Proposal: Add public APIs for creating FrameworkTemplate and derived types

[jeromelaban]

Proposal: Add public APIs for FrameworkTemplate and derived types

Some scenarios for libraries are complexified when trying to create FrameworkTemplate and derived types, having to use XamlReader to create them.

Summary

Add the ability to create DataTemplate, ControlTemplate, and ItemsPanelTemplate types from code:

var dataTemplate = new DataTemplate(() => new TextBlock() { Text = "My text" });
var itemsPanelTemplate = new ItemsPanelTemplate(() => new StackPanel());
var controlTemplate = new ControlTemplate(typeof(ContentControl), () => new ContentPresenter());

Rationale

This will simplify the creation of Framework Templates and potentially improve performance by remove the parsing of literal XAML.

[ranjeshj]

@jeromelaban Love the idea. FYI. Somewhat related to this - we wanted to simplify the story for virtualization in ItemsRepeater and introduced IElementFactory sometime back so that you are not required to provide a DataTemplate.

[chrisglein]

Totally agree. Some types are magically restricted to markup and should be code accessible to unblock and also enable non-markup programming.

[francoistanguay]

@jeromelaban Love the idea. FYI. Somewhat related to this - we wanted to simplify the story for virtualization in ItemsRepeater and introduced IElementFactory sometime back so that you are not required to provide a DataTemplate.

Would IElementFactory be used by ContentControl as well?

Otherwise pretty much back to square one,

Not being able to instantiate templates from C# is the main and almost only blocker to a C# first syntax.

[hawkerm]

I was creating a new control wrapping ItemsControl and I had to give it a Style in XAML purely because I can't define it's ItemsPanel in code-behind with an ItemsPanelTemplate.

[michael-hawker]

FYI @MikeHillberg this is the issue around DataTemplate's being XAML only we were just discussing on the WinUI Community Call.

[francoistanguay]

As food for thoughts, even though IELementFactory would work for ItemsControl-based controls, we need to consider other controls like ContentControl.

[MikeHillberg]

Thanks for tagging this from the call @michael-hawker.

IElementFactory is actually supported by ItemsRepeater today, but not ItemsControl or ContentControl.

DataTemplate implements the interface, so what we need to do is have it re-expose the functionality. I don't think it's possible today to derive from DataTemplate and override the IElementFactory implementation, but we could either enable that or add an event/virtual to DataTemplate.

[VincentH-Net]

@MikeHillberg (@jeromelaban) The UNO implementation realizes this by adding these members to DataTemplate :

public DataTemplate(Func<View?>? factory);
public DataTemplate(object? owner, FrameworkTemplateBuilder? factory);
public static implicit operator DataTemplate?(Func<View?>? obj);

I find them pleasant to use. Would this be an option?

[saint4eva]

FYI @MikeHillberg this is the issue around DataTemplate's being XAML only we were just discussing on the WinUI Community Call.

This blocker should be resolved as soon possible.