Lifecycle of fragments in mvvmCross - xamarin.android

Question

I have the following problem when using fragments in mvvmCross. I borrowed the code from https://github.com/slodge/MvvmCross-Tutorials/tree/master/Fragments. The Framepresenter Class which is in https://github.com/slodge/MvvmCross-Tutorials/blob/master/Fragments/FragmentSample.UI.Droid/Setup.cs resolved requests to show viewmodels in the show method
public class CustomPresenter
: MvxAndroidViewPresenter
, ICustomPresenter
{
private Dictionary _dictionary = new Dictionary();
public override void Show(MvxViewModelRequest request)
{
IFragmentHost host;
if (_dictionary.TryGetValue(request.ViewModelType, out host))
{
if (host.Show(request))
{
return;
}
}
base.Show(request);
}
...
In the first run all works well. If I close the app with the "Back" Button of android and open it again I will get a null pointer exception in the Show method of the base class (MvxAndroidViewPresenter). This is because the top level activity does not exist any more. The property Activity in MvxAndroidViewPresenter is returning null. How can I recreate the top level activity when restarting the app?
I get the following exception
06-26 16:06:28.102 I/MonoDroid( 6791): UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Views.MvxAndroidViewPresenter.Show (Cirrious.MvvmCross.ViewModels.MvxViewModelRequest) [0x0000f] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Views\MvxAndroidViewPresenter.cs:29
06-26 16:06:28.102 I/MonoDroid( 6791): at ManCockpit.UI.Droid.CockpitFragmentPresenter.Show (Cirrious.MvvmCross.ViewModels.MvxViewModelRequest) [0x00040] in d:\Projekte\ManCockpitV2\ManCockpit.UI.Droid\CockpitFragmentPresenter.cs:41
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Views.MvxAndroidViewDispatcher/c__DisplayClass1.b__0 () [0x00000] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Views\MvxAndroidViewDispatcher.cs:27
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Views.MvxAndroidViewDispatcher.ShowViewModel (Cirrious.MvvmCross.ViewModels.MvxViewModelRequest) [0x00015] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Views\MvxAndroidViewDispatcher.cs:27
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.ViewModels.MvxNavigatingObject.ShowViewModelImpl (System.Type,Cirrious.MvvmCross.ViewModels.IMvxBundle,Cirrious.MvvmCross.ViewModels.IMvxBundle,Cirrious.MvvmCross.ViewModels.MvxRequestedBy)
06-26 16:06:28.102 I/MonoDroid( 6791): at (wrapper dynamic-method) objec06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.ViewModels.MvxNavigatingObject.ShowViewModel (System.Type,Cirrious.MvvmCross.ViewModels.IMvxBundle,Cirrious.MvvmCross.ViewModels.IMvxBundle,Cirrious.MvvmCross.ViewModels.MvxRequestedBy)
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.ViewModels.MvxNavigatingObject.ShowViewModel (Cirrious.MvvmCross.ViewModels.IMvxBundle,Cirrious.MvvmCross.ViewModels.MvxBundle,Cirrious.MvvmCross.ViewModels.MvxRequestedBy)
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.ViewModels.MvxAppStart`1.Start (object)
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Views.MvxSplashScreenActivity.TriggerFirstNavigate () [0x00007] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Views\MvxSplashScreenActivity.cs:82
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Views.MvxSplashScreenActivity.InitializationComplete () [0x00001] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Views\MvxSplashScreenActivity.cs:76
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Platform.MvxAndroidSetupSingleton.InitialiseFromSplashScreen (Cirrious.MvvmCross.Droid.Views.IMvxAndroidSplashScreenActivity) [0x00036] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Platform\MvxAndroidSetupSingleton.cs:76
06-26 16:06:28.102 I/MonoDroid( 6791): at Cirrious.MvvmCross.Droid.Views.MvxSplashScreenActivity.OnCreate (Android.OS.Bundle) [0x00014] in d:\Projekte\MvvmCross\mvvmcross-20130621\Cirrious\Cirrious.MvvmCross.Droid\Views\MvxSplashScreenActivity.cs:47
06-26 16:06:28.102 I/MonoDroid( 6791): at ManCockpit.UI.Droid.SplashScreenActivity.OnCreate (Android.OS.Bundle) [0x0002e] in d:\Projekte\ManCockpitV2\ManCockpit.UI.Droid\SplashScreenActivity.cs:105
06-26 16:06:28.102 I/MonoDroid( 6791): at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_ (intptr,intptr,intptr) [0x00010] in /Users/builder/data/lanes/monodroid-lion-bs1/0cc7ae3b/source/monodroid/src/Mono.Android/platforms/android-15/src/generated/Android.App.Activity.cs:1859
06-26 16:06:28.102 I/MonoDroid( 6791): at (wrapper dynamic-method) object.08473369-0d89-4344-a57c-40e97793bab8 (intptr,intptr,intptr)

Answer 1

This looks like this is nothing to do with Fragments - but is instead a known error - https://github.com/slodge/MvvmCross/issues/316
The fix for this hasn't yet been released to nuget.
To workaround it, you can either build the fix yourself. Or you can override the behaviour in your splashscreen class by adding:
private bool _isResumed;
protected override void OnResume()
{
_isResumed = true;
base.OnResume();
}
protected override void OnPause()
{
_isResumed = false;
base.OnPause();
}
public override void InitializationComplete()
{
if (!_isResumed)
return;
TriggerFirstNavigate();
}