以下代码来自于《Android Developer's Cookbook》第58-61页。本书将该代码放在了消息传递的背景下介绍,它描述道:“计时器在后台线程中运行,因此不会阻塞UI线程,但需要在时间更改时更新UI。”我有些困惑,因为我没有看到两个线程。在我看来,主UI线程将一个可运行消息发布到其自己的消息队列中(然后该消息再以延迟时间重新发布自己)。我是否漏掉了什么?
package com.cookbook.background_timer;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.SystemClock;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class BackgroundTimer extends Activity {
//keep track of button presses, a main thread task
private int buttonPress=0;
TextView mButtonLabel;
//counter of time since app started, a background task
private long mStartTime = 0L;
private TextView mTimeLabel;
//Handler to handle the message to the timer task
private Handler mHandler = new Handler();
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
if (mStartTime == 0L) {
mStartTime = SystemClock.uptimeMillis();
mHandler.removeCallbacks(mUpdateTimeTask);
mHandler.postDelayed(mUpdateTimeTask, 100);
}
mTimeLabel = (TextView) findViewById(R.id.text);
mButtonLabel = (TextView) findViewById(R.id.trigger);
Button startButton = (Button) findViewById(R.id.trigger);
startButton.setOnClickListener(new View.OnClickListener() {
public void onClick(View view){
mButtonLabel.setText("Pressed " + ++buttonPress + " times");
}
});
}
private Runnable mUpdateTimeTask = new Runnable() {
public void run() {
final long start = mStartTime;
long millis = SystemClock.uptimeMillis() - start;
int seconds = (int) (millis / 1000);
int minutes = seconds / 60;
seconds = seconds % 60;
mTimeLabel.setText("" + minutes + ":" + String.format("%02d",seconds));
mHandler.postDelayed(this, 200);
}
};
@Override
protected void onPause() {
mHandler.removeCallbacks(mUpdateTimeTask);
super.onPause();
}
@Override
protected void onResume() {
super.onResume();
mHandler.postDelayed(mUpdateTimeTask, 100);
}
}