Add Support for .NET Hot Reload

[TheCodeTraveler]

Description

This PR implements MetadataUpdateHandler which is required for libraries wishing to support C# Hot Reload.

While C# Hot Reload does update the running/active C# code, it will not automatically refresh the visible .NET MAUI UI. To refresh the displayed UI, .NET MAUI devs are required to write additional code specific to their app that invalidates the current view.

For this initial implementation, developers will need to do two things:

1. Implement ICommunityToolkitHotReloadHandler

• In ICommunityToolkitHotReloadHandler.OnHotReload(), specify how to update the visible page
• Shell example below

  class HotReloadHandler : ICommunityToolkitHotReloadHandler
  {
      public async void OnHotReload(IReadOnlyList<Type> types)
      {
          foreach (var type in types)
          {
            if (type.IsSubclassOf(typeof(Page)))
            {
              if (Application.Current?.MainPage is AppShell shell)
  	    {
                    await Application.Current.MainPage.Dispatcher.DispatchAsync(async () =>
                    {
                      await Shell.Current.GoToAsync(type.Name, false);
                      Shell.Current.Navigation.RemovePage(shell.CurrentPage); 
                    });
              }
  
              break;
            }
          }
      }
  }

2. In MauiProgram.CreateMauiApp(), add your implementation of ICommunityToolkitHotReloadHandler:

   builder.Services.AddSingleton<ICommunityToolkitHotReloadHandler, HotReloadHandler>();

Linked Issues

• Fixes Hot Reload Support #71

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: [CommunityToolkit.Maui.Markup] .NET Hot Reload Docs MicrosoftDocs/CommunityToolkit#322

Additional information

While C# Hot Reload does update the running/active C# code, it will not refresh the .NET MAUI UI. To refresh the displayed UI, .NET MAUI devs are required to write additional code specific to their app that invalidates the

For this initial implementation of C# Hot Reload support, I recommend that we do not provide a default implementation of ICommunityToolkitHotReloadHandler because it is near impossible to account for every application architecture.

I have included in this PR (and I recommend including in the docs) a sample implementation of ICommunityToolkitHotReloadHandler.

In future releases, once we are comfortable with a stable implementation of ICommunityToolkitHotReloadHandler, we can wire it up automatically for the users in UseMauiCommunityToolkitMarkup().

[pictos]

I liked the interface idea and let devs opt-in to use the feature. I didn't have my PC with me to test everything, so the comment about the debug flag is a guess

[VincentH-Net]

I like that the ICommunityToolkitHotReloadHandler interface allows devs to inspect the updated types via the IReadOnlyList<Type> types parameter. This allows devs to control the rebuild (+rebuild order) of instances that a page depends upon but that are not created by the page, e.g. a style that is created from C#.

To illustrate, here is a reload handler for WPF in C# Markup 2 - the same logic applies.

I also applaud that no standard handler implementation is shipped; I expect that for non-trivial Apps this logic will need to be customized by the app developer.

Imo it would be fine to keep the handler at a simple implementation in docs, as the PR description proposes, or perhaps as part of a (dotnet new?) template

[VincentH-Net]

await Shell.Current.GoToAsync(type.Name, false);

In the PR description, the Shell example, where is the type variable defined that is referenced in above line?

[TheCodeTraveler]

Thanks Vincent!

where is the type variable defined that is referenced in above line

Whoops - that was just a lazy copy/paste edit from the source code into the PR description. Fixed ✅