Api request: DataTemplateInclude

Discussed in https://github.com/AvaloniaUI/Avalonia/discussions/8426

<div type='discussions-op-text'>

^{Originally posted by yll690 July 1, 2022} In WPF I can put the DataTemplates in ResourceDictionary and merge it in other application. But in Avalonia there are many limits:

• DataTemplates can’t be put in ResourceDictionary without key. It’s also not work when has a key like vm:TestItem.
• DataTemplates property is readonly.
• DataTemplates can not merge each other like ResourceDictionary or Styles.

So is there any way to share DataTemplates across projects?</div>

The simplest way seems implements a DataTemplateInclude like StyleInclude.

[yll690]

I have a new idea. How about adding a DataTemplates property to Styles? Avalonia can also search every Styles.DataTemplates like searching Application.DataTemplates and Controls.DataTemplates. Then we can use StyleInclude instead of DataTemplateInclude and the DataTemplate can apply to models automatically, too.

[trommlbomml]

Is there any huge drawback of having this?

Resources > ResourceInclude Styles > StyleInclude DataTemplates > ?

Yes, because you cannot use the very handy feature defining data templates automatically applied by DataType in separate files. You need to define them all in app.xaml or the corresponding window, which is not very useful. that would also remove the need to define a ViewLocator as from the default samples as you can stick with automatically loaded datatemplates by datatype.