maui/docs/design/HandlerResolution.md

Handler Resolution

Introduction

Handlers are the platform components used to render a cross platform View on the screen. Every platform registers a handler against a .NET Maui type.

Registering a Handler in Code

builder.ConfigureMauiHandlers(handlers =>
       {
          handlers.AddHandler(typeof(Button), typeof(ButtonHandler));
       }

Types used in the resolution of Handlers to Views

MauiFactory

public interface IMauiFactory : IServiceProvider

public class MauiHandlersFactory : MauiFactory, IMauiHandlersFactory

MauiFactory started out as our home grown Dependency Resolver. Prior to going all in on the Ms.Ext.ServiceProvider we were using MauiFactory. The initial intent behind MauiFactory was primarily performance based. We were concerned that the Ms.Ext.ServiceProvider would cause startup performance issues so we created a very simple resolver. We do not have any metrics that prove/disprove the performance of MauiFactory vs MS.Ext.ServiceProvider. At this point the MauiFactory does fulfill some needs that don't seem to quite fit inside the IServiceProvider box.

Features of the MauiFactory

• Handler registration can theoretically happen at any time because it happens via the IMauiHandlerFactory. All you have to do is grab the IMauiHandlersCollection from IMauiHandlerFactory and add an additional Handler. This isn't a common use case but we do make use of this feature here.
• MauiFactory allows you to register two types that have no relationship with each other. Button and ButtonHandler don't implement any of the same interfaces, Button isn't the "Concrete Implementation" of ButtonHandler.
• MauiFactory has support for ctor resolution but we currently have it disabled in all cases.
  • Handlers will currently attempt to instantiate through Extensions.DependencyInjection.ActivatorUtilities.CreateInstance if a default constructor hasn't been created. So the ctor resolution feature of MauiFactory probably doesn't have any currently useful purpose.
• MauiFactory currently doesn't support Scoped Services which is the main reason why we switched to Ms.Ext.DI for our main implementation. .NET MAUI Blazor requires Scoped Services and we've started using Scoped Services as well for multi-window.
• MauiFactory retrieves all base types from the requested type and all implemented interfaces. It first iterates over base types and then if nothing is found it loops through the interfaces. The interface behavior currently leads to some odd behavior because everything implements IView. This means that if a handler isn't registered then MauiFactory just returns a random handler because technically every single handler is registered against a cross platform view that implementsIView. https://github.com/dotnet/maui/issues/1298
  • We should probably remove the interface matching part of MauiFactory

IMauiHandlersFactory

IMauiHandlersFactory is the interface used to request handlers for a View

public class MauiHandlersFactory : MauiFactory, IMauiHandlersFactory

builder.Services.AddSingleton<IMauiHandlersFactory, MauiHandlersFactory>

public interface IMauiHandlersFactory : IMauiFactory
{
    Type? GetHandlerType(Type iview);
    IElementHandler? GetHandler(Type type);
    IElementHandler? GetHandler<T>() where T : IElement;
    IMauiHandlersCollection GetCollection();
}

Access to the HandlerFactory is provided through the IMauiContext

public interface IMauiContext
{
    IServiceProvider Services { get; }

    IMauiHandlersFactory Handlers { get; }
}