How to execute background task when Android app is

问题:

My Android 4+ app is connected to a custom web service that is used to sync data every few minutes. To make sure, that the online data is always up to date, I want to trigger the sync when ever the app is closed / send to background.

Under iOS this is quite easy:

• Listen to applicationDidEnterBackground: in the AppDelegate
• Use beginBackgroundTaskWithName: to start you long running background task and to avoid being suspended while the task is still running

How to do this on Android?

First problem is, that there is nothing equivalent to applicationDidEnterBackground:. All solution I found so far propose to use the main Activities onPause() method. However this called any time the main Activity is paused, which is also the case when another Activity is started within the app. The is true for the onActivityPaused() method from ActivityLifecycleCallbacks interface.

So, how to detect that the app (not just an Activity) is closed or send to the background?

Second problem is, that I did not find any information on how to run background task. All solutions point to simply start an AsyncTask, but is this really the correct solution?

AsyncTask will start a new task, but will this task also run, when the app is closed/inactive? What do I have to do, to prevent the app and the task from being suspended after a few seconds? How can I make sure, that the task can complete, before the app is completely suspended?

回答1:

Following code will help you to post your content when your app is closed or in background:

import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.Handler;
import android.os.IBinder;
import android.widget.Toast;

public class SendDataService extends Service {
    private final LocalBinder mBinder = new LocalBinder();
    protected Handler handler;
    protected Toast mToast;

    public class LocalBinder extends Binder {
        public SendDataService getService() {
            return SendDataService .this;
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

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

    }

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

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        handler = new Handler();
        handler.post(new Runnable() {
            @Override
            public void run() { 

// write your code to post content on server
            }
        });
        return android.app.Service.START_STICKY;
    }

}

More explanation of my code is:

Service runs in background even if your application is in background but remember it always runs on main thread because of which I created a separate thread to perform your background operations.:

Service is started as sticky one as even if because of any reason your service got destroyed in background it will automatically get restarted.

More details can be found here: https://developer.android.com/guide/components/services.html

And to check if your app is in foreground/Background following code will help:

  private boolean isAppOnForeground(Context context) {
    ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
    List<RunningAppProcessInfo> appProcesses = activityManager.getRunningAppProcesses();
    if (appProcesses == null) {
      return false;
    }
    final String packageName = context.getPackageName();
    for (RunningAppProcessInfo appProcess : appProcesses) {
      if (appProcess.importance == RunningAppProcessInfo.IMPORTANCE_FOREGROUND && appProcess.processName.equals(packageName)) {
        return true;
      }
    }
    return false;
  }
}

// Use like this:
boolean foregroud = new ForegroundCheckTask().execute(context).get();

Happy Coding!!!!

回答2:

You can use services for what you want to achieve. A service will keep on running in the background even when the activity component has been destroyed, provided you have not invoked stopService(..) or stopSelf() methods.

In the service you can make an Async network call, preferably using retrofit, and then you can update you local storage like Sqlite DB with the latest data fetched from your web service.

Here is the official link, you can just use an unbounded service for what you want to achieve.

回答3:

The accepted answer is not correct.

Unfortunately, author believes that posting a new Runnable task to the Handler() creates a separate thread - "I created a separate thread to perform your background operations" :

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    handler = new Handler();
    handler.post(new Runnable() {
        @Override
        public void run() { 
           // write your code to post content on server
        }
    });
    return android.app.Service.START_STICKY;
}

onStartCommand() callback is always called on the main thread, and the new Handler() is attached to current thread (which is "main"). So the Runnable task is posted at the end of main thread queue.

To fix the issue and execute the Runnable in a really separate thread you can use AsyncTask as the simplest solution:

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    AsyncTask.execute(new Runnable() {
        @Override
        public void run() {
            // ...
        }
    });
    return android.app.Service.START_STICKY;
}

Think yourself before you copy-past anything...