MvvmCross Realm access from incorrect thread - xamarin.android

Question

Exception
Realm access from incorrect thread in MainViewModel
Application Flow
SplashScreen> MainActivity(Exception)
[Activity(MainLauncher = true
, Icon = "#mipmap/ic_launcher"
, Theme = "#style/Theme.Splash"
, NoHistory = true
, ScreenOrientation = ScreenOrientation.Portrait)]
public class SplashScreen : MvxSplashScreenActivity
{
public SplashScreen()
: base(Resource.Layout.SplashScreen)
{
}
}
MainActivity
public class MainActivity : MvxAppCompatActivity<MainViewModel>,ViewPager.IOnPageChangeListener,View.IOnTouchListener
{
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
SetContentView(Resource.Layout.activity_tutorial);
if (ViewModel.IsCompletedOrNot)
ViewModel.OpenMainViewModel.Execute();
}
MainViewModel
[AddINotifyPropertyChangedInterface]
public class MainViewModel : MvxViewModel
{
private Realm _realm;
private bool isCompleted = false;
public TutorialViewModel(IMvxNavigationService navigationService)
{
_realm = Mvx.Resolve<Realm>();
}
public bool IsCompletedOrNot{
get
{
if (_realm.All<IsFirstTimeAppStartUpRealm>().Count() > 0)
{
isCompleted=true;
}else{
isCompleted = false;
}
return isCompleted;
}
}
}
App.CS
var key = ConfigManager.Settings?.DatabaseEcryption?.EncryptionKey;
if (key != null && key.Length > 0)
{
config.EncryptionKey = key;
}
Mvx.RegisterSingleton<Realm>(() => Realm.GetInstance(config));
Realm _realm = Mvx.Resolve<Realm>();
int count = _realm.All<IsFirstTimeAppStartUpRealm>().Count();
//RegisterCustomAppStart<CustomAppStart>();
// App start
if (count>0)
{
RegisterNavigationServiceAppStart<MainViewModel>();
}
else
{
RegisterNavigationServiceAppStart<OtherViewModel>();
}
The below line throws the exception.
_realm.All<IsFirstTimeAppStartUpRealm>().Count() > 0
App always crashes when it comes through SplashScreen and it works fine if started from MainActivity.

Answer 1

MvvmCross does not guarantee that App start is run on the UI thread. I will most likely run on a ThreadPool Thread.
In order to marshal a piece of code to the main thread, you can resolve the IMvxMainThreadAsyncDispatcher (>= 6.1.x) or IMvxMainThreadDispatcher and request an Action to run on the main thread:
var dispatcher = Mvx.Resolve<IMvxMainThreadAsyncDispatcher>();
int count;
await dispatcher.ExecuteOnMainThreadAsync(() =>
{
count = _realm.All<IsFirstTimeAppStartUpRealm>().Count();
});

Answer 2

I have a reasonably nice way of removing these code smells from my applications. I've just recently started using Realm (liking it so far), but I have always used ReactiveProperty to notify my view layer of VM changes - and it's really nice.
https://github.com/runceel/ReactiveProperty
ReactiveProperty is a Rx framework for .NET, which wraps your properties in an instance that produces your INotifyPropertyChanged events as needed. You chain all these properties together as they depend on each other, and events propagate throughout them. You no longer have these long lists of "notify of this, notify of that" after a property change.
Instead you declare how all your members inter-depend, in a single section of your code (typically your constructor)
As a result, you can place the root of the chain on the Realm thread, and all dependent notifications are published on that thread.
So, my ViewModels look something like this (pseudocode):
class VM
{
public ReactiveProperty<AppStartInfo> Entity { get; set; }
public ReactiveProperty<bool> IsFirstLaunch { get; set; }
public VM(){
var syncCtx = SynchronizationContext.Current;
Entity = new ReactiveProperty<AppStartInfo>();
// this property will fire its notifications on the syncCtx.
// remember to bind your view to "IsFirstLaunch.Value"
IsFirstLaunch = Entity.SubscribeOn(syncCtx).Select(x => x.IsFirstLaunch).ToReactiveProperty()
}
public async Task Init()
{
// let's get our realm instance on to the syncCtx.
syncCtx.Post(() => {
Entity.Value = Realm.Find(typeof(AppStartInfo), 0); // or whatever you need.
});
}
}