Running a repeating task in background on a real t

问题:

I'm writing an application which is continuously listening and checking the sensors (almost all available) and saving that data into the database in the device.

I need to make some calculations every X second with that data and throw a new event if the calculations check says so. I'm thinking about requesting to have the device plugged in while using the application (regarding battery drain).

What's the best approach for the task that needs to make the calculations and throw the event? Timer? Threads? AsynkTask? AlarmManager? Another approach?

I want to keep getting sensors data and saving them to the database despite if the application is not on foreground...it should save the values as long as the application is not stopped by the user. One option for that is wake locks (PARTIAL_WAKE_LOCK, which keeps CPU running).

I'd like to hear different opinions. Thanks in advance! Guillermo.

回答1:

You can use AlarmManager to setup the repeating tasks (this is the Android prefered way of setting future/repeating tasks). To make the calculations use a Service (if you think calculations are going to be expensive, then think about moving them to a separate worker thread or use IntentService).

Regarding the wake lock (from the AlarmManager reference):

The Alarm Manager holds a CPU wake lock as long as the alarm receiver's onReceive() method is executing. This guarantees that the phone will not sleep until you have finished handling the broadcast. Once onReceive() returns, the Alarm Manager releases this wake lock. This means that the phone will in some cases sleep as soon as your onReceive() method completes. If your alarm receiver called Context.startService(), it is possible that the phone will sleep before the requested service is launched. To prevent this, your BroadcastReceiver and Service will need to implement a separate wake lock policy to ensure that the phone continues running until the service becomes available.

回答2:

This is a modified snippet of a service I wrote to log CPU frequency some time ago. It lacks the Application and the Activity part, but illustrates how I wrote the Service to keep logging every ten seconds. It does not log when the phone goes into deep sleep, so if you want to log without interruptions, then you will need to acquire PARTIAL_WAKE_LOCKs, but consider that battery life will be severely reduced by that.

public class YOURCLASS_Service extends Service {
    private long mStartTime = 0L;
    private final Handler mHandler = new Handler();
    private Runnable mUpdateTimeTask;
    private YOURAPP app;

    @Override
    public void onCreate() {
        super.onCreate();
        app = (YOURAPP) getApplicationContext();
    }

    @Override
    public void onDestroy() {
        Toast.makeText(this, "Service finished.", Toast.LENGTH_SHORT).show();
        stopLog ();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if (app.isRunning())
            return START_STICKY;
        try {
            File file = new File(Environment.getExternalStorageDirectory(), "yourlog.csv");
            OutputStreamWriter out = new OutputStreamWriter(new FileOutputStream(file, false));
            out.write("Log title");
            out.close();
        } catch (java.io.IOException e) {
              stopLog ();
              Toast.makeText(this, "Error creating log file. Aborting.", Toast.LENGTH_SHORT).show();
        }

        mUpdateTimeTask = new Runnable() {
            public void run() {
                long millis = SystemClock.uptimeMillis() - mStartTime;
                int seconds = (int) (millis / 1000);
                int minutes = seconds / 60;
                seconds     = seconds % 60;

                readYourSensors ();
                   if (!writeLog (str)) stopLog();
                   mHandler.postAtTime(this, mStartTime + (((minutes * 60) + seconds + 10) * 1000));
                   mHandler.postDelayed (mUpdateTimeTask, 10000);
        }};
        mStartTime = SystemClock.uptimeMillis();
        mHandler.removeCallbacks(mUpdateTimeTask);
        mHandler.postDelayed(mUpdateTimeTask, 100);

        Notification notification = new Notification(R.drawable.notification_icon, "App title", System.currentTimeMillis());
        Intent notificationIntent = new Intent(this, YOURCLASS.class);
        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
        notification.setLatestEventInfo(getApplicationContext(), "App title", "Please see /sdcard/yourlog.csv", contentIntent);
        startForeground(startId, notification);

        app.isRunning(true);
        return START_STICKY;
    }

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

    public void stopLog () {
            mHandler.removeCallbacks(mUpdateTimeTask);
    }
}