Avoid reconstructing handler when it is already available

[cshung]

Description of Change

The root cause of this bugs is this seemly harmless call to AlertRequestHelper.PageIsInThisContext accidentally disposed the GestureManager through this stack.

...
Microsoft.Maui.Controls.Platform.GestureManager.Dispose
Microsoft.Maui.Controls.View.OnHandlerChangingCore
Microsoft.Maui.Controls.Element.SetHandler
Microsoft.Maui.Controls.Element.set_Handler
Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler
Microsoft.Maui.Platform.ElementExtensions.ToHandler
Microsoft.Maui.Platform.ElementExtensions.ToPlatform
Microsoft.Maui.Handlers.ContentViewHandler.UpdateContent
Microsoft.Maui.Handlers.ContentViewHandler.MapContent
Microsoft.Maui.PropertyMapper<Microsoft.Maui.IContentView,Microsoft.Maui.Handlers.IContentViewHandler>.
Microsoft.Maui.PropertyMapper.UpdatePropertyCore
Microsoft.Maui.PropertyMapper.UpdateProperties
Microsoft.Maui.Handlers.ElementHandler.SetVirtualView
Microsoft.Maui.Handlers.ViewHandler.SetVirtualView
Microsoft.Maui.Handlers.ViewHandler<Microsoft.Maui.IContentView,Microsoft.Maui.Platform.ContentViewGroup>.SetVirtualView
Microsoft.Maui.Handlers.ContentViewHandler.SetVirtualView
Microsoft.Maui.Handlers.ViewHandler<Microsoft.Maui.IContentView,Microsoft.Maui.Platform.ContentViewGroup>.SetVirtualView
Microsoft.Maui.Controls.Element.SetHandler
Microsoft.Maui.Controls.Element.set_Handler
Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler
Microsoft.Maui.Platform.ElementExtensions.ToHandler
Microsoft.Maui.Platform.ElementExtensions.ToPlatform
Microsoft.Maui.Controls.Platform.AlertManager.AlertRequestHelper.PageIsInThisContext
...

When the GestureManager is disposed of, the associated OnTouchListener is unregistered. Later on, it will register the OnTouchListener for a label again. But that label is not the same instance as the one that is currently showing, that is why touching the label for the second time won't lead to the event listener.

My observation is that all this heavy lifting just to check if the page is in the current activity is a lot of waste. We could have just reused the existing Handler instance, that is what inspired the fix.

Issues Fixed

Fixes #5425

[mattleibow]

@PureWeen Is this supposed to happen? Should ToHandler be checking to see if the maui context is the same and returning the same instance?

Or is that designed to always create and this was a bad use of it?

[PureWeen]

@mattleibow @cshung

ToPlatform should be checking this and not creating a new handler already if the IMauiContext has changed.

maui/src/Core/src/Platform/ElementExtensions.cs

Line 62 in d1db28c

if (handler?.MauiContext != null && handler.MauiContext != context)

If the IMauiContext has changed then the handler needs to be recreated.

This is the mechanism that makes it work if the Activity gets changed out from underneath you.

[PureWeen]

@cshung I'm pretty sure the fix here is to just change the call to this

bool PageIsInThisContext(IView page)
{
	var platformView = page.ToPlatform();

It's passing the wrong MauiContext into ToPlatform there which causes the Page to recreate the handler. That page already has a handler so ToPlatform will just return it if you don't pass in a MauiContext

I'm not too sure why this broke in p14. The MauiContext has been passed into the code since we created AlertManager. I'm guessing the work we did to rewrite these extensions made it so two wrongs no longer were making a right

[cshung]

@cshung I'm pretty sure the fix here is to just change the call to this

I verified that this alternative fix worked just as well, and is much safer.