什么是Handler
handler通俗讲就是在各个进程之间发送数据的处理对象。在任何进程中，只要获得了另一个进程的handler则可以通过 handler.sendMessage(message)方法向那个进程发送数据。基于这个机制，我们在处理多线程的时候可以新建一个thread，这 个thread拥有UI线程中的一个handler。当thread处理完一些耗时的操作后通过传递过来的handler向UI线程发送数据，由UI线程去更新界面。
Handler类简介
在Android平台中，新启动的线程是无法访问Activity里的Widget的，当然也不能将运行状态外送出来，这就需要有Handler机 制进行消息的传递了，Handler类位于android.os包下，主要的功能是完成Activity的Widget与应用程序中线程之间的交互。接下 来对该类中常用的方法进行介绍，如下表所示。
Handler类的常用方法
方法签名 描 述 public void handleMessage (Message msg) 子类对象通过该方法接收信息 public final boolean sendEmptyMessage (int what) 发送一个只含有 what 值的消息 public final boolean sendMessage (Message msg) 发送消息到 Handler ， 通过 handleMessage 方法接收 public final boolean hasMessages (int what) 监测消息队列中是否还 有 what 值的消息 public final boolean post (Runnable r) 将一个线程添加到消息队列
开发带有Handler类的程序步骤如下。
在Activity或Activity的Widget中开发Handler类的对象，并重写handleMessage方法。
在新启动的线程中调用sendEmptyMessage或者sendMessage方法向Handler发送消息。
Handler类的对象用handleMessage方法接收消息，然后根据消息的不同执行不同的操作。
在Android 中Handler和Message、Thread有着很密切的关系。Handler 主要是负责Message的分发和处理。但是这个Message从哪里来的呢？Message 由一个消息队列进行管理，而消息队列却由一个Looper进行管理。Android系统中Looper负责管理线程的消息队列和消息循环，具体实现请参考Looper的源码。 可以通过Loop.myLooper()得到当前线程的Looper对象，通过Loop.getMainLooper()可以获得当前进程的主线程的 Looper对象。Android系统的消息队列和消息循环都是针对具体线程的，一个线程可以存在（当然也可以不存在）一个消息队列和一个消 息循环（Looper），特定线程的消息只能分发给本线程，不能进行跨线程，跨进程通讯。但是创建的工作线程默认是没有消息循环和消息队列的，如果想让该 线程具有消息队列和消息循环，需要在线程中首先调用Looper.prepare()来创建消息队列，然后调用Looper.loop()进入消息循环。
虽说 特定线程的消息只能分发给本线程，不能进行跨线程通讯，但是由于可以通过获得线程的Looper对象来进行曲线的实现不同线程间消息的传递，代码如下
package com.mytest.handlertest;
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup.LayoutParams;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;
public class HandlerTest extends Activity implements OnClickListener{
private String TAG = "HandlerTest";
private boolean bpostRunnable = false;
private NoLooperThread noLooperThread = null;
private OwnLooperThread ownLooperThread = null;
private ReceiveMessageThread receiveMessageThread =null;
private Handler mOtherThreadHandler=null;
private EventHandler mHandler = null;
private Button btn1 = null;
private Button btn2 = null;
private Button btn3 = null;
private Button btn4 = null;
private Button btn5 = null;
private Button btn6 = null;
private TextView tv = null;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LinearLayout layout = new LinearLayout(this);
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(250, 50);
layout.setOrientation(LinearLayout.VERTICAL);
btn1 = new Button(this);
btn1.setId(101);
btn1.setText("message from main thread self");
btn1.setOnClickListener(this);
layout.addView(btn1, params);
btn2 = new Button(this);
btn2.setId(102);
btn2.setText("message from other thread to main thread");
btn2.setOnClickListener(this);
layout.addView(btn2,params);
btn3 = new Button(this);
btn3.setId(103);
btn3.setText("message to other thread from itself");
btn3.setOnClickListener(this);
layout.addView(btn3, params);
btn4 = new Button(this);
btn4.setId(104);
btn4.setText("message with Runnable as callback from other thread to main thread");
btn4.setOnClickListener(this);
layout.addView(btn4, params);
btn5 = new Button(this);
btn5.setId(105);
btn5.setText("main thread's message to other thread");
btn5.setOnClickListener(this);
layout.addView(btn5, params);
btn6 = new Button(this);
btn6.setId(106);
btn6.setText("exit");
btn6.setOnClickListener(this);
layout.addView(btn6, params);
tv = new TextView(this);
tv.setTextColor(Color.WHITE);
tv.setText("");
params = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.WRAP_CONTENT);
params.topMargin=10;
layout.addView(tv, params);
setContentView(layout);
receiveMessageThread = new ReceiveMessageThread();
receiveMessageThread.start();
}
class EventHandler extends Handler{
public EventHandler(Looper looper){
super(looper);
}
public EventHandler(){
super();
}
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);
Log.e(TAG, "CurrentThread id:----------+>" + Thread.currentThread().getId());
switch(msg.what){
case 1:
tv.setText((String)msg.obj);
break;
case 2:
tv.setText((String)msg.obj);
noLooperThread.stop();
break;
case 3:
//不能在非主线程的线程里面更新UI，所以这里通过log打印信息
Log.e(TAG,(String)msg.obj);
ownLooperThread.stop();
break;
default:
Log.e(TAG,(String)msg.obj);
break;
}
}
}
//ReceiveMessageThread has his own message queue by execute Looper.prepare();
class ReceiveMessageThread extends Thread {
@Override
public void run(){
Looper.prepare();
mOtherThreadHandler= new Handler(){
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);
Log.e(TAG,"-------+>"+(String)msg.obj);
Log.e(TAG, "CurrentThread id:----------+>" + Thread.currentThread().getId());
}
};
Log.e(TAG, "ReceiveMessageThread id:--------+>" + this.getId());
Looper.loop();
}
}
class NoLooperThread extends Thread {
private EventHandler mNoLooperThreadHandler;
@Override
public void run() {
Looper myLooper = Looper.myLooper();
Looper mainLooper= Looper.getMainLooper();
String msgobj;
if(null == myLooper){
//这里获得的是主线程的Looper,由于NoLooperThread没有自己的looper所以这里肯定会被执行
mNoLooperThreadHandler = new EventHandler(mainLooper);
msgobj = "NoLooperThread has no looper and handleMessage function executed in main thread!";
} else{
mNoLooperThreadHandler = new EventHandler(myLooper);
msgobj = "This is from NoLooperThread self and handleMessage function executed in NoLooperThread!";
}
mNoLooperThreadHandler.removeMessages(0);
if(bpostRunnable == false){
//send message to main thread
Message msg = mNoLooperThreadHandler.obtainMessage(2, 1, 1, msgobj);
mNoLooperThreadHandler.sendMessage(msg);
Log.e(TAG, "NoLooperThread id:--------+>" + this.getId());
}else{
//下面new出来的实现了Runnable接口的对象中run函数是在Main Thread中执行，不是在NoLooperThread中执行 记得 null == myLooper么
//注意Runnable是一个接口，它里面的run函数被执行时不会再新建一个线程
//您可以在run上加断点然后在eclipse调试中看它在哪个线程中执行
mNoLooperThreadHandler.post(new Runnable(){
public void run() {
// TODO Auto-generated method stub
tv.setText("update UI through handler post runnalbe mechanism!");
Log.e(TAG, "update UI id:--------+>" + Thread.currentThread().getId());
noLooperThread.stop();
}
});
}
}
}
class OwnLooperThread extends Thread{
private EventHandler mOwnLooperThreadHandler = null;
@Override
public void run() {
Looper.prepare();
Looper myLooper = Looper.myLooper();
Looper mainLooper= Looper.getMainLooper();
String msgobj;
if(null == myLooper){
mOwnLooperThreadHandler = new EventHandler(mainLooper);
msgobj = "OwnLooperThread has no looper and handleMessage function executed in main thread!";
}else{
mOwnLooperThreadHandler = new EventHandler(myLooper);
msgobj = "This is from OwnLooperThread self and handleMessage function executed in NoLooperThread!";
}
mOwnLooperThreadHandler.removeMessages(0);
//给自己发送消息
Message msg = mOwnLooperThreadHandler.obtainMessage(3,1,1,msgobj);
mOwnLooperThreadHandler.sendMessage(msg);
Looper.loop();
}
}
public void onClick(View v) {
// TODO Auto-generated method stub
switch(v.getId()){
case 101:
//主线程发送消息给自己
Looper looper = Looper.myLooper();//get the Main looper related with the main thread
//如果不给任何参数的话会用当前线程对应的Looper(这里就是Main Looper)为Handler里面的成员mLooper赋值
mHandler = new EventHandler(looper);
// 清除整个MessageQueue里的消息
mHandler.removeMessages(0);
String obj = "This main thread's message and received by itself!";
Message msg = mHandler.obtainMessage(1,1,1,obj);
// 将Message对象送入到main thread的MessageQueue里面
mHandler.sendMessage(msg);
break;
case 102:
//other线程发送消息给主线程
bpostRunnable = false;
noLooperThread = new NoLooperThread();
noLooperThread.start();
break;
case 103:
//other thread获取它自己发送的消息
tv.setText("please look at the error level log for other thread received message");
ownLooperThread = new OwnLooperThread();
ownLooperThread.start();
break;
case 104:
//other thread通过Post Runnable方式发送消息给主线程
bpostRunnable = true;
noLooperThread = new NoLooperThread();
noLooperThread.start();
break;
case 105:
//主线程发送消息给other thread
if(null!=mOtherThreadHandler){
tv.setText("please look at the error level log for other thread received message from main thread");
String msgObj = "message from mainThread";
Message mainThreadMsg = mOtherThreadHandler.obtainMessage(1, 1, 1, msgObj);
mOtherThreadHandler.sendMessage(mainThreadMsg);
}
break;
case 106:
finish();
break;
}
}
}