Handler是整个消息系统的核心，是Handler向MessageQueue发送的Message，最后Looper也是把消息通知给1.Handler，所以就从Handler讲起。
Handler的构造函数有很多，但本质差不多：
public Handler() {
this(null, false);
}public Handler(Callback callback, boolean async) {
//自动绑定当前线程的looper
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");//从这可以看出，创建Handler必须有Looper
}
mQueue = mLooper.mQueue; //Looper的MessageQueue
mCallback = callback; //一个回掉接口
mAsynchronous = async;
}//这个是创建给定Looper的Handler
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
2.在看Looper的源代码：
public static Looper myLooper() {
return sThreadLocal.get();
}在一个子线程中创建Looper的一般步骤：（这是我自己写的，不是源代码）
class MyThread extends Thread{
public Handler handler;
public Looper looper;
public void run() {
Looper.prepare();//创建一个looper
looper = Looper.myLooper();
handler = new Handler(){
@Override
public void handleMessage(Message msg) {
System.out.println("currentThread->"+Thread.currentThread());
}
};
Looper.loop();//让消息循环起来
}
}下面就看看Looper.prepare,Looper.loop方法：
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {//sThreadLocal使得线程能够保持各自独立的一个对象。
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}Looper.prepare();
public static void loop() {
final Looper me = myLooper();
if (me == null) { //如果Looper为空
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
。。。。
for (;;) {
Message msg = queue.next(); // 循环下一个
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
。。。。
msg.target.dispatchMessage(msg); //分发消息，msg.target就是Handler
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
。。。。
msg.recycle(); //回收msg到msgPool
}
}从这些代码可以看出Looper不断检查MessagePool是否有《==Message，有的话就通过Handler的dispatchMessage(msg)发送出去，利用Handler与外界交互。3.Message的源代码：
public static Message obtain() { //得到Message对象
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
sPoolSize--;
return m;
}
}
return new Message(); //没有就新建
}handler.obtainMessage()方法：
public final Message obtainMessage()
{
return Message.obtain(this); //通过Message的obtain方法
} public static Message obtain(Handler h) { //就是这个方法
Message m = obtain(); //最终调用的还是obtain方法
m.target = h; //target是handler
return m;
}看了上边的源代码，相信你一定对Handler，message，Looper有了一定了解，对编程中常遇到的方法，知道是怎么用的啦。其实学android一定要常看源码，源码很有用。
4.下边就是上代码，实例分析：
<pre name="code" class="java">package com.example.handler_01;
import android.Manifest.permission;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Handler.Callback;
import android.os.Message;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends Activity implements OnClickListener{
private TextView textView;
private Button button;
private Handler handler = new Handler(new Callback() {//拦截消息
public boolean handleMessage(Message msg) { //截获handler的发送的消息
Toast.makeText(getApplicationContext(), ""+1, 1).show();
//return false;
return false;//若返回true，则证明截获，下面的handleMessage就不会执行！
}
}){
public void handleMessage(Message msg) {
Toast.makeText(getApplicationContext(), ""+2, 1).show();
Person person = (Person)msg.obj;
System.out.println(person.toString());
}
};
private MyRunnable myRunnable=new MyRunnable();
private ImageView imageView;
private int images[]={R.drawable.a1,R.drawable.a2,R.drawable.a3};
private int index;
class MyRunnable implements Runnable{ //不断的更新图片，3张轮换
@Override
public void run() {
index++;
index=index%3; //不断循环
imageView.setImageResource(images[index]);
handler.postDelayed(myRunnable, 1000); //每隔一段时间执行myRunnable
System.out.println("MyRunnable中的线程："+Thread.currentThread());//运行在当前主线程！
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textView = (TextView) findViewById(R.id.textview);
imageView = (ImageView) findViewById(R.id.imageView1);
button = (Button) findViewById(R.id.button1);
button.setOnClickListener(this);
new Thread(){
@Override
public void run() {
try {
Thread.sleep(2000);
/*Message message = new Message();
message.arg1=88;*/
Message message = handler.obtainMessage();
Person person = new Person();
person.age=20;
person.name="chaochao";
message.obj=person;
handler.sendMessage(message);//在子线程中向主线程发消息。
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
handler.postDelayed(myRunnable, 1000);
}
@Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.button1:
handler.removeCallbacks(myRunnable);
//handler.sendEmptyMessage(1);
break;
default:
break;
}
}
class Person{
public int age;
public String name;
public String toString() {
return "name="+name+" age="+age;
}
}
}布局很简单，就不上代码啦。
</pre><p></p><pre>
运行结果：
这次代码先写到这，饿啦去吃饭，在后边再详细解析Handler的用法。。
转载请注明出处：http://blog.csdn.net/jycboy