How Google calendar or Instagram make their apps all the time up and running even after force stop

Question

Looks like force stop should prevent app from running and it's even disable all app's alarms. However I found that notification in Google Calendar still shown fine even after force stop, and I all the time see Instagram app running, even when I kill it, it's just restart automatically and it's once again there.

So, what is a way to make app running constantly? I'm doing app with reminders and need to show notifications in specific time, regardless how app was previously closed before.

Answer 1

If you start a service in Application Class than your service will be always running even though if a user terminates or force stop from task manager, it will run again.

To create service specifically in Android studio Right click on app from Project Explorer and then New > Service > Service

Create a service:

public class ServiceName extends Service {

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

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
      // do your jobs here
      return super.onStartCommand(intent, flags, startId);
        }
    } 

Now create an Application Class and start Service in Application Class

public class App extends Application {

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

        startService(new Intent(this, ServiceName.class));
    }
}  

Answer 2

I see two options there.

First one is to handle any exception via Thread.setDefaultUncaughtExceptionHandler() that is shipping with Java, it's not from Android SDK.

To be concise, you make your custom Application class implement UncaughtExceptionHandler and register it as a listener to any exception in the application. As soon as some crash happens this callback will be fired from where you can delay a job to happen in near future (e.g. spawn AlarmManager to launch the app again in 50ms).

---

The second option is that you can launch your components in different processes. From docs:

android:process

The name of the process where the service is to run. Normally, all components of an application run in the default process created for the application. It has the same name as the application package. The element's process attribute can set a different default for all components. But component can override the default with its own process attribute, allowing you to spread your application across multiple processes.

A crash that happens on different process won't make the process that hosts your UI to crash.

Answer 3

Well for starters they have a job scheduler system that schedules jobs to executed, these jobs can be stopped , started or resumed. They implement a mechanism to detect crash, think java-script where if a application crashes it can be restarted (nodemon or forever) , in android there are services, which can be started or resume, There is this special service behavior to restart crashed services.

START_REDELIVER_INTENT- tells the system to restart the service after the crash and also redeliver the intents that were present at the time of crash.

The Android 5.0 Lollipop (API 21) release introduces a job scheduler API via the JobScheduler class. This API allows to batch jobs when the device has more resources available. In general this API can be used to schedule everything that is not time critical for the user.

You can also mix Alarms, broadcast receivers and threading coupled with reactive programming to perform the job.Alarms (based on the AlarmManager class) give you a way to perform time-based operations outside the lifetime of your application. For example, you could use an alarm to initiate a long-running operation, such as starting a service once a day to download a weather forecast.

You can attach observables to certain tasks performing specific operations, You can implement asynchronous i/o, computational operations, or even “infinite” streams of data by designing your own Observable.