Android消息处理机制(2)

角色综述（回顾）:

???(1)UI thread?通常就是main thread，而Android启动程序时会替它建立一个MessageQueue。

(2)当然需要一个Looper对象，来管理该MessageQueue。

(3)我们可以构造Handler对象来push新消息到Message Queue里；或者接收Looper(从Message Queue取出)所送来的消息。

(4)线程A的Handler对象可以传递给别的线程，让别的线程B或C等能送讯息来给线程A(存于A的Message Queue里)。

(5)线程A的Message Queue里的消息，只有线程A所属的对象可以处理。

?

子线程传递消息给主线程

public?class?Activity2?extends?Activity?implements?OnClickListener{

???????Button?button?=?null;

???????TextView?text?=?null;

???????MyHandler?mHandler?=?null;

???????Thread?thread?;

???????@Override

???????protected?void?onCreate(Bundle savedInstanceState) {

??????????????super.onCreate(savedInstanceState);

??????????????setContentView(R.layout.activity1);????????

??????????????button?= (Button)findViewById(R.id.btn);

??????????????button.setOnClickListener(this);

??????????????text?= (TextView)findViewById(R.id.content);

???????}

???????public?void?onClick(View v) {

??????????????switch?(v.getId()) {

??????????????case?R.id.btn:

?????????????????????thread?=?new?MyThread();

?????????????????????thread.start();

?????????????????????break;

??????????????}????????????

???????}?????

???????private?class?MyHandler?extends?Handler{?????????????

??????????????public?MyHandler(Looper looper){

?????????????????????super(looper);

??????????????}

??????????????@Override

??????????????public?void?handleMessage(Message msg) {//处理消息

?????????????????????text.setText(msg.obj.toString());

??????????????}????????????

???????}

???????private?class?MyThread?extends?Thread{

??????????????@Override

??????????????public?void?run() {

?????????????????????Looper curLooper = Looper.myLooper();

?????????????????????Looper mainLooper = Looper.getMainLooper();

?????????????????????String msg ;

?????????????????????if(curLooper==null){

????????????????????????????mHandler?=?new?MyHandler(mainLooper);

????????????????????????????msg =?"curLooper is null";

?????????????????????}else{

????????????????????????????mHandler?=?new?MyHandler(curLooper);

????????????????????????????msg =?"This is curLooper";

?????????????????????}

?????????????????????mHandler.removeMessages(0);

?????????????????????Message m =?mHandler.obtainMessage(1, 1, 1, msg);

?????????????????????mHandler.sendMessage(m);

??????????????}????????????

???????}

}

说明：

Android会自动替主线程建立Message Queue。在这个子线程里并没有建立Message Queue。所以，myLooper值为null，而mainLooper则指向主线程里的Looper。于是，执行到：

mHandler = new MyHandler (mainLooper);

此mHandler属于主线程。

???mHandler.sendMessage(m);

就将m消息存入到主线程的Message Queue里。mainLooper看到Message Queue里有讯息，就会作出处理，于是由主线程执行到mHandler的handleMessage()来处理消息。

下一节将会写一个关于应多线程请求网络数据的例子。