Android requestLayout 跟 invalidate
这两个方法很多人 搞不太清楚，这里小结一下：
View的流程图
对于标题提及的两个方法 调用invalidate()或者requestLayout()会触发哪些方法，一图道破天机。
源代码
现在来看看具体的代码： android.view.ViewRootImpl
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void invalidate() {
mDirty.set(0, 0, mWidth, mHeight);
if (!mWillDrawSoon) {
scheduleTraversals();
}
}这两个方法不约而同的调用了scheduleTraversals()，区别就是requsetLayout检查当前是否在主线程中，并且置位mLayoutRequsted = true，而invalidate方法没有检查。
接着我们进一步追踪下去看看scheduleTraversals()
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
}
}这里会post一个runnable请求就是mTraversalRunnable
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}进入doTraversal()方法：
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "performTraversals");
try {
performTraversals();
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}注意这个方法中，我们重点关注performTraversals()方法：
这个方法是ViewRootImpl最复杂的方法，将近800行代码Android View 分析（下）
这篇博文表粗略的分析了这个方法，这里不打算详细分析这个方法，我们仅仅做粗线条的了解， 进一步验证博文最上面的图是否正确：
ViewRootImpl这个类的L1767行：
if (!mStopped) {
boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
(relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
|| mHeight != host.getMeasuredHeight() || contentInsetsChanged) {
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed! mWidth="
+ mWidth + " measuredWidth=" + host.getMeasuredWidth()
+ " mHeight=" + mHeight
+ " measuredHeight=" + host.getMeasuredHeight()
+ " coveredInsetsChanged=" + contentInsetsChanged);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
// Implementation of weights from WindowManager.LayoutParams
// We just grow the dimensions as needed and re-measure if
// needs be
int width = host.getMeasuredWidth();
int height = host.getMeasuredHeight();
boolean measureAgain = false;
if (lp.horizontalWeight > 0.0f) {
width += (int) ((mWidth - width) * lp.horizontalWeight);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (lp.verticalWeight > 0.0f) {
height += (int) ((mHeight - height) * lp.verticalWeight);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (measureAgain) {
if (DEBUG_LAYOUT) Log.v(TAG,
"And hey let's measure once more: width=" + width
+ " height=" + height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
layoutRequested = true;
}
}
}只要没有stop 并且
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()|| mHeight != host.getMeasuredHeight() || contentInsetsChanged) 这个条件成立：
就会引发 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)
经过Debug可知：
requsetLayout触发performTraversal方法，进而调用performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)，进一步的调用performLayout方法，此时layoutRequested = true
invalidate触发performTraversal方法，因为上面的条件不成立，所以不会调用performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)，同样也不会调用performLayout方法。
final boolean didLayout = layoutRequested && !mStopped;
boolean triggerGlobalLayoutListener = didLayout
|| mAttachInfo.mRecomputeGlobalAttributes;
if (didLayout) {
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
...
}所以到这里可以确认的是：
**requsetLayout方法会调用performMeasure和performLayout方法
而invalidate方法则一个都不会调用。**
我们接着往下看是否会接着调用performDraw方法
if (!cancelDraw && !newSurface) {
if (!skipDraw || mReportNextDraw) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
performDraw();
}
} 由上面的代码可知：需满足以下几个条件才会触发performDraw方法
if (!cancelDraw && !newSurface) {
if (!skipDraw || mReportNextDraw)
}一般情况下 以上条件都会满足，除非设置了明显的标志位，所以performDraw方法会执行。
我们来大概看一下上面的几个标志位：
cancelDraw 默认是false
skipDraw 默认是false
newSurface 位于：
if (!hadSurface) {
if (mSurface.isValid()) {
// If we are creating a new surface, then we need to
// completely redraw it. Also, when we get to the
// point of drawing it we will hold off and schedule
// a new traversal instead. This is so we can tell the
// window manager about all of the windows being displayed
// before actually drawing them, so it can display then
// all at once.
newSurface = true;
mFullRedrawNeeded = true;
mPreviousTransparentRegion.setEmpty();
if (mAttachInfo.mHardwareRenderer != null) {
try {
hwInitialized = mAttachInfo.mHardwareRenderer.initialize(
mSurface);
} catch (OutOfResourcesException e) {
handleOutOfResourcesException(e);
return;
}
}
}
} 首先最开始的判断分支不会成立，因为 hadSurface为true，所以newSurface = false
最后一个标志位mReportNextDraw：
// Remember if we must report the next draw.
if ((relayoutResult & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
mReportNextDraw = true;
}这里在执行判断该标志位之前就置位为true。所以一般会进入performDraw方法，
我们小结一下：
requsetLayout方法会触发performMeasure和performLayout，而performDraw方法是否触发视情况而定
如果以下条件满足
if (!cancelDraw && !newSurface) {
if (!skipDraw || mReportNextDraw)
}则触发performDraw方法，反之，不会触发performDraw方法。
而invalidate方法则只会触发performDraw方法，因为一般情况下条件都会满足：
if (!cancelDraw && !newSurface) {
if (!skipDraw || mReportNextDraw)
}至此，上面的流程图我们得到验证。
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