##View的post()和postDelayed()方法详解
###子线程中不能对UI进行操作，这我们都知道，但是可以通过view.post(Runnable)的方式进行UI操作，这是为什么呢？我们通过分析源码的方式来了解一下
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
getRunQueue().post(action);
return true;
}
view的post()将Runnable action传递给attachInfo.Handler,所以其本质都是Handler、Looper、MessageQueue、Message的异步消息处理机制，接着再查看mAttachInfo的来源，源码定位到
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
mAttachInfo = info;
…
}
####在view的绘制过程中，第一次会调用DecorView的dispatchAttachedToWindow，所以这也就解释了view.post()可以解决在oncreate()中获取view的宽高为0的问题
` getRunQueue().post(action); `当attachInfo为空时会走此方法，getRunQueue()的源码：
private HandlerActionQueue getRunQueue() {
if (mRunQueue == null) {
mRunQueue = new HandlerActionQueue();
}
return mRunQueue;
}
再看HandlerActionQueue的源码：
public void post(Runnable action) {
postDelayed(action, 0);
}
public void postDelayed(Runnable action, long delayMillis) {
final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
synchronized (this) {
if (mActions == null) {
mActions = new HandlerAction[4];
}
mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
mCount++;
}
}
HandlerActionQueue会将runnable缓存起来，最大缓存数为4，缓存起来的Runnable返回到上面的dispatchAttachedToWindow中会被调用
