android中利用Handler来更新UI

刚刚开始接触android线程编程的时候，习惯好像java一样，试图用下面的代码解决问题

new Thread( new Runnable() {   
    public void run() {   
         myView.invalidate();  
     }          
}).start();

然而发现这样是不行的，因为它违背了单线程模型：Android UI操作并不是线程安全的并且这些操作必须在UI线程中执行。查阅了文档和apidemo后，发觉常用的方法是利用Handler来实现UI线程的更新的。

下面代码的功能很简单：画一个圆出来，每隔0.1秒，圆向10移动10个像素。但可以清楚展示利用Handler更新UI的流程。

首先创建简单的View,代码如下：

package com.ray.handler;
 
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Point;
import android.graphics.drawable.Drawable;
import android.view.View;
 
public class BounceView extends View {
    float x = 40;
     
    public BounceView(Context context) { 
         super(context); 
    } 
 
    @Override 
    protected void onDraw(Canvas canvas) { 
        x+=10;
        Paint mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setColor(Color.GREEN);
        canvas.drawCircle(x, 40, 40, mPaint);
    }
}

创建Activity，代码如下：

package com.ray.handler;
 
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.Window;
 
public class TestHandler extends Activity {
     protected static final int GUIUPDATEIDENTIFIER = 0x101; 
      
     Thread myRefreshThread = null; 
     BounceView myBounceView = null; 
  
     Handler myHandler = new Handler() {
          public void handleMessage(Message msg) { 
               switch (msg.what) { 
                    case TestHandler.GUIUPDATEIDENTIFIER: 
                         myBounceView.invalidate();
                         break; 
               } 
               super.handleMessage(msg); 
          } 
     };
     public void onCreate(Bundle savedInstanceState) { 
          super.onCreate(savedInstanceState); 
          this.requestWindowFeature(Window.FEATURE_NO_TITLE); 
  
          this.myBounceView = new BounceView(this);
          this.setContentView(this.myBounceView); 
          new Thread(new myThread()).start();
     } 
  
     class myThread implements Runnable { 
          public void run() {
               while (!Thread.currentThread().isInterrupted()) {  
                      
                    Message message = new Message(); 
                    message.what = TestHandler.GUIUPDATEIDENTIFIER; 
                     
                    TestHandler.this.myHandler.sendMessage(message); 
                    try { 
                         Thread.sleep(100);  
                    } catch (InterruptedException e) { 
                         Thread.currentThread().interrupt(); 
                    } 
               } 
          } 
     } 
}