Handling Unbalanced calls to begin / end appearance when popping controller - ios

Question

I'm trying to add some error handling (to cope with loses of network connectivity when initializing my view models as well as elsewhere) by having it publish a message that is picked up by my view models that will then do a ChangePresentation with a PresentationHint that causes my presenter (derived from MvxTouchViewPresenter) to do this:
this.MasterNavigationController.PopToRootViewController(false);
This occasionally works, but a lot of the time it doesn't, getting stuck on whatever view it was currently on and I see the message Unbalanced calls to begin/end appearance transitions for <MyView: 0x...>. I believe this is because sometimes the message is getting thrown before the view that was loading has had time to finish loading (the actual loading of data is asynchronous and fires up on another thread - hence the problem).
So my question is, is there a way to synchronize this so that instead of immediately popping to the root, it will finish what it's doing, then pop to the root? Or is there some better way to handle this?

Answer 1

There's not quite enough code in your question to work out what's happening. There are lots of other questions on StackOverflow containing that error message - it may be worth looking through them to see if there is a nice solution to your issue. e.g. UINavigationController popToRootViewController, and then immediately push a new view
If you do want to detect 'finish loading' then on way to do this is to listen to the ViewDidAppear messages in the ViewControllers that are being shown. In mvx, this is easy to do as all ViewController's support a ViewDidAppearCalled event that you could easily hook up in your custom presenter:
private readonly Queue<Action> _pendingActions = new Queue<Action>();
private bool _isBusy;
public override void Show(Cirrious.MvvmCross.Touch.Views.IMvxTouchView view)
{
if (_isBusy)
{
_pendingActions.Enqueue(() => Show(view));
return;
}
_isBusy = true;
var eventSource = view as IMvxEventSourceViewController;
eventSource.ViewDidAppearCalled += OnViewAppeared;
base.Show(view);
}
private void OnViewAppeared(object sender, MvxValueEventArgs<bool> mvxValueEventArgs)
{
_isBusy = false;
var eventSource = sender as IMvxEventSourceViewController;
eventSource.ViewDidAppearCalled -= OnViewAppeared;
if (!_pendingActions.Any())
return;
var action = _pendingActions.Dequeue();
action();
}
public override void ChangePresentation(Cirrious.MvvmCross.ViewModels.MvxPresentationHint hint)
{
if (_isBusy)
{
_pendingActions.Enqueue(() => ChangePresentation(hint));
return;
}
base.ChangePresentation(hint);
}
Note: this code requires 3.0.13 or later to work (there was a bug for ViewDidAppear in some view controllers in earlier versions)
If you are using a simple UINavigationController, another way to achieve a similar effect is to use the Delegate on that controller - see https://developer.apple.com/library/ios/documentation/uikit/reference/UINavigationControllerDelegate_Protocol/Reference/Reference.html