OnAppearing still called even if I go back from second page

[yunusefendi52]

Description

Suppose I have two pages with NavigationPage, then I override OnAppearing method in the second page:

protected override async void OnAppearing()
{
    base.OnAppearing();
    await Task.Delay(10000);
    Debug.WriteLine("OnAppearing");
}

The weird thing is that, if I navigate from first page to second page and then go back to first page as much as I can before 10000 delay , the Debug.WriteLine("OnAppearing") code still be called, so am I missing something here?

Expected Behavior

The Debug.WriteLine("OnAppearing") code not be called even if go back from second page

Actual Behavior

The Debug.WriteLine("OnAppearing") still be called

Basic Information

• Version with issue: 2.5.0.122203 in this reproduction (but I have a project that use 3.0.0.354232-pre3 which has the same issue)
• IDE: Visual Studio Community 2017 15.5.3
• Platform Target Frameworks:
  • iOS: 11.6.1.2
  • Android: 8.1.3.0
  • UWP: 16299

Gif how to test

Reproduction Link

App4.zip

[ChaseFlorell]

That's because it's async void. The void method his the await and immediately returns. It's not stopping you from navigating back, but the rest of the code is still executing.

[yunusefendi52]

What should I do if the code is

protected override async void OnAppearing()
{
    base.OnAppearing();
    HttpResponseMessage response = await httpClient.GetAsync("https://github.com/xamarin/Xamarin.Forms");
    Debug.WriteLine(response.StatusCode);
}

and the GetAsync is wait more than 5 seconds (due to slow internet connection) and user go back on 2 or 3 seconds? so what should I do?

[PureWeen]

@5yunus2efendi That's kind of a big question

I think reading through the documentation on Tasks would really help you out
https://docs.microsoft.com/en-us/dotnet/standard/parallel-programming/task-cancellation

The short answer is that await is really just a way to help you deal with async code. So what your code above is doing is kicking off a separate background process and then when that process finishes it picks up where it left off.

All of this happens within the contained scope of that method. So if you need those results to just be thrown away you would need to deal with that after the await comes back or just cancel the Task when OnDisappearing fires

[ChaseFlorell]

Create a Cancellation token OnAppearing and cancel it OnDiappearing. Pass it into your async code.

Still smelly and you should look at something like Prism to handle navigation, move your logic to the ViewModel.

[yunusefendi52]

@PureWeen sorry, I didn't mean to ask like that.

I actually already implemented the code using cancellation, I just thought there will be a better implementation from XF, anyway, thanks guys!

[PureWeen]

@5yunus2efendi aw ok.

Here's a discussion about extending Commands in forms to handle tasks
#1857

Which I think addresses what you're asking about a bit better.