Added InvokeOnMainThreadAsync to consolidate GoToAsync calls

[MaxFelinger]

Description of Change

Two of three GoToAsync calls were already wrapped by InvokeOnMainThreadAsync. We had a navigation issue because the third GoToAsync call in ShellNavigationManager is not surrounded by InvokeOnMainThreadAsync.
I do realise that this is a temporary fix and there should be a better fix. Nevertheless this consolidation is better than keeping it broken.

Issues Resolved

Fixes a navigation call that won't be executed otherwise.
We had this issue in the OnResume method. A specific scenario could be that the user has moved the app into the background while being at an important process step. The app could be terminated while it's in the background. To let the user proceed from where he has left off, we added a "ReentryService" that allows the user to jump to an important point where he left off. The ReentryService basically loads some stuff from SecureStorage (asynchronously) and invoked GoToAsync at its end.

I think we also had this issue when we used PushNotification, but I cannot remember the full scenario.

Platforms Affected

• Core/XAML (all platforms)

Testing Procedure

I'm not sure how to test this case properly. I hope one of you reviewers knows how to test it.

[jfversluis]

@PureWeen any thoughts about this one?

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[PureWeen]

@MaxFelinger do you have a stack trace of your crash?

[MaxFelinger]

@PureWeen My bad it's not an exception that gets thrown, the navigation just won't be executed. I've adjusted the description of this PR.

To make things simple for reproduction purposes, someone could use this sample:

// Inside OnResume()
Task.Run(async () =>
{
    // Navigation won't be executed
    await Shell.Current.GoToAsync("AnyRelativeRoute");
    
    /* 
    Device.InvokeOnMainThreadAsync(() =>
    {
        // Navigation will be executed
        await Shell.Current.GoToAsync("AnyRelativeRoute");
    });
    */
});

[MaxFelinger]

@jfversluis Thanks for the additional information.
Regarding the comment. It's actually not from me. I've copied the whole invocation part (including the comment) from above (lines 157 - 161).
// TODO get rid of this hack and fix so if there's a stack the current page doesn't display await Device.InvokeOnMainThreadAsync(() => { return _shell.CurrentItem.CurrentItem.GoToAsync(navigationRequest, queryData, animate, isRelativePopping); });
I was hoping @PureWeen would know how to UnitTest this scenario.

Reproducing this issue: Invoke an async method from OnResume and inside this async method invoke GoToAsync.

[PureWeen]

@PureWeen My bad it's not an exception that gets thrown, the navigation just won't be executed. I've adjusted the description of this PR.

To make things simple for reproduction purposes, someone could use this sample:

// Inside OnResume()
Task.Run(async () =>
{
    // Navigation won't be executed
    await Shell.Current.GoToAsync("AnyRelativeRoute");
    
    /* 
    Device.InvokeOnMainThreadAsync(() =>
    {
        // Navigation will be executed
        await Shell.Current.GoToAsync("AnyRelativeRoute");
    });
    */
});

If you change that initial Task.Run to a Device.InvokeOnMainThread does it work? I think there's a few possible reasons why GotoAsync might not work when executed from a task pool thread vs the ui thread

[MaxFelinger]

@PureWeen yes, when I exchange Task.Run by Device.InvokeOnMainThreadAsync the navigation does work.

PS: The posted code is just a sample. The real code that we use in our project does async stuff (load from SecureStorage and a few more things) first and navigates afterwards. Sure we can surround the navigation call by Device.InvokeOnMainThreadAsync, but when I look into the Shell implementation and see that three GoToAsync invocations (very close to each other) and only one of them is not surrounded by Device.InvokeOnMainThreadAsync, I get curious and ask myself "Why aren't all three statements surrounded by Device.InvokeOnMainThreadAsync?".

[jfversluis]

Now that we're so close to the sunsetting of Xamarin.Forms unfortunately we won't be able to take this in anymore, we're really sorry about that. Nevertheless, thank you so much for your time and effort that you have put into this PR.

Please have a look at the evolution of Xamarin.Forms, .NET MAUI. A lot of development has been going on there. Hopefully this issue was already fixed in that codebase. If not, feel free to port this over to there.

Again, thank you so much for being a contributor and Xamarin.Forms user!