结论：view.post()方法在整个view树的performMeasure, performLayout, performDraw执行完后，才被主线程轮询到，才得到执行。
基于android sdk-23的源码分析，文章分成两个部分，实际上我是先写第二部分了再写第一部分的。
第一部分
看一下view.post的内部。
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;
}
如果在Activity#onCreate()中直接调用view.post，attachInfo==null，因为此时view还没有attach到Window上。那么看ViewRootImpl.getRunQueue().post(action)。
/**
* The run queue is used to enqueue pending work from Views when no Handler is
* attached. The work is executed during the next call to performTraversals on
* the thread.
* @hide
*/
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 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 {
Runnable action;
long delay;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
HandlerAction that = (HandlerAction) o;
return !(action != null ? !action.equals(that.action) : that.action != null);
}
@Override
public int hashCode() {
int result = action != null ? action.hashCode() : 0;
result = 31 * result + (int) (delay ^ (delay >>> 32));
return result;
}
}
}
这里省略了一个remove的方法。RunQueue就是在view.post(Runnable)的时候，把Runnable放到数组mAction里，然后当ActivityThread的performTraversal()里的getRunQueue().executeActions(attachInfo.mHandler);时，就把所有数组中的Runnable发送到主线程的Looper里去轮询。当然这些都会在performTraversal()执行完之后才能执行。
第二部分
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();//专门用来执行ViewRootImpl里的doTraversal()，而doTraversal()里面有performTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();//紧接着就new出来一个实例
mTraversalRunnable在ViewRootImpl里面总共也就出现了两次
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
另一个是void unscheduleTraversals()，先不看。
那么在这里出现的地方这一句mChoreographer.postCallback( Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);，
Choreographer.n 编舞者
他在ViewRootImpl的构造方法中实例化：mChoreographer = Choreographer.getInstance();
//获取单例
public static Choreographer getInstance() {
return sThreadInstance.get();
}
//看下sThreadInstance
private static final ThreadLocal<Choreographer> sThreadInstance =
new ThreadLocal<Choreographer>() {
@Override
protected Choreographer initialValue() {
Looper looper = Looper.myLooper();
if (looper == null) {
throw new IllegalStateException("The current thread must have a looper!");
}
return new Choreographer(looper);
}
};
//看下编舞者的私有构造方法
private Choreographer(Looper looper) {
mLooper = looper;
mHandler = new FrameHandler(looper);//用looper实例化了Handler。
mDisplayEventReceiver = USE_VSYNC ? new FrameDisplayEventReceiver(looper) : null;
mLastFrameTimeNanos = Long.MIN_VALUE;
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
for (int i = 0; i <= CALLBACK_LAST; i++) {
mCallbackQueues[i] = new CallbackQueue();
}
}
很明显的是由ThreadLocal来控制的线程单例类，拿到的Looper也是线程单例的Looper，而创建Choreographer的ViewRootImpl是在主线程创建的，因此拿到的就是主线程的Looper。
看回Choreographer的postCallback();，最终进来调用的postCallbackDelayedInternal();
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG_FRAMES) {
Log.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + delayMillis);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
//在此处用主线程的looper，最后把这个Runnable发到主线程里去，排队等待被调用mHandler.handleMessage()；最后被执行。
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
总之这个ViewRootImpl里的doTraversal()方法由ViewRootImpl的scheduleTraversal()最终放到
Android主线程的Looper里面去轮询排队执行了。
后面就是doTraversal()里面的performTraversal()了。
private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
//注：该方法有800行代码，已省略
// Execute enqueued actions on every traversal in case a detached view enqueued an action
getRunQueue().executeActions(attachInfo.mHandler);
...
performMeasure();//从DecorView开始完成View树的测量
...
performLayout();//从DecorView开始完成View树的布局
...
performDraw();//从DecorView开始绘制View树
}
注意：getRunQueue().executeActions(attachInfo.mHandler)里面会遍历数组然后把所有的view.post(Runnable)里的Runnable都用和主线程Looper关联的Hanlder#sendMessage出去，放到主线程Looper里轮询，等待调用。那么由于当前的performTraversals()本身就是由主线程Looper回调给刚才的编舞者Choreographer里面去执行的，因此主线程一定会等待performTraversals()整个方法执行完，才去接着执行由view.post()推送到主线程的Runnable。因此整个View树都完成了测量，布局，绘制。然后view.post()里面百分百的可以拿到view的宽高了。
thanks
通过View.post()获取View的宽高引发的两个问题：1post的Runnable何时被执行，2为何View需要layout两次；以及发现Android的一个小bug - CSDN博客
