Glide解密-Android图片加载生命周期管理

在前面Glide鉴赏文章中，我们从整体架构和加载图片的完整流程对Glide进行了详细的分析，希望大家能对Glide有大概的理解，常言道：取其精华，去其糟粕，与其深陷在源码中，不如看看真正值得我们学习的是什么

Glide有个特性是可以依赖页面生命周期动态控制加载图片，比如在页面不可见时暂停加载，页面恢复时继续加载图片。其实现思想如下：

1 获取FragmentManager

在Glide.with流程中，调用RequestManagerRetriever#get这一步，根据传入Context类型和当前线程获取当前页面的FragmentManager

• 若是在子线程或传入的Context为Application，最终调用RequestManagerRetriever#getApplicationManager，传入的是Application Context构建RequestManager，生命周期和App一致。
• 若是在主线程，根据Context类型，获取到当前页面的FragmentManager，如下源码所示

  @NonNull
  public RequestManager get(@NonNull FragmentActivity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      FragmentManager fm = activity.getSupportFragmentManager();
      return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
  }

  @NonNull
  public RequestManager get(@NonNull Fragment fragment) {
    Preconditions.checkNotNull(
        fragment.getContext(),
        "You cannot start a load on a fragment before it is attached or after it is destroyed");
    if (Util.isOnBackgroundThread()) {
      return get(fragment.getContext().getApplicationContext());
    } else {
      FragmentManager fm = fragment.getChildFragmentManager();
      return supportFragmentGet(fragment.getContext(), fm, fragment, fragment.isVisible());
    }
  }

2 构建SupportRequestManagerFragment

在上面获取到的FragmentManager中，先查找是否已经存在当前页面对应的SupportRequestManagerFragment，若不存在，则进行构建，并将其添加到上面获取到的FragmentManager中。对于SupportRequestManagerFragment,我们需要关注其内部这个对象ActivityFragmentLifecycle

ActivityFragmentLifecycle是用于添加依赖生命周期监听的对象。在SupportRequestManagerFragment构造时创建，只要实现了LifecycleListener接口的对象，都可以添加到弱引用的Set中，防止重复添加和内存泄漏。在SupportRequestManagerFragment的onStart，onStop，onDestroy会回调ActivityFragmentLifecycle对象的对应方法，从而对ActivityFragmentLifecycle内部添加的监听对象也进行遍历，调用各自的生命周期方法

public class SupportRequestManagerFragment extends Fragment {
  private final ActivityFragmentLifecycle lifecycle;
  private RequestManager requestManager;

  public SupportRequestManagerFragment() {
    this(new ActivityFragmentLifecycle());
  }

  ActivityFragmentLifecycle getGlideLifecycle() {
    return lifecycle;
  }

  public void setRequestManager(@Nullable RequestManager requestManager) {
    this.requestManager = requestManager;
  }

  public RequestManager getRequestManager() {
    return requestManager;
  }

  @Override
  public void onStart() {
    super.onStart();
    lifecycle.onStart();
  }
    // ...onStop, onDestory
}

class ActivityFragmentLifecycle implements Lifecycle {
  private final Set<LifecycleListener> lifecycleListeners =
      Collections.newSetFromMap(new WeakHashMap<LifecycleListener, Boolean>());
  private boolean isStarted;
  private boolean isDestroyed;

  @Override
  public void addListener(@NonNull LifecycleListener listener) {
    lifecycleListeners.add(listener);
    if (isDestroyed) {
      listener.onDestroy();
    } else if (isStarted) {
      listener.onStart();
    } else {
      listener.onStop();
    }
  }

  @Override
  public void removeListener(@NonNull LifecycleListener listener) {
    lifecycleListeners.remove(listener);
  }

  void onStart() {
    isStarted = true;
    for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
      lifecycleListener.onStart();
    }
  }
    //...onStop，onDestroy
}

3 构建RequestManager和绑定监听

在构建了SupportRequestManagerFragment后，也是先从中获取RequestManager，即当前页面的请求管理器。若没有，则传入上面SupportRequestManagerFragment内部的ActivityFragmentLifecycle对象current.getGlideLifecycle()，构建出RequestManager，在RequestManager构造方法中，会将自身添加进传入的ActivityFragmentLifecycle对象里。如此，就将RequestManager和SupportRequestManagerFragment的生命周期方法绑定了

  // RequestManagerRetriever.java
  private RequestManager supportFragmentGet(@NonNull Context context, @NonNull FragmentManager fm,
      @Nullable Fragment parentHint, boolean isParentVisible) {
    SupportRequestManagerFragment current =
        getSupportRequestManagerFragment(fm, parentHint, isParentVisible);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

以页面不可见时暂停加载为例，SupportRequestManagerFragment会执行onStop，流程如下图

public class RequestManager implements ComponentCallbacks2, LifecycleListener, ModelTypes<RequestBuilder<Drawable>> {
    public RequestManager(
      @NonNull Glide glide,
      @NonNull Lifecycle lifecycle,
      @NonNull RequestManagerTreeNode treeNode,
      @NonNull Context context) {
          //...
      }
    @Override
    public synchronized void onStop() {
      pauseRequests();
      targetTracker.onStop();
    }
}