Xamarin MvvmCross: iOS how to monitor the app has move to foreground/background

Question

I have an MvvmCross application, and I am using the MvvmCross ViewModel Lifecycle functions to realize certain actions when the view appears, moves to background, and moves to foreground:

public override async void ViewAppeared()
public override async void ViewAppearing()
public override void ViewDisappearing()
public override void ViewDisappeared()
public override void ViewDestroy(bool viewFinishing)

Those functions work great in my Android device. But for iOS they do not get fired when the application moves to background or to foreground (although, except for ViewDestroy, they fire when navigating between the screens in the app in iOS)

1)Is that the intended behavior, or I am missing something?

2)If so, what is the approach we have to follow, when there are actions that we need to do when the app moves to foreground/background (like stopping timers)? Should we maybe have two implementations one for android, and one for ios? I also tried the ViewDidDisappear method in the MvxBaseViewController, still it is not activated when the app moves to background. There is a way in Xamarin/MvvmCross to hook into the native ios applicationDidEnterBackground?

Edit: I have tried Ranjit´s answer, but it seems to be a problem subscribing to the message. Here is my test code:

AppDelegate.cs:

public override void DidEnterBackground(UIApplication application)
    {
        base.DidEnterBackground(application);
        var message = new LocationMessage(
            this,
            34
        );
        _messenger = Mvx.IoCProvider.Resolve<IMvxMessenger>();

        _messenger.Publish(message);
    }

Base class:

public abstract class GenericMvxViewModel : MvxViewModel
{
    private IMvxMessenger _messenger;
 
    protected GenericMvxViewModel()
    {
        // other stuff
        _messenger = Mvx.IoCProvider.Resolve<IMvxMessenger>();
        _messenger.Subscribe<LocationMessage>(OnLocationMessage);
    }

    protected virtual void OnLocationMessage(LocationMessage locationMessage){} 
}

ViewModel:

public  class MyClassViewModel : GenericMvxViewModel
{
    protected override void OnLocationMessage(LocationMessage locationMessage)
    {
        Debug.WriteLine(locationMessage.Lat);
    }
}

The message is published in the AppDelegate.cs, but the OnLocationMessage method in the viewmodel is never executed.

Also I was wondering how to unsubscribe properly the message. ViewDestroy seems the most natural place, but as mentioned before it is never called on iOS

As far as I know. You have to use DidEnterBackground and WillEnterForeground override method of AppDelegate only. To make common, you can publish event (IMvxMessenger) from android and iOS and subscribe it from common base ViewModel and make sure you unsubscribe the message properly. — Ranjit, Apr 25 '21 at 15:57

Ranjit · Accepted Answer · 2021-04-27T16:02:53.633

Your code should work. I am using same kind of function in my app it was working fine

GenericMvxViewModel Code

        private MvxSubscriptionToken _locationEventToken;

        public override void ViewAppeared()
        {
            SubscribeBaseLocationEvent();
            base.ViewAppeared();
        }

        public override void ViewDisappeared()
        {
            if (StaticStorage.IsApplicationInForeground)
            {
                UnSubscribeBaseLocationEvent();
            }
            base.ViewDisappeared();
        }

        public void SubscribeBaseLocationEvent()
        {
            if (_locationEventToken == null)
            {
                _locationEventToken = Messenger.Subscribe<LocationMessage>(OnLocationMessage);
            }
        }

        public void UnSubscribeBaseLocationEvent()
        {
            if (_locationEventToken != null)
            {
                Messenger.Unsubscribe<LocationMessage>(_locationEventToken);
                _locationEventToken = null;
            }
        }

AppDelegate Code

  public override void DidEnterBackground(UIApplication application)
        {
            base.DidEnterBackground(application);
            StaticStorage.IsApplicationInForeground = false;
            _messenger.Publish(new LocationMessage( this, 34 ));
        }

        public override void WillEnterForeground(UIApplication application)
        {
            StaticStorage.IsApplicationInForeground = true;
        }

Android

 protected override void OnResume()
        {
            StaticStorage.IsApplicationInForeground = true;
            base.OnResume();
        }

        protected override void OnStop()
        {
            StaticStorage.IsApplicationInForeground = false;
            base.OnStart();
        }

When application is moving from one view to other view, we need to unsubscribe the event. But not when application moves to background. So, IsApplicationInForeground flag will help to solve this issue for android. Because for android when application goes to background ViewDisappeared will be called.

In my case. I have one common activity which holds remaining all views of fragment. So, I have added this code in common activity. Not sure in your case how you are using. But implementation will be similar.

Xamarin MvvmCross: iOS how to monitor the app has move to foreground/background

1 Answers1