Android中的Handler机制

直接在UI线程中开启子线程来更新TextView显示的内容，运行程序我们会发现，如下错误：android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.翻译过来就是：只有创建这个控件的线程才能去更新该控件的内容。

所有的UI线程要去负责View的创建并且维护它，例如更新冒个TextView的显示，都必须在主线程中去做，我们不能直接在UI线程中去创建子线程，要利用消息机制：handler，如下就是handler的简单工作原理图：

Android中的Handler机制
既然android给我们提供了Handler机制来解决这样的问题，请看如下代码：

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public class HandlerTestActivity extends Activity { 

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    private TextView tv; 

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    private static final int UPDATE = 0; 

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    private Handler handler = new Handler() { 

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        @Override

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        public void handleMessage(Message msg) { 

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            // TODO 接收消息并且去更新UI线程上的控件内容 

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            if (msg.what == UPDATE) { 

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                // Bundle b = msg.getData(); 

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                // tv.setText(b.getString("num")); 

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                tv.setText(String.valueOf(msg.obj)); 

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            } 

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            super.handleMessage(msg); 

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        } 

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    }; 

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    /** Called when the activity is first created. */

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    @Override

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    public void onCreate(Bundle savedInstanceState) { 

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        super.onCreate(savedInstanceState); 

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        setContentView(R.layout.main); 

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        tv = (TextView) findViewById(R.id.tv); 

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        new Thread() { 

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            @Override

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            public void run() { 

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                // TODO 子线程中通过handler发送消息给handler接收，由handler去更新TextView的值 

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                try { 

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                    for (int i = 0; i < 100; i++) { 

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                        Thread.sleep(500); 

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                        Message msg = new Message(); 

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                        msg.what = UPDATE; 

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                        // Bundle b = new Bundle(); 

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                        // b.putString("num", "更新后的值：" + i); 

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                        // msg.setData(b); 

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                        msg.obj = "更新后的值：" + i; 

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                        handler.sendMessage(msg); 

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                    } 

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                } catch (InterruptedException e) { 

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                    e.printStackTrace(); 

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                } 

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            } 

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        }.start(); 

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    } 

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}

我们就通过Handler机制来处理了子线程去更新UI线程控件问题，Andrid开发中要经常用到这种机制。