handler.obtainMessage()理解

在handler.obtainMessage()的参数是这样写的:
Message android.os.Handler.obtainMessage(int what, int arg1, int arg2, Object obj)

public final Message obtainMessage (int what, int arg1, int arg2, Object obj)
Since: API Level 1
Same as obtainMessage(), except that it also sets the what, obj, arg1,and arg2 values on the returned Message.

Parameters
what  Value to assign to the returned Message.what field.
arg1  Value to assign to the returned Message.arg1 field.
arg2  Value to assign to the returned Message.arg2 field.
obj  Value to assign to the returned Message.obj field.

而Handler中obtainMessage与new Message的区别：

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。