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Android优化————布局优化

绘画原理

Android的绘制主要是借助cpu和gpu结合刷新机制共同完成的

  • cpu:负责执行measure、layout等方法,计算显示内容
  • gpu:负责栅格化(将UI元素绘制在屏幕上) image

绘制过程使用skia库(2D),硬件本质是采用openGL库进行绘制

16ms内渲染一次,否则会掉帧

布局加载原理

Android中的布局加载入口为setContentView(),分析如下:

 @Override
    public void setContentView(@LayoutRes int layoutResID) {
        getDelegate().setContentView(layoutResID);
    }
    
    //AppCompatDelegate.java
    public abstract void setContentView(@LayoutRes int resId);
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查看抽象接口实现

 @Override
    public void setContentView(int resId) {
        ensureSubDecor();
        //获取content跟布局
        ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
        //移除所有布局
        contentParent.removeAllViews();
        //加载新布局
        LayoutInflater.from(mContext).inflate(resId, contentParent);
        //接口状态通知
        mOriginalWindowCallback.onContentChanged();
    }
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进入inflate方法:

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
        return inflate(resource, root, root != null);
    }
    
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        if (DEBUG) {
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                    + Integer.toHexString(resource) + ")");
        }

        final XmlResourceParser parser = res.getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }
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getLayout方法返回一个XmlResourceParser对象:

    public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException {
        return loadXmlResourceParser(id, "layout");
    }
    
    @NonNull
    XmlResourceParser loadXmlResourceParser(@AnyRes int id, @NonNull String type)
            throws NotFoundException {
        final TypedValue value = obtainTempTypedValue();
        try {
            final ResourcesImpl impl = mResourcesImpl;
            impl.getValue(id, value, true);
            if (value.type == TypedValue.TYPE_STRING) {
                return impl.loadXmlResourceParser(value.string.toString(), id,
                        value.assetCookie, type);
            }
            throw new NotFoundException("Resource ID #0x" + Integer.toHexString(id)
                    + " type #0x" + Integer.toHexString(value.type) + " is not valid");
        } finally {
            releaseTempTypedValue(value);
        }
    }
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进入loadXmlResourceParser:

 @NonNull
    XmlResourceParser loadXmlResourceParser(@NonNull String file, @AnyRes int id, int assetCookie,
            @NonNull String type)
            throws NotFoundException {
        if (id != 0) {
            try {
                synchronized (mCachedXmlBlocks) {
                    final int[] cachedXmlBlockCookies = mCachedXmlBlockCookies;
                    final String[] cachedXmlBlockFiles = mCachedXmlBlockFiles;
                    final XmlBlock[] cachedXmlBlocks = mCachedXmlBlocks;
                    // First see if this block is in our cache.
                    final int num = cachedXmlBlockFiles.length;
                    for (int i = 0; i < num; i++) {
                        if (cachedXmlBlockCookies[i] == assetCookie && cachedXmlBlockFiles[i] != null
                                && cachedXmlBlockFiles[i].equals(file)) {
                            return cachedXmlBlocks[i].newParser();
                        }
                    }

                    // Not in the cache, create a new block and put it at
                    // the next slot in the cache.
                    final XmlBlock block = mAssets.openXmlBlockAsset(assetCookie, file);
                    if (block != null) {
                        final int pos = (mLastCachedXmlBlockIndex + 1) % num;
                        mLastCachedXmlBlockIndex = pos;
                        final XmlBlock oldBlock = cachedXmlBlocks[pos];
                        if (oldBlock != null) {
                            oldBlock.close();
                        }
                        cachedXmlBlockCookies[pos] = assetCookie;
                        cachedXmlBlockFiles[pos] = file;
                        cachedXmlBlocks[pos] = block;
                        return block.newParser();
                    }
                }
            } catch (Exception e) {
                final NotFoundException rnf = new NotFoundException("File " + file
                        + " from xml type " + type + " resource ID #0x" + Integer.toHexString(id));
                rnf.initCause(e);
                throw rnf;
            }
        }

        throw new NotFoundException("File " + file + " from xml type " + type + " resource ID #0x"
                + Integer.toHexString(id));
    }
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加载指定布局文件的xml,生成XMLBlock:

 /*package*/ final XmlBlock openXmlBlockAsset(int cookie, String fileName)
        throws IOException {
        synchronized (this) {
            if (!mOpen) {
                throw new RuntimeException("Assetmanager has been closed");
            }
            long xmlBlock = openXmlAssetNative(cookie, fileName);
            if (xmlBlock != 0) {
                XmlBlock res = new XmlBlock(this, xmlBlock);
                incRefsLocked(res.hashCode());
                return res;
            }
        }
        throw new FileNotFoundException("Asset XML file: " + fileName);
    }
    
    private native final long openXmlAssetNative(int cookie, String fileName);
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最终指向了native方法

获取到XMLResourceParser后,进行渲染:

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            View result = root;

            try {
                // Look for the root node.
                int type;
                while ((type = parser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty
                }

                if (type != XmlPullParser.START_TAG) {
                    throw new InflateException(parser.getPositionDescription()
                            + ": No start tag found!");
                }

                final String name = parser.getName();

                if (DEBUG) {
                    System.out.println("**************************");
                    System.out.println("Creating root view: "
                            + name);
                    System.out.println("**************************");
                }
                //如果是merge标签,查看是否是当前布局的父节点,不是的话抛出异常
                if (TAG_MERGE.equals(name)) {
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }

                    rInflate(parser, root, inflaterContext, attrs, false);
                } else {
                    // Temp is the root view that was found in the xml
                    //获取xml中的根节点
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);
                    }
                ...
                }
            }
        }
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使用XmlPull解析布局,如果是merge标签,merge节点不是当前布局的父节点,则抛出异常,进入CreateViewFromTag:

  private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
        return createViewFromTag(parent, name, context, attrs, false);
    }
    
    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        
        ......
        try {
            View view;
            if (mFactory2 != null) {
                view = mFactory2.onCreateView(parent, name, context, attrs);
            } else if (mFactory != null) {
                view = mFactory.onCreateView(name, context, attrs);
            } else {
                view = null;
            }

            if (view == null && mPrivateFactory != null) {
                view = mPrivateFactory.onCreateView(parent, name, context, attrs);
            }

            if (view == null) {
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) {
                        view = onCreateView(parent, name, attrs);
                    } else {
                        view = createView(name, null, attrs);
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }
        ...
        }
    }
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使用mFactory2、mFactory、mPrivateFactory创建view,最终时调用createView方法,内部采用反射创建节点,过多的反射会造成性能问题,可以进行优化。

获取界面布局耗时

  1. 手动埋点,打印时间
  2. AOP打印setContView的时间
  3. 重写LayoutInflaterCompat.setFactory2方法,打印每一个控件的耗时时间
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {

    // 使用LayoutInflaterCompat.Factory2全局监控Activity界面每一个控件的加载耗时,
    // 也可以做全局的自定义控件替换处理,比如:将TextView全局替换为自定义的TextView。
    LayoutInflaterCompat.setFactory2(getLayoutInflater(), new LayoutInflater.Factory2() {
        @Override
        public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {

            if (TextUtils.equals(name, "TextView")) {
                // 生成自定义TextView
            }
            long time = System.currentTimeMillis();
            // 1
            View view = getDelegate().createView(parent, name, context, attrs);
            LogHelper.i(name + " cost " + (System.currentTimeMillis() - time));
            return view;
        }

        @Override
        public View onCreateView(String name, Context context, AttributeSet attrs) {
            return null;
        }
    });
    
    //也可以直接调用这个方法
//    LayoutInflater.from(this).setFactory2(new LayoutInflater.Factory2() {
//            @Override
//            public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
//
//                if (TextUtils.equals(name, "TextView")) {
//                    // 生成自定义TextView
//                }
//                long time = System.currentTimeMillis();
//                // 1
//                View view = getDelegate().createView(parent, name, context, attrs);
//                AppLog.E(name + " cost " + (System.currentTimeMillis() - time));
//                return view;
//            }
//
//            @Override
//            public View onCreateView(String name, Context context, AttributeSet attrs) {
//               return null;
//            }
//        });

    // 2、setFactory2方法需在super.onCreate方法前调用,否则无效  
    super.onCreate(savedInstanceState);
    setContentView(getLayoutId());
    unBinder = ButterKnife.bind(this);
    mActivity = this;
    ActivityCollector.getInstance().addActivity(this);
    onViewCreated();
    initToolbar();
    initEventAndData();
}

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优化工具

Lint

Android Studio自带工具,可以进行代码校验,发现代码结构/质量问题

Layout Inspector

Android Studio推荐布局检测工具,可以查看整个布局的层级,进而优化处理

GPU过度绘制

手机中打开开发者选项-开启GPU过度绘制

布局优化的必要性

  1. 减少页面卡顿,提高流畅度
  2. 减少线上bug产出

总体原则

  1. 避免层间嵌套
  2. 减少绘制时间,三个方法的执行时间

优化方法

  1. 使用include标签重用公共布局

  2. 使用merge减少视图层级

    当使用的是merge时,连续两个布局相似会合并,减少层级。

  3. 使用viewStub延迟加载,减少资源浪费

  4. 简单布局使用LinearLayout,复杂布局使用RelativeLayout或者ConstraintLayout减少层级嵌套。

  5. 善用控件属性

    • TextView实现图片+文字显示
    • 使用LinearLayout自带的分割线
  6. 使用space控件

  7. 尽量少使用wrap_content,增加计算成本,绘制过久

主要布局比较

名称优点缺点
RelativeLayout减少层级嵌套onDraw执行两次,耗时
LinearLayout不使用weight,onDraw执行一次布局时容易层级嵌套
FrameLayout
ConstraintLayout减少层级 + 比例布局耗时

总结:性能好的布局,FrameLayout和LinearLayout 功能复杂,需要层级嵌套使用RelativeLayout或者ConstraintLayout。

优先考虑层级问题,在考虑单个布局性能问题

文章分类
Android
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