null;
        }

        EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
        if (active != null) {
            cb.onResourceReady(active);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Loaded resource from active resources", startTime, key);
            }
            return null;
        }

        EngineJob current = jobs.get(key);
        if (current != null) {
            current.addCallback(cb);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Added to existing load", startTime, key);
            }
            return new LoadStatus(cb, current);
        }

        EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
        DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
                transcoder, diskCacheProvider, diskCacheStrategy, priority);
        EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
        jobs.put(key, engineJob);
        engineJob.addCallback(cb);
        engineJob.start(runnable);

        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Started new load", startTime, key);
        }
        return new LoadStatus(cb, engineJob);
    }

忽略缓存相关的代码，首先创建了个EngineJob对象，然后创建了一个DecodeJob对象，然后把这两个对象穿进去创建了EngineRunnable对象，之后start()，ok所以我们现在就来重点看一下 EngineRunnable的run()方法。

@Override
    public void run() {
        if (isCancelled) {
            return;
        }

        Exception exception = null;
        Resource<?> resource = null;
        try {
            resource = decode();
        } catch (Exception e) {
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                Log.v(TAG, "Exception decoding", e);
            }
            exception = e;
        }

        if (isCancelled) {
            if (resource != null) {
                resource.recycle();
            }
            return;
        }

        if (resource == null) {
            onLoadFailed(exception);
        } else {
            onLoadComplete(resource);
        }
    }

后面我们可以看到一个是请求取消了的情况，一个是加载失败了，一个是加载完成了，所以基本上记载的过程应该都是在decode()这个方法里面，进去：

private Resource<?> decode() throws Exception {
        if (isDecodingFromCache()) {
            return decodeFromCache();
        } else {
            return decodeFromSource();
        }
    }

顾名思义，一个是从缓存里获取图片，不然就去执行decodeFromSource()方法，ok，我们这里先走流程，先看decodeFromSouce()方法：

private Resource<?> decodeFromSource() throws Exception {
        return decodeJob.decodeFromSource();
    }

再进入到decodeFromSource()方法进入到DecodeJob类里：

public Resource<Z> decodeFromSource() throws Exception {
        Resource<T> decoded = decodeSource();
        return transformEncodeAndTranscode(decoded);
    }

private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
        long startTime = LogTime.getLogTime();
        Resource<T> transformed = transform(decoded);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transformed resource from source", startTime);
        }

        writeTransformedToCache(transformed);

        startTime = LogTime.getLogTime();
        Resource<Z> result = transcode(transformed);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transcoded transformed from source", startTime);
        }
        return result;
    }

private Resource<T> decodeSource() throws Exception {
        Resource<T> decoded = null;
        try {
            long startTime = LogTime.getLogTime();
            final A data = fetcher.loadData(priority);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Fetched data", startTime);
            }
            if (isCancelled) {
                return null;
            }
            decoded = decodeFromSourceData(data);
        } finally {
            fetcher.cleanup();
        }
        return decoded;
    }

在这里，我们看到了fetcher.loadData()方法，这里的fetcher其实就是httpUrlFetcher的实体对象，接着往下看:

@Override
    public InputStream loadData(Priority priority) throws Exception {
        return loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/, glideUrl.getHeaders());
    }

private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl, Map<String, String> headers)
            throws IOException {
        if (redirects >= MAXIMUM_REDIRECTS) {
            throw new IOException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
        } else {
            // Comparing the URLs using .equals performs additional network I/O and is generally broken.
            // See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.
            try {
                if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
                    throw new IOException("In re-direct loop");
                }
            } catch (URISyntaxException e) {
                // Do nothing, this is best effort.
            }
        }
        urlConnection = connectionFactory.build(url);
        for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
          urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
        }
        urlConnection.setConnectTimeout(2500);
        urlConnection.setReadTimeout(2500);
        urlConnection.setUseCaches(false);
        urlConnection.setDoInput(true);

        // Connect explicitly to avoid errors in decoders if connection fails.
        urlConnection.connect();
        if (isCancelled) {
            return null;
        }
        final int statusCode = urlConnection.getResponseCode();
        if (statusCode / 100 == 2) {
            return getStreamForSuccessfulRequest(urlConnection);
        } else if (statusCode / 100 == 3) {
            String redirectUrlString = urlConnection.getHeaderField("Location");
            if (TextUtils.isEmpty(redirectUrlString)) {
                throw new IOException("Received empty or null redirect url");
            }
            URL redirectUrl = new URL(url, redirectUrlString);
            return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
        } else {
            if (statusCode == -1) {
                throw new IOException("Unable to retrieve response code from HttpUrlConnection.");
            }
            throw new IOException("Request failed " + statusCode + ": " + urlConnection.getResponseMessage());
        }
    }

哦？我们终于看到了跟网络通讯有关的东西了。在这里，我们只是拿到了一个inputStream的输入流而已。ok，我们再回去，回到DecodeJob的decodeSource()方法，我们发现这个方法其实起到了解码的作用:

private Resource<T> decodeFromSourceData(A data) throws IOException {
        final Resource<T> decoded;
        if (diskCacheStrategy.cacheSource()) {
            decoded = cacheAndDecodeSourceData(data);
        } else {
            long startTime = LogTime.getLogTime();
            decoded = loadProvider.getSourceDecoder().decode(data, width, height);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Decoded from source", startTime);
            }
        }
        return decoded;
    }

这里的loadProvider当然还是之前的FixdLoadProvider，它的sourceDecoder其实就是ImageVideoGifDrawableLoadProvider里的sourceDecoder，也就是GifBitmapWrapperResourceDecoder的一个对象，我们顺着这条路，来看GifBitmapWrapperResourceDecoder这个类的decode()方法：

private GifBitmapWrapper decode(ImageVideoWrapper source, int width, int height, byte[] bytes) throws IOException {
        final GifBitmapWrapper result;
        if (source.getStream() != null) {
            result = decodeStream(source, width, height, bytes);
        } else {
            result = decodeBitmapWrapper(source, width, height);
        }
        return result;
    }

private GifBitmapWrapper decodeStream(ImageVideoWrapper source, int width, int height, byte[] bytes)
            throws IOException {
        InputStream bis = streamFactory.build(source.getStream(), bytes);
        bis.mark(MARK_LIMIT_BYTES);
        ImageHeaderParser.ImageType type = parser.parse(bis);
        bis.reset();

        GifBitmapWrapper result = null;
        if (type == ImageHeaderParser.ImageType.GIF) {
            result = decodeGifWrapper(bis, width, height);
        }
        // Decoding the gif may fail even if the type matches.
        if (result == null) {
            // We can only reset the buffered InputStream, so to start from the beginning of the stream, we need to
            // pass in a new source containing the buffered stream rather than the original stream.
            ImageVideoWrapper forBitmapDecoder = new ImageVideoWrapper(bis, source.getFileDescriptor());
            result = decodeBitmapWrapper(forBitmapDecoder, width, height);
        }
        return result;
    }

我们看到，这里通过调用decodeStream()方法来读取数据，根据ImageHeaderParser.ImageType来判断这张图到底是gif图还是普通图片，gif图的解码过程我看了半天其实还没搞清楚，有点复杂，等我搞清楚了再来，我们这里先看静图的解码过程吧

private GifBitmapWrapper decodeBitmapWrapper(ImageVideoWrapper toDecode, int width, int height) throws IOException {
        GifBitmapWrapper result = null;

        Resource<Bitmap> bitmapResource = bitmapDecoder.decode(toDecode, width, height);
        if (bitmapResource != null) {
            result = new GifBitmapWrapper(bitmapResource, null);
        }

        return result;
    }

这里调用了一个bitmapDecoder.decode(toDecode, width, height)方法，这其实就是ImageVideoBitmapDecoder的decode()方法

 public Resource<Bitmap> decode(ImageVideoWrapper source, int width, int height) throws IOException {
        Resource<Bitmap> result = null;
        InputStream is = source.getStream();
        if (is != null) {
            try {
                result = streamDecoder.decode(is, width, height);
            } catch (IOException e) {
                if (Log.isLoggable(TAG, Log.VERBOSE)) {
                    Log.v(TAG, "Failed to load image from stream, trying FileDescriptor", e);
                }
            }
        }

        if (result == null) {
            ParcelFileDescriptor fileDescriptor = source.getFileDescriptor();
            if (fileDescriptor != null) {
                result = fileDescriptorDecoder.decode(fileDescriptor, width, height);
            }
        }
        return result;
    }

我们看到这里又是调用了streamDecoder.decode(is, width, height);的方法，定位一下其实streamDecoder就是StreamBitmapDecoder类，ok，继续：

@Override
    public Resource<Bitmap> decode(InputStream source, int width, int height) {
        Bitmap bitmap = downsampler.decode(source, bitmapPool, width, height, decodeFormat);
        return BitmapResource.obtain(bitmap, bitmapPool);
    }

oh shit，终于看到bitmap了，ok，来看decode()方法：

 @Override
    public Bitmap decode(InputStream is, BitmapPool pool, int outWidth, int outHeight, DecodeFormat decodeFormat) {
        final ByteArrayPool byteArrayPool = ByteArrayPool.get();
        final byte[] bytesForOptions = byteArrayPool.getBytes();
        final byte[] bytesForStream = byteArrayPool.getBytes();
        final BitmapFactory.Options options = getDefaultOptions();

        // Use to fix the mark limit to avoid allocating buffers that fit entire images.
        RecyclableBufferedInputStream bufferedStream = new RecyclableBufferedInputStream(
                is, bytesForStream);
        // Use to retrieve exceptions thrown while reading.
        // TODO(#126): when the framework no longer returns partially decoded Bitmaps or provides a way to determine
        // if a Bitmap is partially decoded, consider removing.
        ExceptionCatchingInputStream exceptionStream =
                ExceptionCatchingInputStream.obtain(bufferedStream);
        // Use to read data.
        // Ensures that we can always reset after reading an image header so that we can still attempt to decode the
        // full image even when the header decode fails and/or overflows our read buffer. See #283.
        MarkEnforcingInputStream invalidatingStream = new MarkEnforcingInputStream(exceptionStream);
        try {
            exceptionStream.mark(MARK_POSITION);
            int orientation = 0;
            try {
                orientation = new ImageHeaderParser(exceptionStream).getOrientation();
            } catch (IOException e) {
                if (Log.isLoggable(TAG, Log.WARN)) {
                    Log.w(TAG, "Cannot determine the image orientation from header", e);
                }
            } finally {
                try {
                    exceptionStream.reset();
                } catch (IOException e) {
                    if (Log.isLoggable(TAG, Log.WARN)) {
                        Log.w(TAG, "Cannot reset the input stream", e);
                    }
                }
            }

            options.inTempStorage = bytesForOptions;

            final int[] inDimens = getDimensions(invalidatingStream, bufferedStream, options);
            final int inWidth = inDimens[0];
            final int inHeight = inDimens[1];

            final int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
            final int sampleSize = getRoundedSampleSize(degreesToRotate, inWidth, inHeight, outWidth, outHeight);

            final Bitmap downsampled =
                    downsampleWithSize(invalidatingStream, bufferedStream, options, pool, inWidth, inHeight, sampleSize,
                            decodeFormat);

            // BitmapFactory swallows exceptions during decodes and in some cases when inBitmap is non null, may catch
            // and log a stack trace but still return a non null bitmap. To avoid displaying partially decoded bitmaps,
            // we catch exceptions reading from the stream in our ExceptionCatchingInputStream and throw them here.
            final Exception streamException = exceptionStream.getException();
            if (streamException != null) {
                throw new RuntimeException(streamException);
            }

            Bitmap rotated = null;
            if (downsampled != null) {
                rotated = TransformationUtils.rotateImageExif(downsampled, pool, orientation);

                if (!downsampled.equals(rotated) && !pool.put(downsampled)) {
                    downsampled.recycle();
                }
            }

            return rotated;
        } finally {
            byteArrayPool.releaseBytes(bytesForOptions);
            byteArrayPool.releaseBytes(bytesForStream);
            exceptionStream.release();
            releaseOptions(options);
        }
    }

在这里，我们对服务器返回的inputStream进行了读取，以及图片的加载， MarkEnforcingInputStream invalidatingStream = new MarkEnforcingInputStream(exceptionStream)这个就是用来读取数据的。这个方法里还操作了很多逻辑，旋转，圆角等，具体就不详述了。ok，这里我们得到了bitmap对象，接下来任务就是就是把bitmap对象去显示出来。我们再次回到GifBitmapWrapperResourceDecoder的decodeBitmapWrapper方法中我们发现这里对bitmap进行了一次封装，同时支持了bitmap图片和gif图片，返回的是GifBitmapWrapper类的对象。ok知道了这个以后我们再回去，回到DecodeJob的decodeFromSource()方法：

public Resource<Z> decodeFromSource() throws Exception {
    Resource<T> decoded = decodeSource();
    return transformEncodeAndTranscode(decoded);
}

再来看transformEncodeAndTranscode(decoded)这个方法：

private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
        long startTime = LogTime.getLogTime();
        Resource<T> transformed = transform(decoded);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transformed resource from source", startTime);
        }

        writeTransformedToCache(transformed);

        startTime = LogTime.getLogTime();
        Resource<Z> result = transcode(transformed);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transcoded transformed from source", startTime);
        }
        return result;
    }

这里前面的代码都没什么用，我们来看一开始transformed是一个Resource对象，经过transcode()方法之后变成了一个Resource对象，其实这里的transformed就是GifBitmapWrapper转变后的GifBitmapWrapper对象。我们再来看transcode()方法:

private Resource<Z> transcode(Resource<T> transformed) {
        if (transformed == null) {
            return null;
        }
        return transcoder.transcode(transformed);
    }

ok，再定位，这里的transcoder其实就是GifBitmapWrapperDrawableTranscoder的一个实例，接下去看它的transcode()方法：

@SuppressWarnings("unchecked")
    @Override
    public Resource<GlideDrawable> transcode(Resource<GifBitmapWrapper> toTranscode) {
        GifBitmapWrapper gifBitmap = toTranscode.get();
        Resource<Bitmap> bitmapResource = gifBitmap.getBitmapResource();

        final Resource<? extends GlideDrawable> result;
        if (bitmapResource != null) {
            result = bitmapDrawableResourceTranscoder.transcode(bitmapResource);
        } else {
            result = gifBitmap.getGifResource();
        }
        // This is unchecked but always safe, anything that extends a Drawable can be safely cast to a Drawable.
        return (Resource<GlideDrawable>) result;
    }

这里transcode()方法直接返回了Resource。ok，接下来，时间回滚，我们又要回去了，回到EngineRunnable的decodeFromSource()方法，刚才的这么多分析，我们最终知道了decode()方法执行以后得到一个Resource对象。接下来就是如何显示的问题了，一个是错误的时候onLoadFailed()，还有一个就是加载完成以后调用的onLoadComplete(resource);方法，我们看里面return了onResourceReady(resource)方法，定位到EngineJob的onResourceReady()方法：

@Override
    public void onResourceReady(final Resource<?> resource) {
        this.resource = resource;
        MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
    }

这里用到了handler，说明之前的EngineRunnable都是在子线程中加载的，handler通知MSG_COMPLETE，然后我们看到了：

private static class MainThreadCallback implements Handler.Callback {

        @Override
        public boolean handleMessage(Message message) {
            if (MSG_COMPLETE == message.what || MSG_EXCEPTION == message.what) {
                EngineJob job = (EngineJob) message.obj;
                if (MSG_COMPLETE == message.what) {
                    job.handleResultOnMainThread();
                } else {
                    job.handleExceptionOnMainThread();
                }
                return true;
            }

            return false;
        }
    }

感觉胜利在望了，已经又回到了主线程来更新UI了，接着来看 job.handleResultOnMainThread()方法：

private void handleResultOnMainThread() {
        if (isCancelled) {
            resource.recycle();
            return;
        } else if (cbs.isEmpty()) {
            throw new IllegalStateException("Received a resource without any callbacks to notify");
        }
        engineResource = engineResourceFactory.build(resource, isCacheable);
        hasResource = true;

        // Hold on to resource for duration of request so we don't recycle it in the middle of notifying if it
        // synchronously released by one of the callbacks.
        engineResource.acquire();
        listener.onEngineJobComplete(key, engineResource);

        for (ResourceCallback cb : cbs) {
            if (!isInIgnoredCallbacks(cb)) {
                engineResource.acquire();
                cb.onResourceReady(engineResource);
            }
        }
        // Our request is complete, so we can release the resource.
        engineResource.release();
    }

这里发现调用了两遍onResouceReady()，ok，我们沿着方法的调用顺序回去，回到GenericRequest的onResourceReady()方法:

public void onResourceReady(Resource<?> resource) {
        if (resource == null) {
            onException(new Exception("Expected to receive a Resource<R> with an object of " + transcodeClass
                    + " inside, but instead got null."));
            return;
        }

        Object received = resource.get();
        if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
            releaseResource(resource);
            onException(new Exception("Expected to receive an object of " + transcodeClass
                    + " but instead got " + (received != null ? received.getClass() : "") + "{" + received + "}"
                    + " inside Resource{" + resource + "}."
                    + (received != null ? "" : " "
                        + "To indicate failure return a null Resource object, "
                        + "rather than a Resource object containing null data.")
            ));
            return;
        }

        if (!canSetResource()) {
            releaseResource(resource);
            // We can't set the status to complete before asking canSetResource().
            status = Status.COMPLETE;
            return;
        }

        onResourceReady(resource, (R) received);
    }

private void onResourceReady(Resource<?> resource, R result) {
        // We must call isFirstReadyResource before setting status.
        boolean isFirstResource = isFirstReadyResource();
        status = Status.COMPLETE;
        this.resource = resource;

        if (requestListener == null || !requestListener.onResourceReady(result, model, target, loadedFromMemoryCache,
                isFirstResource)) {
            GlideAnimation<R> animation = animationFactory.build(loadedFromMemoryCache, isFirstResource);
            target.onResourceReady(result, animation);
        }

        notifyLoadSuccess();

        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Resource ready in " + LogTime.getElapsedMillis(startTime) + " size: "
                    + (resource.getSize() * TO_MEGABYTE) + " fromCache: " + loadedFromMemoryCache);
        }
    }

这里有两个onResourceReady()方法，第一个取得了封装的图片对象，然后传给了第二个onResouceReady()方法，注意这里有一个： target.onResourceReady(result, animation);

定位这个target，发现其实就是GlideDrawableImageViewTarget的实例，来看这个类：

public class GlideDrawableImageViewTarget extends ImageViewTarget<GlideDrawable> {
    private static final float SQUARE_RATIO_MARGIN = 0.05f;
    private int maxLoopCount;
    private GlideDrawable resource;

    public GlideDrawableImageViewTarget(ImageView view) {
        this(view, GlideDrawable.LOOP_FOREVER);
    }

    public GlideDrawableImageViewTarget(ImageView view, int maxLoopCount) {
        super(view);
        this.maxLoopCount = maxLoopCount;
    }

    @Override
    public void onResourceReady(GlideDrawable resource, GlideAnimation<? super GlideDrawable> animation) {
        if (!resource.isAnimated()) {

            float viewRatio = view.getWidth() / (float) view.getHeight();
            float drawableRatio = resource.getIntrinsicWidth() / (float) resource.getIntrinsicHeight();
            if (Math.abs(viewRatio - 1f) <= SQUARE_RATIO_MARGIN
                    && Math.abs(drawableRatio - 1f) <= SQUARE_RATIO_MARGIN) {
                resource = new SquaringDrawable(resource, view.getWidth());
            }
        }
        super.onResourceReady(resource, animation);
        this.resource = resource;
        resource.setLoopCount(maxLoopCount);
        resource.start();
    }

    /**
     * Sets the drawable on the view using
     * {@link android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
     *
     * @param resource The {@link android.graphics.drawable.Drawable} to display in the view.
     */
    @Override
    protected void setResource(GlideDrawable resource) {
        view.setImageDrawable(resource);
    }

    @Override
    public void onStart() {
        if (resource != null) {
            resource.start();
        }
    }

    @Override
    public void onStop() {
        if (resource != null) {
            resource.stop();
        }
    }
}

注意这里的onSourceReady()方法，super.onResourceReady(resource, animation); 我们来到它的父类ImageViewTarget类：

 @Override
    public void onResourceReady(Z resource, GlideAnimation<? super Z> glideAnimation) {
        if (glideAnimation == null || !glideAnimation.animate(resource, this)) {
            setResource(resource);
        }
    }

    protected abstract void setResource(Z resource);

这是个抽象方法，所以我们去子类寻找实现的逻辑，也就是GlideDrawableImageViewTarget类：

@Override
    protected void setResource(GlideDrawable resource) {
        view.setImageDrawable(resource);
    }

酷哦。到这里也就显示出来了。

厉害了，写了这么多，本来想把Glide的缓存一并讲进去了，但实在太长了，算了，另起一篇了只能。接下去会写一篇Glide的缓存机制，Fresco源码解析（都说Fresco性能上比Glide要优秀，但是到底优秀在哪里。我觉得有必要深究一下）。