Handler sendMessage 与 obtainMessage (sendToTarget)比较

obtainmessage（）是从消息池中拿来一个msg 不需要另开辟空间new

new需要重新申请，效率低，obtianmessage可以循环利用；

//use Handler.obtainMessage(),instead of msg = new Message();  
//because if there is already an Message object,that not be used by   
//any one ,the system will hand use that object,so you don't have to   
//create and object and allocate memory.  
//it  is also another example of object recycling and reusing in android.  
    Message msg = mHandler.obtainMessage();  
    msg.what = UPDATE_LISTVIEW;  
    msg.obj = current + "/" + total + "songs";  
    //this method is called from worker Thread,so we cannot update UI from here.  
    msg.sendToTarget();

在看下面代码：

Message msg = handler.obtainMessage();  
                        msg.arg1 = i;  
                        msg.sendToTarget();   
  
  
Message msg=new Message();  
    msg.arg1=i;  
    handler.sendMessage(msg);

第一种写法是message 从handler 类获取，从而可以直接向该handler 对象发送消息，第二种写法是直接调用 handler 的发送消息方法发送消息。