Renderer to Handler and Handler to Renderer Shims

[PureWeen]

Description of Change

• Wire up our Registrar so that everything registered via assembly scanning can be auto auto shimmed or use a handler
• Create HandlerToRendererShim for cases where people are still in the Context of a Forms app and will call Platform.CreateRenderer
• Create RendererToHandlerShim for cases where the user has registered a renderer that only implements IVisualElementRenderer

API Changes

public class RendererToHandlerShim : Xamarin.Platform.Handlers.AbstractViewHandler<Xamarin.Platform.IView, Android.Views.View>

public class HandlerToRendererShim : IVisualElementRenderer

FormsHandler.Init()

This is an xplat way to register the handlers as we add handlers you can register the new handler here and it'll be used for CG.

https://github.com/xamarin/Xamarin.Forms/pull/12546/files#diff-5a7d3f99f1c9eb4b20a63099a3a317f3a39ba74e719355aa92a8803836209fe0R7

Platforms Affected

• iOS
• Android

Examples

Legacy Apps

Given the following code

			SetMainPage(new ContentPage()
			{
				Content = new StackLayout()
				{
					Children = {
						new Button()
					}
				}
			});

Button can be registered to either a Handler or a Renderer and the registrar will react accordingly. The ContentPage calls CreateRenderer on its content which will return an IVisualElementRenderer. If the Content is of type IViewHandler then it gets wrapped with a HandlerToRendererShim so that ContentPage can still render the control successfully

Future Apps

If you just register a Shim against the handler then the Shim will reach out to the old registrar for the type

			Xamarin.Forms.Internals.Registrar.Registered.Register(typeof(Xamarin.Forms.Button), typeof(Xamarin.Forms.Platform.Android.FastRenderers.ButtonRenderer));
			RegistrarHandlers.Handlers.Register<Xamarin.Forms.Button>((type) =>
			{
				RendererToHandlerShim rendererToHandlerShim = new RendererToHandlerShim();
				return rendererToHandlerShim;
			});

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[PureWeen]

@hartez is it alright that I moved these to VisualElement?

[PureWeen]

/azp run

[azure-pipelines]

No pipelines are associated with this pull request.

[jsuarezruiz]

@PureWeen Could we add some tests to validate the use of RendererToHandlerShim and RendererToHandlerShim?

[rmarinho]

@PureWeen can we rebase and add some tests to make sure this works fine?

[PureWeen]

@rmarinho @jsuarezruiz

So in theory our Control Gallery tests are the UI Tests for the shims.
You Shim a Handler into the Control.Gallery and then all the Control Gallery tests pass.

Also, we can't really do any tests for RendererToHandlerShim until we have the Handler.Testing project setup or the Appium project

At that point we can write some tests to verify that the renderers shim correctly into handlers.