View的绘制流程 - onLayout()源码分析

前言

View绘制流程系列文章
View的绘制流程 - onMeasure()源码分析
View的绘制流程 - onLayout()源码分析
View的绘制流程 - onDraw()源码分析

结论


View的绘制流程都是从ViewRootImpl中的requestLayout()方法开始进去的,performMeasure()、performLayout()、performDraw(),而如果代码中又写了这样的代码:addView()、setVisibility()等方法,意思就是会重新执行requestLayout(),意思就是会重新执行View的绘制流程,这个时候执行View的绘制流程时不会和第一次一样去执行所有的逻辑,比如说你自己addView(),一次性添加了10个View,那么它有可能等你添加完毕之后才去执行 View的绘制流程的:

performLayout()调用方法顺序如下:
在 ViewRootImpl中的 performLayout()方法 ->
然后调用View中的 layout()方法 ->
然后调用View中的 onLayout()方法 ,这里还是和上节课讲解 onMeasure()源码分析举的例子一样,还是用LinearLayout垂直的布局包裹3个 TextView ->
摆放子布局, for循环所有子View,前提是子View不为 GONE,然后调用 child.layout()方法

下边进行分析,最下边的结论可以不看,因为和上边这个一样,下边仅用于分析流程。

1. 说明


上节课我们看了下View绘制流程中的 onMeasure()方法,那么这节课我们就来看下onLayout()方法
View的绘制流程的入口就是 ViewRootImpl中的 requestLayout()方法,源码如下,从requestLayout()中点击进入View的 layout()方法:

ViewRootImpl源码如下:

    /**
     * We have one child
     */
    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                mView = view;

                mAttachInfo.mDisplayState = mDisplay.getState();
                mDisplayManager.registerDisplayListener(mDisplayListener, mHandler);

                // Schedule the first layout -before- adding to the window
                // manager, to make sure we do the relayout before receiving
                // any other events from the system.
                requestLayout();
            }
        }
    }

@Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true;
            scheduleTraversals();
        }
    }
void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }
final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }
void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
            performTraversals();
        }
    }
private void performTraversals() {
        // cache mView since it is used so much below...
        final View host = mView;

        if (DBG) {
            System.out.println("======================================");
            System.out.println("performTraversals");
            host.debug();
        }

                    if (measureAgain) {
                        if (DEBUG_LAYOUT) Log.v(mTag,
                                "And hey let's measure once more: width=" + width
                                + " height=" + height);
                        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    }
            }
        } else 
        if (didLayout) {
            performLayout(lp, mWidth, mHeight);
            }

            performDraw();
        } else {
            
        }

        mIsInTraversal = false;
    }
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
          int desiredWindowHeight) {
      final View host = mView;
      if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
          Log.v(mTag, "Laying out " + host + " to (" +
                  host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
      }

      Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
      try {
          host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

          mInLayout = false;
          int numViewsRequestingLayout = mLayoutRequesters.size();
          if (numViewsRequestingLayout > 0) {
                      false);
              if (validLayoutRequesters != null) {
                  // Process fresh layout requests, then measure and layout
                  int numValidRequests = validLayoutRequesters.size();
                  for (int i = 0; i < numValidRequests; ++i) {
                      final View view = validLayoutRequesters.get(i);
                      Log.w("View", "requestLayout() improperly called by " + view +
                              " during layout: running second layout pass");
                      view.requestLayout();
                  }
                  measureHierarchy(host, lp, mView.getContext().getResources(),
                          desiredWindowWidth, desiredWindowHeight);
                  mInLayout = true;
                  host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight())
                  if (validLayoutRequesters != null) {
                      final ArrayList finalRequesters = validLayoutRequesters;
                      // Post second-pass requests to the next frame
                      getRunQueue().post(new Runnable() {
                          @Override
                          public void run() {
                              int numValidRequests = finalRequesters.size();
                              for (int i = 0; i < numValidRequests; ++i) {
                                  final View view = finalRequesters.get(i);
                                  Log.w("View", "requestLayout() improperly called by " + view +
                                          " during second layout pass: posting in next frame");
                                  view.requestLayout();
                              }
                          }
                      });
                  }
  }
 public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;

        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
    }

performLayout()调用方法顺序如下:
在 ViewRootImpl中的 performLayout()方法 ->
然后调用View中的 layout()方法 ->
然后调用View中的 onLayout()方法 ,这里还是和上节课讲解 onMeasure()源码分析举的例子一样,还是用LinearLayout垂直的布局包裹3个 TextView ->
摆放子布局, for循环所有子View,前提是子View不为 GONE,然后调用 child.layout()方法

你可能感兴趣的:(View的绘制流程 - onLayout()源码分析)