MvvmCross ViewModel lifecycle during rotation

Question

I have been using Mvvmcross to develop Android application. I am dealing with the issue of ViewModel lifecycle during a rotation. It seems that generally ViewModel is preserved during a rotation. However this is not the case when I present ViewModels in MvxTabActivity. When the rotation happens it always calls a ViewModel constructor.

I have used similar code structure as in N+1 tutorial https://github.com/slodge/NPlus1DaysOfMvvmCross/tree/master/N-25-Tabbed.

Is there a way to modify this tutorial to keep ViewModels in memory during rotation when using MvxTabActivity?

Answer 1

The default ViewModel caching which attempts to workaround the Android rotation behaviour is based around IMvxSingleViewModelCache - so it's not too surprising it can't cope with multiple Activities and multiple ViewModels.

For where this interface is declared and used, see https://github.com/slodge/MvvmCross/search?q=IMvxSingleViewModelCache&ref=cmdform

If this behaviour is troubling you, then you should be able to work around it by one of :

1. Use fragment based tabs rather than Activity based ones

Android handles Fragment lifecycle differently to Activity ones.

2. Or continue using activity based tabs, but implement your own IMvxSingleViewModelCache

It should be simple, for example, to identify your child view models with by their 'Child' naming convention.

With this done you can then implement something like:

public class MyCustomViewModelCache
    : IMvxSingleViewModelCache
{
    private const string BundleCacheKey = "__mvxVMCacheKey";

    private int _counter;

    private IMvxViewModel _currentViewModel;

    public void Cache(IMvxViewModel toCache, Bundle bundle)
    {
        if (toCache != null
            && toCache.GetType().Name.StartsWith("Child"))
        {
            // don't worry about caching child view models
            return;
        }

        _currentViewModel = toCache;
        _counter++;

        if (_currentViewModel == null)
        {
            return;
        }

        bundle.PutInt(BundleCacheKey, _counter);
    }

    public IMvxViewModel GetAndClear(Bundle bundle)
    {
        var storedViewModel = _currentViewModel;
        _currentViewModel = null;

        if (bundle == null)
            return null;

        var key = bundle.GetInt(BundleCacheKey);
        var toReturn = (key == _counter) ? storedViewModel : null;
        return toReturn;
    }
}

This class based on MvxSingleViewModelCache.cs with just one small addition.

You can register an instance of this class as the IMvxSingleViewModelCache singleton during the InitializeLastChance of your Setup.

  Mvx.RegisterSingleton<IMvxSingleViewModelCache>(new MyCustomViewModelCache());
    

With this done, the home/tab activity should (I think) continue to work - and it'll pass the viewmodels down to the tab children after rotation.

(Other possibilities for IMvxSingleViewModelCache are possible - e.g. it could cache multiple view models - but please don't let it cache too many view models for too long or you may run into 'out of memory' conditions)

3. Or switch the Android rotation handling off

If you add the android:configChanges="orientation" flag (or it's monodroid equivalent Attribute) then you can just handle the rotation yourself.