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布局优化

移动端 2021/05/31

本文于 909 天之前发表，文中内容可能已经过时。

[TOC]

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：

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@Override
   public void setContentView(@LayoutRes int layoutResID) {
       getDelegate().setContentView(layoutResID);
   }
   
   //AppCompatDelegate.java
   public abstract void setContentView(@LayoutRes int resId);

查看抽象接口实现
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@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();
   }

进入inflate方法：
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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();
        }
    }

getLayout方法返回一个XmlResourceParser对象：
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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);
    }
}

进入loadXmlResourceParser：
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@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));
   }

加载指定布局文件的xml，生成XMLBlock：
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/*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);

最终指向了native方法

获取到XMLResourceParser后，进行渲染：

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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);
                }
            ...
            }
        }
    }

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：

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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;
              }
          }
      ...
      }
  }

使用mFactory2、mFactory、mPrivateFactory创建view，如果null的情况下，就会调用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
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    @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();
    }

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

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