解惑Android的post()方法究竟运行在哪个线程中

Android中我们常用的post()方法大致有两种情况：

1.如果post方法是handler的，则Runnable执行在handler依附线程中，可能是主线程，也可能是其他线程

下面是Handler里面的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，则一定是运行在主线程中的，因为所有view都自带一个handler，所有handler都有post方法，所以它的Runnable是运行在主线程中的

下面是View中的post方法

    /**
     * <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);
        }
        // Assume that post will succeed later
        ViewRootImpl.getRunQueue().post(action);
        return true;
    }

例如：Imageview自带一个handler，它有postDelayed方法，由于imageview是主线程上的，所以Runable是运行在主线程中的代码。

imageview.postDelayed(new Runnable() {  

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

        }  
    }, 2000);