前言
我们知道view有一系列post方法，用于在非UI线程中发出一些页面处理。view还有另外一个postInvalidate方法，同样在非UI线程中发起重绘。
同样是在非UI线程向UI线程发出消息，但是这里面有很大的区别。
1、postInvalidate
先来看看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 window final AttachInfo attachInfo = mAttachInfo; if (attachInfo != null) { attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds); } } 复制代码
可以看到当mAttachInfo为null的时候，这个流程就直接结束了。而mAttachInfo则是当view被DetachedFromWindow的时候会被置为null，代码如下：
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(); } } 复制代码
所以当view被从页面上移除后，postInvalidate就无效了。
当mAttachInfo不为null的时候，则执行mViewRootImpl的dispatchInvalidateDelayed函数，代码如下：
public void dispatchInvalidateDelayed(View view, long delayMilliseconds) { Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view); mHandler.sendMessageDelayed(msg, delayMilliseconds); } 复制代码
直接用mHandler发出了消息。
2、post
下面再来看看post
public boolean post(Runnable action) { final AttachInfo attachInfo = mAttachInfo; if (attachInfo != null) { return attachInfo.mHandler.post(action); } // Assume that post will succeed later ViewRootImpl.getRunQueue().post(action); return true; } 复制代码
同样当mAttachInfo不为null的时候，直接使用mHandler发出消息。
但是！注意但是！当mAttachInfo为null时，并不直接结束流程，而是将runnable存入了一个RunQueue。RunQueue是一个队列，部分代码如下：
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 { ... } } 复制代码
当RunQueue的executeActions函数被调用时，会遍历队列再去用handler发送消息。
那么executeActions什么时候被调用？
在ViewRootImpl的performTraversals函数中，如下：
private void performTraversals() { ... // Execute enqueued actions on every traversal in case a detached view enqueued an action getRunQueue().executeActions(mAttachInfo.mHandler); ... } 复制代码
而performTraversals在doTraversal函数中被调用
void doTraversal() { if (mTraversalScheduled) { mTraversalScheduled = false; mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier); if (mProfile) { Debug.startMethodTracing("ViewAncestor"); } performTraversals(); if (mProfile) { Debug.stopMethodTracing(); mProfile = false; } } } 复制代码
doTraversal则在ViewRootImpl中一个Runnable对象mTraversalRunnable中执行
final class TraversalRunnable implements Runnable { @Override public void run() { doTraversal(); } } final TraversalRunnable mTraversalRunnable = new TraversalRunnable(); 复制代码
mTraversalRunnable则在ViewRootImpl的scheduleTraversals函数中被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(); } } 复制代码
而scheduleTraversals则在很多地方被执行，比如：
void handleAppVisibility(boolean visible) { if (mAppVisible != visible) { mAppVisible = visible; scheduleTraversals(); if (!mAppVisible) { WindowManagerGlobal.trimForeground(); } } } void handleGetNewSurface() { mNewSurfaceNeeded = true; mFullRedrawNeeded = true; scheduleTraversals(); } ... 复制代码
这里就不一一列举了，大家有兴趣可以自己去源码里搜索一下。
总结一下就是当view被从页面上移除后，通过post系列函数传的消息并不会立刻用handler发出去，而是先将其存入一个队列里。当view再次被添加到页面上时，会从队列中的取出消息再用handler发出去。
3、总结
所以当我们使用
post(new Runnable() { @Override public void run() { invalidate(); } }); 复制代码
它其实与postInvalidate还是有区别的。
