[listview] fixes for various null/empty DataTemplate

[jonathanpeppers]

Description of Change

Fixes #11203

The following breaks XAML Hot Reload:

1. Setup a working ListView and DataTemplate

2. Delete the DataTemplate's body (as if I'm about to type a completely new view there).

3. Trigger a XAML Hot Reload. This can happen automatically if you just pause typing.

4. MAUI crashes at runtime:

in Android.Views.ViewGroup.Layout at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net7.0/android-33/mcw/Android.Views.ViewGroup.cs:3369,5
in Microsoft.Maui.Controls.Handlers.Compatibility.VisualElementRenderer<Microsoft.Maui.Controls.ListView>.OnLayout at D:\a\_work\1\s\src\Controls\src\Core\Compatibility\Handlers\Android\VisualElementRenderer.cs:54,6
in Microsoft.Maui.Controls.Handlers.Compatibility.ListViewRenderer.OnLayout at D:\a\_work\1\s\src\Controls\src\Core\Compatibility\Handlers\ListView\Android\ListViewRenderer.cs:298,4

You can also create this problem in C#, like I did in a unit test:

listView.ItemTemplate = new DataTemplate(() => /* valid template */);
listView.ItemTemplate = new DataTemplate(); // broken
listView.ItemTemplate = new DataTemplate(() => null); //broken

I could also get a slightly different crash:

System.InvalidCastException: Specified cast is not valid.
at Microsoft.Maui.Controls.Internals.TemplatedItemsList`2[[Microsoft.Maui.Controls.ItemsView`1[[Microsoft.Maui.Controls.Cell, Microsoft.Maui.Controls, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]], Microsoft.Maui.Controls, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Controls.Cell, Microsoft.Maui.Controls, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].ActivateContent(Int32 index, Object item)

The "invalid cast" appears to be due to the changes in c57858f. It returns a new Label() in some cases, which does not work with a ListView. You need a Cell instead of View in that case.

The fixes appear to be in two places:

• VisualElementRenderer.OnLayout: use is, also simplifies the code

• TemplatedItemsList.ActivateContent : use is and call the default data template.

Now my tests pass, and I can't reproduce the issue in XAML Hot Reload either!

Manually testing, I'm able to alternative commenting out the two XAML elements:

This didn't work before.

Issues Fixed

Fixes: #11203

[mandel-macaque]

Looks good, maybe invert the if (I'm not a UI developer)

[rmarinho]

Failing test on iOS

NavigatingBackViaBackButtonFiresNavigatedEvent

[jonathanpeppers]

@rmarinho what test is this?

I can't even find it in the repo:

> git grep NavigatingBackViaBackButtonFiresNavigatedEvent
# nothing

I don't see how the latest commit broke it c1133aa, as it should produce identical C# code.

It was green before, so is this broken by a change in main?

[PureWeen]

@rmarinho what test is this?

I can't even find it in the repo:

> git grep NavigatingBackViaBackButtonFiresNavigatedEvent
# nothing

I don't see how the latest commit broke it c1133aa, as it should produce identical C# code.

It was green before, so is this broken by a change in main?

@jonathanpeppers CI merges main before running tests. It looks like main broke with one of the merges from yesterday. Taking a look now

[jonathanpeppers]

@rmarinho so maybe we can merge this one? It seems like it should be an improvement, adding is checks.

[mandel-macaque]

@rmarinho @PureWeen there is a way (I made it work) to ensure that vsts use the commit of the PR and not the merge with main. It has its pros an cons. But you can do that and then make the merge block if it is not updated with main