Android 平台上，界面元素在定时器的响应函数里刷新。

在 Android 平台上，界面元素不能在定时器的响应函数里刷新。

以下这段代码中，mButton 的文本并不变化。

public class AndroidTimerDemo extends Activity {
private Button mButton;
private Timer mTimer;
private TimerTask mTimerTask;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

mButton = (Button) findViewById(R.id.Button01);

mTimer = new Timer();

mTimerTask = new TimerTask() {

@Override
public void run() {
Log.d("AndroidTimerDemo", "timer");
Calendar cal = Calendar.getInstance();
mButton.setText(cal.toString());
}
};

mTimer.schedule(mTimerTask, 1000, 1000);
}
}
在 Android 平台上，UI 单元必须在 Activity 的 context 里刷新。为了达到想要的效果，可以使用 Message Handler。在定时器响应函数里发送条消息，在 Activity 里响应消息并更新文本。

public class AndroidTimerDemo extends Activity {
protected static final int UPDATE_TEXT = 0;
private Button mButton;
private Timer mTimer;
private TimerTask mTimerTask;
private Handler mHandler;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

mButton = (Button) findViewById(R.id.Button01);

mTimer = new Timer();

mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
switch (msg.what){
case UPDATE_TEXT:
Calendar cal = Calendar.getInstance();
mButton.setText(cal.toString());
break;
}
}
};

mTimerTask = new TimerTask() {

@Override
public void run() {
Log.d("AndroidTimerDemo", "timer");
mHandler.sendEmptyMessage(UPDATE_TEXT);
/*
// It doesn't work updating the UI inside a timer.
Calendar cal = Calendar.getInstance();
mButton.setText(cal.toString());
*/
}
};

mTimer.schedule(mTimerTask, 1000, 1000);
}
}

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