目錄

前言

1、postInvalidate

2、post

3、總結

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前言

我們知道view有一系列post方法，用于在非UI線程中發出一些頁面處理。view還有另外一個postInvalidate方法，同樣在非UI線程中發起重繪。

同樣是在非UI線程向UI線程發出消息，但是這里面有很大的區別。
?

1、postInvalidate

先來看看postInvalidate

public void postInvalidate() {postInvalidateDelayed(0); }public void postInvalidateDelayed(long delayMilliseconds) {// We try only with the AttachInfo because there's no point in invalidating// if we are not attached to our windowfinal AttachInfo attachInfo = mAttachInfo;if (attachInfo != null) {attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds);} }

可以看到當mAttachInfo為null的時候，這個流程就直接結束了。而mAttachInfo則是當view被DetachedFromWindow的時候會被置為null，代碼如下：

void dispatchDetachedFromWindow() {AttachInfo info = mAttachInfo;if (info != null) {int vis = info.mWindowVisibility;if (vis != GONE) {onWindowVisibilityChanged(GONE);}}onDetachedFromWindow();onDetachedFromWindowInternal();InputMethodManager imm = InputMethodManager.peekInstance();if (imm != null) {imm.onViewDetachedFromWindow(this);}ListenerInfo li = mListenerInfo;final CopyOnWriteArrayList<OnAttachStateChangeListener> listeners =li != null ? li.mOnAttachStateChangeListeners : null;if (listeners != null && listeners.size() > 0) {for (OnAttachStateChangeListener listener : listeners) {listener.onViewDetachedFromWindow(this);}}if ((mPrivateFlags & PFLAG_SCROLL_CONTAINER_ADDED) != 0) {mAttachInfo.mScrollContainers.remove(this);mPrivateFlags &= ~PFLAG_SCROLL_CONTAINER_ADDED;}mAttachInfo = null;if (mOverlay != null) {mOverlay.getOverlayView().dispatchDetachedFromWindow();} }

所以當view被從頁面上移除后，postInvalidate就無效了。
當mAttachInfo不為null的時候，則執行mViewRootImpl的dispatchInvalidateDelayed函數，代碼如下：

public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);mHandler.sendMessageDelayed(msg, delayMilliseconds); }

直接用mHandler發出了消息。

2、post

下面再來看看post

public boolean post(Runnable action) {final AttachInfo attachInfo = mAttachInfo;if (attachInfo != null) {return attachInfo.mHandler.post(action);}// Assume that post will succeed laterViewRootImpl.getRunQueue().post(action);return true; }

同樣當mAttachInfo不為null的時候，直接使用mHandler發出消息。
但是！注意但是！當mAttachInfo為null時，并不直接結束流程，而是將runnable存入了一個RunQueue。RunQueue是一個隊列，部分代碼如下：

static final class RunQueue {private final ArrayList<HandlerAction> mActions = new ArrayList<HandlerAction>();void post(Runnable action) {postDelayed(action, 0);}void postDelayed(Runnable action, long delayMillis) {HandlerAction handlerAction = new HandlerAction();handlerAction.action = action;handlerAction.delay = delayMillis;synchronized (mActions) {mActions.add(handlerAction);}}void removeCallbacks(Runnable action) {...}void executeActions(Handler handler) {synchronized (mActions) {final ArrayList<HandlerAction> actions = mActions;final int count = actions.size();for (int i = 0; i < count; i++) {final HandlerAction handlerAction = actions.get(i);handler.postDelayed(handlerAction.action, handlerAction.delay);}actions.clear();}}private static class HandlerAction {...} }

當RunQueue的executeActions函數被調用時，會遍歷隊列再去用handler發送消息。
那么executeActions什么時候被調用？
在ViewRootImpl的performTraversals函數中，如下：

private void performTraversals() {...// Execute enqueued actions on every traversal in case a detached view enqueued an actiongetRunQueue().executeActions(mAttachInfo.mHandler);... }

而performTraversals在doTraversal函數中被調用

void doTraversal() {if (mTraversalScheduled) {mTraversalScheduled = false;mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);if (mProfile) {Debug.startMethodTracing("ViewAncestor");}performTraversals();if (mProfile) {Debug.stopMethodTracing();mProfile = false;}} }

doTraversal則在ViewRootImpl中一個Runnable對象mTraversalRunnable中執行

final class TraversalRunnable implements Runnable {@Overridepublic void run() {doTraversal();} } final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

mTraversalRunnable則在ViewRootImpl的scheduleTraversals函數中被post出去

void scheduleTraversals() {if (!mTraversalScheduled) {mTraversalScheduled = true;mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);if (!mUnbufferedInputDispatch) {scheduleConsumeBatchedInput();}notifyRendererOfFramePending();pokeDrawLockIfNeeded();} }

而scheduleTraversals則在很多地方被執行，比如：

void handleAppVisibility(boolean visible) {if (mAppVisible != visible) {mAppVisible = visible;scheduleTraversals();if (!mAppVisible) {WindowManagerGlobal.trimForeground();}} }void handleGetNewSurface() {mNewSurfaceNeeded = true;mFullRedrawNeeded = true;scheduleTraversals(); }...

這里就不一一列舉了，大家有興趣可以自己去源碼里搜索一下。
總結一下就是當view被從頁面上移除后，通過post系列函數傳的消息并不會立刻用handler發出去，而是先將其存入一個隊列里。當view再次被添加到頁面上時，會從隊列中的取出消息再用handler發出去。

3、總結

所以當我們使用

post(new Runnable() {@Overridepublic void run() {invalidate();} });

它其實與postInvalidate還是有區別的。

總結

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