Handler sendMessage 与 obtainMessage (sendToTarget)比较

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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；

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//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();

//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：

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Message msg = handler.obtainMessage();

msg.arg1 = i;

msg.sendToTarget();

Message msg=new Message();

msg.arg1=i;

handler.sendMessage(msg);

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。