Example of listening to connectivity status in Android and reacting to going offline. (Subscription, Observable, Observer, PublishSubject are from RxJava.)

1_RxJava-connectivity-status-example.md

Example of using RxJava to listen to connectivity status in Android

You could use the same approach to listen to any status, but this example includes network connectivity specifics too (ConnectionChangeReceiver and AndroidUtils.isConnected).

AndroidUtils.java

public class AndroidUtils {
  
    public static boolean isConnected(Context context) {
        ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo activeNetwork = manager.getActiveNetworkInfo();
        return activeNetwork != null && activeNetwork.isConnected();
    }
    
}

BaseActivity.java

// Superclass for all activities
public abstract class BaseActivity extends Activity {
    private static final String TAG = BaseActivity.class.getSimpleName();
  
    private Subscription onlineSubscription;
    
    @Override
    protected void onResume() {
        super.onResume();

        onlineSubscription = NetworkManager.getInstance().onlineSignal().subscribe(new GoOfflineObserver());

        if (!AndroidUtils.isConnected(this)) {
            // Launch offline alert 
        }
    }
    
    @Override
    protected void onPause() {
        // When we're in the background, we do NOT want network status changes to bring our app to foreground again
        if (onlineSubscription != null) {
            onlineSubscription.unsubscribe();
        }
        super.onPause();
    }

    private class GoOfflineObserver implements rx.Observer<Boolean> {
        @Override
        public void onCompleted() {
        }

        @Override
        public void onError(Throwable throwable) {
            Log.e(TAG, "Error with onlineSignal", throwable);
        }

        @Override
        public void onNext(Boolean online) {
            if (!online) {
                // Launch offline alert 
            }
        }
    }
    
}

ConnectionChangeReceiver.java

/**
 * onReceive is invoked when connection is lost or regained. 
 * 
 * This needs to be configured in AndroidManifest.xml, inside <application> element: 
 * 
  <receiver
    android:name="com.company.product.network.ConnectionChangeReceiver"
    android:label="NetworkConnection">
      <intent-filter>
        <action android:name="android.net.conn.CONNECTIVITY_CHANGE"/>
      </intent-filter>
   </receiver>
 */
public class ConnectionChangeReceiver extends BroadcastReceiver {
    private static final String TAG = ConnectionChangeReceiver.class.getSimpleName();

    @Override
    public void onReceive(Context context, Intent intent) {
        boolean online = AndroidUtils.isConnected(context);
        NetworkManager.getInstance().onlineObserver().onNext(online);
        Log.i(TAG, String.format("Switching to %s mode", online ? "ONLINE" : "OFFLINE"));
    }
    
}

NetworkManager.java

// Class with all of the app's network code... Here omitting everything except onlineSubject 
public class NetworkManager {
  
  private PublishSubject<Boolean> onlineSubject = PublishSubject.create();
  
  public Observable<Boolean> onlineSignal() {
      return onlineSubject;
  }

  Observer<Boolean> onlineObserver() {
      return onlineSubject;
  }
    
}