Android View之View.post()分析

你們都知道每一個View都有一個post()和postDelayed()方法，那麼這兩個方法是作什麼用呢?何時須要用呢？

咱們帶着這兩個問題來分析一下：

首先咱們看下這兩個方法的源碼,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;

}

從源碼中咱們很清楚的看到，兩種狀況：

1.當mAttachInfo != null 時，post方法最終是由attachInfo中的mHandler調用post來處理，從而保證在ui線程中執行，因此從根本上來以後的整個流程仍是Handler 的整個處理機制流程（Handler的處理機制流程，這裏不作分析），那麼mAttachInfo又是何時賦值的呢？搜索一下源碼看到：

/**

*

@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);

}

能夠看到mAttachInfo是在，dispatchAttachedToWindow中調用的，而dispatchAttachedToWindow 是在 ViewRootImpl 類的performTraversals 調用的，而這個方法在view初始化的時候會被調用。

2.mAttachInfo==null時，調用getRunQueue().post(action)，咱們來看下這個getRunQueue()的源碼：

/**

* 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;

}

這裏看到最終調用的是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++;
}

}

這裏咱們看到基本上就是經過HandlerActionQueue緩存起來，緩存起來何時執行呢？咱們找下HandlerActionQueue的源碼裏：

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;
}

}

緩存起來的runnable最終仍是經過handler來執行,直接找到執行的代碼：

仍是經過dispatchAttachedToWindow來執行。

這兩個方法：

當打開一個activity時若是調用post方法時尚未開始執行dispatchAttachedToWindow就先調用getRunQueue().post(action)緩存起來，當執行到dispatchAttachedToWindow時再執行handler的post，若是到達就直接執行handler.post。

總結：post()和postDelayed()用來更新ui,postDelayed能夠設置延時。