Android中的Handler机制

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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：

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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publicclass HandlerTestActivity extendsActivity {
    privateTextView tv;
    privatestatic final int UPDATE = 0;
    privateHandler handler = newHandler() {
 
        @Override
        publicvoid handleMessage(Message msg) {
            // TODO 接收消息并且去更新UI线程上的控件内容
            if(msg.what == UPDATE) {
                // Bundle b = msg.getData();
                // tv.setText(b.getString("num"));
                tv.setText(String.valueOf(msg.obj));
            }
            super.handleMessage(msg);
        }
    };
 
    /** Called when the activity is first created. */
    @Override
    publicvoid onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        tv = (TextView) findViewById(R.id.tv);
 
        newThread() {
            @Override
            publicvoid run() {
                // TODO 子线程中通过handler发送消息给handler接收，由handler去更新TextView的值
                try{
                    for(inti = 0; i < 100; i++) {
                        Thread.sleep(500);
                        Message msg = newMessage();
                        msg.what = UPDATE;
                        // Bundle b = new Bundle();
                        // b.putString("num", "更新后的值：" + i);
                        // msg.setData(b);
                        msg.obj = "更新后的值："+ i;
                        handler.sendMessage(msg);
                    }
                } catch(InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }.start();
    }
 
}

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。

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