解惑Android的post()方法究竟執行在哪個執行緒中

Android中我們常用的post()方法大致有兩種情況：

1.如果post方法是handler的，則Runnable執行在handler依附執行緒中，可能是主執行緒，也可能是其他執行緒

下面是Handler裡面的post方法

    /**
     * Causes the Runnable r to be added to the message queue.
     * The runnable will be run on the thread to which this handler is 
     * attached. 
     *  
     * @param r 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.
     */
    public final boolean post(Runnable r){
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

2.如果post方法是View的，則一定是執行在主執行緒中的，因為所有view都自帶一個handler，所有handler都有post方法，所以它的Runnable是執行在主執行緒中的

下面是View中的post方法

    /**
     * 
Causes the Runnable to be added to the message queue.
     * The runnable will be run on the user interface thread.
     *
     * @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);
        }
        // Assume that post will succeed later
        ViewRootImpl.getRunQueue().post(action);
        return true;
    }

例如：Imageview自帶一個handler，它有postDelayed方法，由於imageview是主執行緒上的，所以Runable是執行在主執行緒中的程式碼。

imageview.postDelayed(new Runnable() {  

        @Override  
        public void run() {  
            Intent mIntent = new Intent(MainActivity.this,  
                    SecondActivity.class);  
            startActivity(mIntent);  
            finish();  

        }  
    }, 2000);