Android视图加载到显示(基于API 29)分析

来到执行resume的方法：handleResumeActivity()

  @Override
  public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
      String reason) {
  //onNewIntent(),以及调用Activity的resume()生命周期函数，都在下面这个方法执行
  final ActivityClientRecord r = perfORMResumeActivity(token, finalStateRequest, reason);
  ...
  //获取到PhoneWindow,
   r.window = r.activity.getWindow();
  //获取DecorView
      View decor = r.window.getDecorView();
  //decorView设置为不可见
      decor.setVisibility(View.INVISIBLE);
  //拿到windowManagerImpl
      ViewManager wm = a.getWindowManager();
      WindowManager.LayoutParams l = r.window.getAttributes();
      a.mDecor = decor;
      l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
      l.softInputMode |= forwardBit;
  ...
  if (!a.mWindowAdded) {
      a.mWindowAdded = true;
  	//WindowManagerImpl的addView方法，传入参数为decorView,和Window的属性
  	//故事就从这里开始了
      wm.addView(decor, l);
  ...	
  //调用Activity里面的makeVisisble()使得视图可见
   if (r.activity.mVisibleFromClient) {
          r.activity.makeVisible();
      }
  //你说了解Handler,那来说说IdleHandler吧。
   Looper.myQueue().addIdleHandler(new Idler());

ok,可以看到上面一系列“熟悉”的流程。我们重点关注上面的wm.addView(DecorView,WindowManager.LayoutParams). 进入WindowManagerImpl.

  @Override
  public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
      applyDefaultToken(params);
  	//单例的WindowMagerGlobal.
      mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
  }

WindowManagerGlobal

  	//创建ViewRootImpl
      root = new ViewRootImpl(view.getContext(), display);
      view.setLayoutParams(wparams);
  	//集合缓存
      mViews.add(view);
      mRoots.add(root);
      mParams.add(wparams);
      // do this last because it fires off messages to start doing things
  	//调用ViewRootImpl的setView方法：
      root.setView(view, wparams, panelParentView);

来到ViewRootImpl的setView,很快我们看到了一个名为requestLayout的方法

  @Override
  public void requestLayout() {
      if (!mHandlingLayoutInLayoutRequest) {
          checkThread();
          mLayoutRequested = true;
          scheduleTraversals();
      }
  }
  //上面的checkThread
  void checkThread() {
  还是那个熟悉的提示，原来这么多年来，提示非主线程不能更新Ui的提示，都在这里孤孤单单，今天我终于来看她了。
      if (mThread != Thread.currentThread()) {
          throw new CalledFromWrongThreadException(
                  "Only the original thread that created a view hierarchy can touch its views.");
      }
  }
  //上面的scheduleTraversals()
  void scheduleTraversals() {
  if (!mTraversalScheduled) {
      mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
      mChoreographer.postCallback(
              Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
   }}
  注意这个mTraversalRunnable
      final class TraversalRunnable implements Runnable {
  @Override
  public void run() {
      doTraversal(); //调用了doThrversal()
  }}

performTraversals中一直往下走，走啊走，你会看到一行代码：

  // Ask host how big it wants to be
  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);	

再走：

   performLayout(lp, mWidth, mHeight);	

再接再厉

   performDraw();

完毕之后，我们看

  private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
      if (mView == null) {
          return;
      }
      Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
      try {
  	//这个mView就是DecorView
          mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
      } finally {
          Trace.traceEnd(Trace.TRACE_TAG_VIEW);
      }
  }

DecorView也是继承自View,来到View中的measure(),我们看到：

  调用了onMeasure,decorView的父类是FragmLayout
  onMeasure(widthMeasureSpec, heightMeasureSpec);

FrameLayout##onMeasure()

  @Override
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int count = getChildCount();
  int maxHeight = 0;
  int maxWidth = 0;
  int childState = 0;
  //注意到，开始循环遍历ViewGroup里面包含的子View
  for (int i = 0; i < count; i++) {
      final View child = getChildAt(i);
  	//不为Gone的不去测算，为InVISibility也需要测算的
      if (mMeasureAllChildren || child.getVisibility() != GONE) {
          measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);}
  .......
  setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
          resolveSizeAndState(maxHeight, heightMeasureSpec,
                  childState << MEASURED_HEIGHT_STATE_SHIFT));

measureChildWithMargins()

  protected void measureChildWithMargins(View child,
      int parentWidthMeasureSpec, int widthUsed,
      int parentHeightMeasureSpec, int heightUsed) {
  //拿到孩子写在布局里面的宽高属性
  final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
  //测算子孩子的大小
  final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
          mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                  + widthUsed, lp.width);
  final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
          mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                  + heightUsed, lp.height);
  //看到再次调用child.measure(),measure中会调用onMeasure()
  child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}

由父类和子类确定子类的模式和大小。

  public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
  int specMode = MeasureSpec.getMode(spec);
  int specSize = MeasureSpec.getSize(spec);
  int size = Math.max(0, specSize - padding);
  int resultSize = 0;
  int resultMode = 0;
  switch (specMode) {
  // Parent has imposed an exact size on us
  case MeasureSpec.EXACTLY:
      if (childDimension >= 0) {
          resultSize = childDimension;
          resultMode = MeasureSpec.EXACTLY;
      } else if (childDimension == LayoutParams.MATCH_PARENT) {
          // Child wants to be our size. So be it.
          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.
          resultSize = size;
          resultMode = MeasureSpec.AT_MOST;
      }
      break;
  // Parent has imposed a maximum size on us
  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);
  }

父布局是Exactly

父布局是AT_MOST

子View写为准确值(100dp),那么模式是Exactly

子View是wrap_content或match_parent,都是AT_MOST模式。

上面两句话读起来有点晦涩难懂，记住Exactly是已经确定了，AT_MOST就是待计算。

父类确定(Exactly)了，子类确定（100dp和match_parent）那么子类也是确定(Exactly)

父类不确定(AT_MOST), 子类确定(100dp)那么是Exactly,否则，子类都不确定。

举一个小栗子，就是ScrollView包裹ListView ,你会发现只显示了一条数据。网上给出的解决方案很多都是： 计算ListView 的高度，然后更改List的LayoutParams。竟然把所有的ListView所有的孩子都拿来累加高度。

这个问题是因为ScrollView,传过来的模式是：UNSPECIFIED,ListView中的onMeasure,对于UNSPECIFIED的处理是：

if (heightMode == MeasureSpec.UNSPECIFIED) {
        heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                getVerticalFadingEdgeLength() * 2;
    }

所以正确的打开方式：

//继承一下ListView，然后，重写onMeasure()方法：
val height = MeasureSpec.makeMeasureSpec(heightMeasureSpec, MeasureSpec.AT_MOST)
super.onMeasure(widthMeasureSpec, height)