View相关知识总结

所有涉及的源码基于Android 10, api 29

Window添加

Window是抽象的概念，实际显示的内容是View, ViewManager是管理View的基本接口, WindowManger继承了ViewManager接口，WindowManagerImpl是WindowManger的实现类, 内部的实际工作交给WindowManagerGlobal处理。

WindowManagerGlobal在addView时, 创建了一个ViewRootImpl对象并调用其setView方法，setView主要包含图上的两个过程：requestLayout和addToDisplay。其中addToDisplay是IPC调用，通过Session最终调用WMS的方法。resquestLayout的相关代码如下：

public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        // 消息屏障, 只处理异步消息
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}
// mTraversalRunnable的定义
final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        // 移除消息屏障
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        // 进入三大流程
        performTraversals();
    }
}
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通过源码可知最终会执行performTraversals进入View的三大流程。

setContentView

最终调用PhoneWindow的setContentView

public void setContentView(View view, ViewGroup.LayoutParams params) {
    // mContentParent就是放置我们定义的XML视图的位置
    if (mContentParent == null) {
        // 创建DecorView, 并初始化mContentParent
        installDecor();
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }
    // Activity切换动画
    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        view.setLayoutParams(params);
        final Scene newScene = new Scene(mContentParent, view);
        transitionTo(newScene);
    } else {
        mContentParent.addView(view, params);
    }
    // 省略
}
// 省略缩减的代码
private void installDecor() {
    if (mDecor == null) {
        // 创建DecorView
        mDecor = generateDecor(-1);
    } else {
        mDecor.setWindow(this);
    }
    if (mContentParent == null) {
        // 初始化mContentParent
        mContentParent = generateLayout(mDecor);
    }
}

public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;

ViewGroup generateLayout(DecorView decor) {
    // 省略
    ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
    // 省略
    return contentParent
}
// 从DecorView中查找id为com.android.internal.R.id.content的ViewGroup
public <T extends View> T findViewById(@IdRes int id) {
    return getDecorView().findViewById(id);
}

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由源码可以看出setContentView只是创建出了DecorView并将我们定义的视图放到id为com.android.internal.R.id.content的ViewGroup中, 还没有通过WindowManager来真正添加视图到窗口上。

Activity onCreate和onResume执行的时机

1. onCreate在ActivityTheard#performLaunchActivity方法中利用Instrumentation#callActivityOnCreate调用Activity的onCreate方法
2. onResume的调用和onCreate基本一样, 在performResumeActivity方法中, 最终也是通过Instrumentation调用了onResume方法

DecorView何时添加

在ActivityTheard的handleResumeActivity方法中, 首先调用了performResumeActivity方法，之后再用WindowManager的addView方法将DecorView添加到Window中。由上面Window添加的过程可以看出最终通过ViewRootImpl完成setView操作, 并触发View的三大流程以及和WMS的IPC通信。

View#post为什么可以获取到视图的宽高

• 为什么onCreate和onResume获取不到
  

  通常我们编写代码时在Activity的onCreate或者onResume方法中无法获取到View的宽高, 而通过View#post则可以获取到。说明onCreate和onResume方法的执行在View测量之前。从上面onCreate和onResume的执行时机以及DecorView被添加的时机可以直到View的测量发生在onCreate和onResume之后, 因此在这些方法中无法获取View的宽高。

• 为什么View#post可以获取到

  public boolean post(Runnable action) {
      // 获取AttachInfo
      final AttachInfo attachInfo = mAttachInfo;
      // AttachInfo不为null, 之间通过Handler放到MessageQueue中
      if (attachInfo != null) {
          return attachInfo.mHandler.post(action);
      }
      // 消息暂时存到HandlerActionQueue中
      getRunQueue().post(action);
      return true;
  }
  
  // 简略代码
  void dispatchAttachedToWindow(AttachInfo info, int visibility) {
      // mAttachInfo赋值
      mAttachInfo = info;
      if (mRunQueue != null) {
          // 将暂存的消息发送到MessageQueue等待执行
          mRunQueue.executeActions(info.mHandler);
          mRunQueue = null;
      }
  }
  
  public void executeActions(Handler handler) {
      synchronized (this) {
          final HandlerAction[] actions = mActions;
          for (int i = 0, count = mCount; i < count; i++) {
              final HandlerAction handlerAction = actions[i];
              handler.postDelayed(handlerAction.action, handlerAction.delay);
          }
          mActions = null;
          mCount = 0;
      }
  }
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  从View#post的源码可以看到mAttachInfo为null的时候会暂存消息, 搜索源码发现mAttachInfo在dispatchAttachedToWindow方法中被复制, 而该方法在performTraversals中被调用，因此当该方法被调用时，View的三大流程已经开始了，等到Looper轮询到我们post的消息的时候，View的宽高以及被测量了。

View三大流程

ViewRootImpl的performTraversals方法中分别调用了performMeasure、performLayout、performDraw方法而进入View的三大流程

Measure

performMeasure直接调用View#measure, 最终会回调具体ViewGroup实现的onMeasure方法, 对于ViewGroup需要先对其所属的子View进行测量。

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
    final int size = mChildrenCount;
    final View[] children = mChildren;
    for (int i = 0; i < size; ++i) {
        final View child = children[i];
        if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
        }
    }
}

protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    // 子View的LayoutParam参数
    final LayoutParams lp = child.getLayoutParams();
    // 根据父View的MeasureSpec和View自己的LayoutParam的到View自己的MeasureSpec
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);
    // 调用子View的measure方法
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

// View#onMeasure的默认实现, 有具体的View或者ViewGroup负责重写
// 参数的两个MeasureSpec都是上面 getChildMeasureSpec 方法计算到的View自己的 MeasureSpec
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
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注意：onMeasure方法中的MeasureSpec参数是当前View自身的MeasureSpec

• MeasureSpec

  View类的一个静态内部类, 用一个32的int数据来表示View的测量模式, 以及对应的测量数值。
  高2位表示测量模式, 低30表示具体数值。
  测量模式有UNSPECIFIED,EXACTLY,AT_MOST三种

• MeasureSpec的计算

  上面提到通过getChildMeasureSpec来计算View的MeasureSpec

  // childDimension：LayoutParams中的width或者height, 就是XML中写的
  public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
      // 父View对应的Mode和Size
      int specMode = MeasureSpec.getMode(spec);
      int specSize = MeasureSpec.getSize(spec);
  
      int size = Math.max(0, specSize - padding); // 去掉padding
  
      int resultSize = 0;
      int resultMode = 0;
      
      // ViewGroup.LayoutParams中MATCH_PARENT定义为-1, WRAP_CONTENT定义为-2
  
      switch (specMode) {
      // Parent has imposed an exact size on us 父View的Mode是EXACTLY
      case MeasureSpec.EXACTLY:
          if (childDimension >= 0) { // XML中写的具体dp数值
              resultSize = childDimension; 
              resultMode = MeasureSpec.EXACTLY;
          } else if (childDimension == LayoutParams.MATCH_PARENT) {
              // Child wants to be our size. So be it. 和父View一样大
              resultSize = size; 
              resultMode = MeasureSpec.EXACTLY;
          } else if (childDimension == LayoutParams.WRAP_CONTENT) {
              // Child wants to determine its own size. It can't be
              // bigger than us.  最大不超过父View
              resultSize = size;
              resultMode = MeasureSpec.AT_MOST;
          }
          break;
  
      // Parent has imposed a maximum size on us  父View指定了一个最大值
      case MeasureSpec.AT_MOST:
          if (childDimension >= 0) { // 
              // Child wants a specific size... so be it
              resultSize = childDimension;
              resultMode = MeasureSpec.EXACTLY;
          } else if (childDimension == LayoutParams.MATCH_PARENT) {
              // Child wants to be our size, but our size is not fixed.
              // Constrain child to not be bigger than us.
              resultSize = size;
              resultMode = MeasureSpec.AT_MOST;
          } else if (childDimension == LayoutParams.WRAP_CONTENT) {
              // Child wants to determine its own size. It can't be
              // bigger than us.
              resultSize = size;
              resultMode = MeasureSpec.AT_MOST;
          }
          break;
  
      // Parent asked to see how big we want to be
      case MeasureSpec.UNSPECIFIED:
          if (childDimension >= 0) {
              // Child wants a specific size... let him have it
              resultSize = childDimension;
              resultMode = MeasureSpec.EXACTLY;
          } else if (childDimension == LayoutParams.MATCH_PARENT) {
              // Child wants to be our size... find out how big it should
              // be
              resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
              resultMode = MeasureSpec.UNSPECIFIED;
          } else if (childDimension == LayoutParams.WRAP_CONTENT) {
              // Child wants to determine its own size.... find out how
              // big it should be
              resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
              resultMode = MeasureSpec.UNSPECIFIED;
          }
          break;
      }
      //noinspection ResourceType
      return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
  }
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Layout

performLayout直接调用View#layout方法, 最终调用了具体ViewGroup的onLayout进行布局操作, 计算中子View在布局中的具体位置后, 再调用具体View的layout来设置View的left、top、right、bottom相关参数。

Draw

performDraw方法，最终会调用具体View的onDraw方法绘制View自身