fix: NavigateBack wrong values on the NavigationStack

[RLittlesII]

What kind of change does this PR introduce?

@garyng provided a solution for fixing the RoutedViewHost Xamarin.Forms implementation.

What is the current behavior?

fixes: #2317

What is the new behavior?

The stack values do not get distorted after the navigation pattern provided.

What might this PR break?

Xamarin.Forms navigation

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
I tested this in a sample and saw that the NavigationStack seems to be correct. I am going to look into creating some unit tests for this behavior so we can make sure it continues to function as expected.

[codecov]

Codecov Report

Merging #2331 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2331   +/-   ##
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  Coverage   54.73%   54.73%           
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  Files         113      113           
  Lines        4334     4334           
  Branches      660      660           
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  Hits         2372     2372           
  Misses       1797     1797           
  Partials      165      165

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[garyng]

@RLittlesII Is your original fix of adding .ObserveOn(Router.Scheduler) still required? After thinking about it, the navigation changed notification might fire on any thread, and Pop*Async needs to be on the UI thread right?

I am not sure what is the default value for Router.Scheduler though...

[RLittlesII]

@garyng I am not sure if its required. I honestly thought my original fix resolved it. Default for Router.Scheduler is RxApp.MainThreadScheduler but any consumer can override that I believe.

That was my thought that the notifications were not returning to the UI thread thus causing the issue.

[garyng]

Ohh okay! I asked because I bumped into the issue where all the page-poppings are executed on other non-ui thread - it weirdly swallowed all the exceptions.

[RLittlesII]

Well. The code changed in the PR works with the previous scenario we were testing. So you're thinking the Popped notifications should happen on the MainThreadScheduler?

[garyng]

I mean this chunk of code:

ReactiveUI/src/ReactiveUI.XamForms/RoutedViewHost.cs

Lines 85 to 97 in 877ddce

	try
	{
	if (popToRoot)
	{
	await PopToRootAsync(true);
	}
	else if (!popToRootPending)
	{
	for (var i = 0; i < x.Delta; ++i)
	{
	await PopAsync(i == x.Delta - 1);
	}
	}

Should be executed on the main ui thread right? Since all the Pop*Asyncs are all ui operations.

[RLittlesII]

Yes. Those are the Xamarin.Forms navigation methods.