What's the best way of handling ViewModel destroying for CancellationToken activation with MvvmCross? - xamarin

Question

I am having an MvvmCross ViewModel, which calls different async methods of my DataService.
Similar to the following:
public class LoginViewModel : MvxViewModel
{
private readonly IIdentityService _dataService;
private CancellationTokenSource _viewModelCancellationTokenSource;
public IMvxCommand GoLogin { get; set; }
public LoginViewModel(IIdentityService identityService)
{
_dataService = identityService;
_viewModelCancellationTokenSource = new CancellationTokenSource();
GoLogin = new MvxCommand(async () => await ProcessLogin());
}
private async Task ProcessLogin()
{
// calling the dataservice which must stop processing
// (to cancel) in case if the ViewModel is being destroyed
await _dataService.AssureIsLoggedIn(data, _viewModelCancellationTokenSource.Token);
await NavigationService.Navigate<LoginNextStepViewModel>();
}
public override void ViewDestroy(bool viewFinishing = true)
{
base.ViewDestroy(viewFinishing);
// not sure if that is a right (and working) place
_viewModelCancellationTokenSource.Cancel();
}
}
So, MvvmCross is quite unclear about the part with the ViewModel destroying. It describes Construction, Init, Reload and Start, but doesn't say any definite regarding the destroying:
Monitoring other View/ViewModel lifecycle event across multiple
platforms is fairly tricky, especially once developers start
experimenting beyond the ‘basic’ presentation models and start using
tabs, splitviews, popups, flyouts, etc
For most viewmodels, it’s common to not try to monitor other lifecyle
events. This is OK since most viewmodels don’t perform any actions and
don’t consume any resources when the view is not present - so these
can just be left to be garbage collected when the system needs the
memory back.
However, besides the custom platform situations, there are still many cases like navigating back from the view model, or (again) navigation away from current viewmodel with its following closing.
So, what's the best way to handle it then?

Answer 1

From your code:
// calling the dataservice which must stop processing
// (to cancel) in case if the ViewModel is being destroyed
The ViewModel won't be destroyed before the async methods finish executing. I think you are confusing the View with the ViewModel.
In case of a login page, you would usually prevent the user from navigating away from it until your server call goes through.
If for some reason you want to cancel then you need to decide what scenarios you want to handle, there is no single universal place. Your options are the view callbacks:
void ViewDisappearing();
void ViewDisappeared();
void ViewDestroy();
and the navigation events:
event BeforeNavigateEventHandler BeforeNavigate;
event AfterNavigateEventHandler AfterNavigate;
event BeforeCloseEventHandler BeforeClose;
event AfterCloseEventHandler AfterClose;
event BeforeChangePresentationEventHandler BeforeChangePresentation;
event AfterChangePresentationEventHandler AfterChangePresentation;