Allow setting the backing classes for UI elements

[sdaves]

This small change allows setting ViewBuilders properties and enables me to replace the implementation of the underlying Xamarin.Forms classes used by Fabulous.

For example, if I need to override a virtual method on MasterDetailPage, this change enables running:

type MasterDetailPageWithoutToolbar() =
    inherit Xamarin.Forms.MasterDetailPage()
    override __.ShouldShowToolbarButton() = false

Fabulous.DynamicViews.ViewBuilders.CreateFuncMasterDetailPage <- fun () ->
    upcast(new MasterDetailPageWithoutToolbar())

This will cause the MasterDetailPageWithoutToolbar class to be loaded when using the standard:

View.MasterDetailPage()

[TimLariviere]

If I understand correctly, you might want to extend this Create%s method instead.
It's the one with the upcast.
Nevermind, just realized it's a concrete method and can't be overwritten. :)

[sdaves]

Yeah, I went with the public setter. I first tried adding a static extension method of the same name in the same namespace, but in my project and not in Fabulous, hoping that would override the default static methods in ViewBuilders(), but that didn't work so I went with the property setters.

[TimLariviere]

Ok, I think it could be a good addition.
It's a bit of an advance case, but this will complete our extension points (like ViewProto for default ViewElements).
And it's the same way that Fabulous uses its internal controls instead of the default implementation.

Do you think you can add some documentation on it?
I think we should stress the fact that it's not the recommended way for custom controls, only good for overridden controls like your example.

[sdaves]

Sure thing will add some docs. Thanks for the feedback!

[TimLariviere]

Retriggering CI

[sdaves]

Looks like that only retriggered AppVeyor. The Travis build link still says its from 8 hours ago.

[SergejDK]

just needs some time.

[sdaves]

Awesome it worked :)

[TimLariviere]

Could you run .\build.cmd (or .\build.sh on macOS) to generate the new Xamarin.Forms.Core.fs based on your modification?

[sdaves]

Built and uploaded!
Do you feel this could fit nicely in your release plans for 0.33.0?

[TimLariviere]

Thanks!
Yes, it will be released in 0.33.0.

[sdaves]

Awesome, looking forward to it 👍

[dsyme]

MIght be good to add some docs for ViewProto too? I couldn't spot any on a quick glance

[sdaves]

@dsyme Were you thinking we should document each member in type ViewProtos() or just a doc for the type?

I noticed some items are assigned in type ViewProtos(), but quite a few are left empty, such as ProtoContentPage and ProtoMasterDetailPage.

ViewProtos members seem to only be called from internal Update methods in the generated Xamarin.Forms.Core.fs file, so I'm not sure if they are intended to be used from outside of that file.

[TimLariviere]

Ah maybe it's not what I thought it was. If that's the case, it needs to be marked as internal.

Nevertheless, I think it would be doable to propose things like this

let view model dispatch =
    View.ContentPage(
        content = View.StackLayout(
            children = [
                View.Label(text="Hello world")
            ]
        )
    )

type App() as app =
   inherit Application()

   ViewBuilders.DefaultLabel <- View.Label(fontFamily="Lobster-Regular")

    let runner =
        Program.mkProgram init update view 
        |> Program.runWithDynamicView app