Android 开发笔记—— Handler与异步

     在开发中，有这样一个应用场景，我们做的Android程序需要每次运行前，通过网络，检查有没有最新版本。这功能目前很多软件都有，这时候检查有没有新版本，会是后台运行的一个异步任务。使用Handler我们可以到达这一效果。



     但是创建Handler并不是创建一个用于异步的线程，相反，Handler会绑定在创建它的对象所占用的线程之上。就是说，Handler和创建Handler的对象公用一个线程，为了证明我们的想法，请看看项目的代码：

; ruler: true]package com.example;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.provider.Settings;
import android.widget.TextView;

public class HandlerActivity extends Activity
{
TextView welcomeTxt;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
welcomeTxt = (TextView) findViewById(R.id.txtView);

MyHandler myHandler = new MyHandler();
myHandler.post(longTimeWork);
//because myHandler and HandlerActivity are in the same thread, so android should finish longTimeWork first
//And then the welcome message will appear.
welcomeTxt.setText("Welcome!");
}

private class MyHandler extends Handler{
public MyHandler(){
}
public MyHandler(Looper looper){
super(looper);
}
@Override
public void handleMessage(Message msg) {
Bundle b = msg.getData();
int age = b.getInt("age");
String name = b.getString("name");
System.out.println("age is " + age + ", name is" + name);
System.out.println("Handler--->" + Thread.currentThread().getId());
System.out.println("handlerMessage");
}
}

private Runnable longTimeWork = new Runnable() {
public void run() {
System.out.println("I will cost long time!");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
}
}
};
}

 
    程序中longTimeWork模拟了一个需要五秒运行的时间的任务，但程序运行后，我们可以看到“Welcome”会在程序运行后大约五秒后才出现，而我们在两个地方打印出来的Thread ID都是一样的（图一）。这足够证明我们的猜想，而且在API中，也可以证实我们的猜想。


（图一）
 
    这明显不能满足文章开头所描述的应用场景的需要，我们必须寻找解决方法。尝试修改代码：
 

package com.example;

import android.app.Activity;
import android.os.*;
import android.provider.Settings;
import android.widget.TextView;

public class HandlerActivity extends Activity
{
TextView welcomeTxt;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
welcomeTxt = (TextView) findViewById(R.id.txtView);

System.out.println("Thread ID in HandlerActivity:"+Thread.currentThread().getId());
HandlerThread handlerThread = new HandlerThread("new Thread");
handlerThread.start();
MyHandler myHandler = new MyHandler(handlerThread.getLooper());
myHandler.post(longTimeWork);
//because myHandler and HandlerActivity are in the same thread, so android should finish longTimeWork first
//And then the welcome message will appear.
welcomeTxt.setText("Welcome!");
}

private class MyHandler extends Handler{
public MyHandler(){
}
public MyHandler(Looper looper){
super(looper);
}
@Override
public void handleMessage(Message msg) {

}
}

private Runnable longTimeWork = new Runnable() {
public void run() {
System.out.println("I will cost long time!");
try {
System.out.println("Thread ID in longTimeWork:"+Thread.currentThread().getId());
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
}
}
};
}

 


（图二）
 
    这次情况不一样了，“Welcome”会马上显示，而且，打印的Thread ID也不一样了（图二）。我这里创建了HandlerThread，这个类是用来创建一条新线程而这个类含有一个Looper，这个Looper可以用来创建Handler，这样创建的Handler在HanderThread的线程上运行。
    到此文章开头的场景也就满足了。