android--handler

app在启动时会产生一个进程和一个线程，线程是主线程，又叫UI线程，更新UI元素必须要在UI线程中更新，否则会报错。

在UI线程中有消息队列，子线程sendMessage到MQ中，looper类取出队列中的消息由UI线程中的Handler的handleMessage方法处理。

public class HandlerTestActivity extends Activity {
    private TextView tv;
    private static final int UPDATE = 0;
    private Handler handler = new Handler() {

        @Override
        public void 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
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        tv = (TextView) findViewById(R.id.tv);

        new Thread() {
            @Override
            public void 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 (int i = 0; i < 100; i++) {
                        Thread.sleep(500);
                        Message msg = new Message();
                        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();
    }

}