Android View之View.post()分析

你们都知道每一个View都有一个post()和postDelayed()方法，那么这两个方法是作什么用呢?何时须要用呢？

咱们带着这两个问题来分析一下：

首先咱们看下这两个方法的源码,首先看post():

/**

*

<p>

Causes the Runnable to be added to the message queue.

* The runnable will be run on the user interface thread.

</p>

*

*

@param

action

The Runnable that will be executed.

*

*

@return

Returns true if the Runnable was successfully placed in to the

* message queue. Returns false on failure, usually because the

* looper processing the message queue is exiting.

*

*

@see

#postDelayed

*

@see

#removeCallbacks

*/

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;

}

从源码中咱们很清楚的看到，两种状况：

1.当mAttachInfo != null 时，post方法最终是由attachInfo中的mHandler调用post来处理，从而保证在ui线程中执行，因此从根本上来以后的整个流程仍是Handler 的整个处理机制流程（Handler的处理机制流程，这里不作分析），那么mAttachInfo又是何时赋值的呢？搜索一下源码看到：

/**

*

@param

info

the {

@link

android.view.View.AttachInfo} to associated with

* this view

*/

void dispatchAttachedToWindow(AttachInfo info, int visibility) {
mAttachInfo = info;
if (mOverlay != null) {
mOverlay.getOverlayView().dispatchAttachedToWindow(info, visibility);

}

能够看到mAttachInfo是在，dispatchAttachedToWindow中调用的，而dispatchAttachedToWindow 是在 ViewRootImpl 类的performTraversals 调用的，而这个方法在view初始化的时候会被调用。

2.mAttachInfo==null时，调用getRunQueue().post(action)，咱们来看下这个getRunQueue()的源码：

/**

* Returns the queue of runnable for this view.

*

*

@return

the queue of runnables for this view

*/

private HandlerActionQueue getRunQueue() {
if (mRunQueue == null) {
mRunQueue = new HandlerActionQueue();
}
return mRunQueue;

}

这里看到最终调用的是HandlerActionQueue的post()方法：

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++;
}

}

这里咱们看到基本上就是经过HandlerActionQueue缓存起来，缓存起来何时执行呢？咱们找下HandlerActionQueue的源码里：

public void executeActions(Handler handler) {
synchronized (this) {
final HandlerAction[] actions = mActions;
for (int i = 0, count = mCount; i < count; i++) {
final HandlerAction handlerAction = actions[i];
handler.postDelayed(handlerAction.action, handlerAction.delay);
}

mActions = null;
mCount = 0;
}

}

缓存起来的runnable最终仍是经过handler来执行,直接找到执行的代码：

仍是经过dispatchAttachedToWindow来执行。

这两个方法：

当打开一个activity时若是调用post方法时尚未开始执行dispatchAttachedToWindow就先调用getRunQueue().post(action)缓存起来，当执行到dispatchAttachedToWindow时再执行handler的post，若是到达就直接执行handler.post。

总结：post()和postDelayed()用来更新ui,postDelayed能够设置延时。