【Android 开发教程】在服务中执行重复任务

本章节翻译自《Beginning-Android-4-Application-Development》，如有翻译不当的地方，敬请指出。

原书购买地址http://www.amazon.com/Beginning-Android-4-Application-Development/dp/1118199545/

除了在service中执行耗时的操作，也可能需要在service中执行重复的任务。举个例子，你想要编写一个闹钟程序，定时地在后台执行一个任务。在这种情况下，你的service就需要判断循环周期是否已经到达。可以使用Timer类去实现。

1. 使用之前的Services工程，做一点修改。

public class MyService extends Service {    int counter = 0;
    static final int UPDATE_INTERVAL = 1000;
    private Timer timer = new Timer();

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // We want this service to continue running until it is explicitly
        // stopped, so return sticky.
        Toast.makeText(this, "Service Started", Toast.LENGTH_LONG).show();
        new DoBackgroundTask().execute(urls);
        return START_STICKY;
    }

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
// We want this service to continue running until it is explicitly
// stopped, so return sticky.

// Toast.makeText(this, "Service Started", Toast.LENGTH_LONG).show();

        doSomethingRepeatedly();

        try {
            new DoBackgroundTask().execute(
                    new URL("http://www.amazon.com/somefiles.pdf"),
                    new URL("http://www.wrox.com/somefiles.pdf"),
                    new URL("http://www.google.com/somefiles.pdf"),
                    new URL("http://www.learn2develop.net/somefiles.pdf"));

        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

return START_STICKY;
}

    private void doSomethingRepeatedly() {
        timer.scheduleAtFixedRate( new TimerTask() {
            public void run() {
                Log.d("MyService", String.valueOf(++counter));
            }
        }, 0, UPDATE_INTERVAL);
    }

    private int DownloadFile(URL url) {
        try {
            //---simulate taking some time to download a file---
            Thread.sleep(5000);
        } catch (InterruptedException e) {
             e.printStackTrace();
        }
        //---return an arbitrary number representing 
        // the size of the file downloaded---
        return 100;
    }

    private class DoBackgroundTask extends AsyncTask<URL, Integer, Long> {
        protected Long doInBackground(URL... urls) {
            int count = urls.length;
            long totalBytesDownloaded = 0;
            for (int i = 0; i < count; i++) {
                totalBytesDownloaded += DownloadFile(urls[i]);
                //---calculate percentage downloaded and
                // report its progress---
                publishProgress((int) (((i+1) / (float) count) * 100));
            }
            return totalBytesDownloaded;
        }

        protected void onProgressUpdate(Integer... progress) {
            Log.d("Downloading files",
                    String.valueOf(progress[0]) + "% downloaded");
            Toast.makeText(getBaseContext(),
                String.valueOf(progress[0]) + "% downloaded",
                Toast.LENGTH_LONG).show();
        }

        protected void onPostExecute(Long result) {
            Toast.makeText(getBaseContext(),
                    "Downloaded " + result + " bytes",
                    Toast.LENGTH_LONG).show();
            stopSelf();
        }
    }    

@Override
public void onDestroy() {
super.onDestroy();

        if (timer != null){
            timer.cancel();
        }

Toast.makeText(this, "Service Destroyed", Toast.LENGTH_LONG).show();
}
}

2. 调试。查看系统的日志。

在这个例子中，首先创建一个Timer对象，然后调用scheduleAtFixedRate()方法：

    private void doSomethingRepeatedly() {        timer.scheduleAtFixedRate( new TimerTask() {            public void run() {                Log.d("MyService", String.valueOf(++counter));            }        }, 0, UPDATE_INTERVAL);    }

传入一个TimerTast对象，这样在run()方法就可以执行重复任务了。同事可以设置两次执行任务的间隔。

最后，在onDestroy()里面调用timer对象的cancel()方法。

@Overridepublic void onDestroy() {super.onDestroy();        if (timer != null){            timer.cancel();        }Toast.makeText(this, "Service Destroyed", Toast.LENGTH_LONG).show();}