使用Microsoft装饰MediatR IRequestHandler

Question

我想装饰一下我的MediatR的IRequestHandlers。为了标记我想要修饰的处理程序，我创建了接口ICommandHandler，它继承自IRequestHandler，ICommand继承自IRequest。

public interface ICommand : IRequest { }
public interface ICommandHandler<in TCommand> : IRequestHandler<TCommand>
    where TCommand : ICommand { }

现在，我希望每个ICommandHandler都用UnitOfWorkCommandHandlerDecorator来装饰。

internal class UnitOfWorkCommandHandlerDecorator<TCommand> : ICommandHandler<TCommand>
    where TCommand : ICommand
{
    private readonly ICommandHandler<TCommand> _decorated;
    private readonly IUnitOfWork _unitOfWork;

    public UnitOfWorkCommandHandlerDecorator(ICommandHandler<TCommand> decorated, IUnitOfWork unitOfWork)
    {
        _decorated = decorated;
        _unitOfWork = unitOfWork;
    }

    public async Task<Unit> Handle(TCommand request, CancellationToken cancellationToken)
    {
        var res = await _decorated.Handle(request, cancellationToken);
        await _unitOfWork.CommitAsync();
        return res;
    }
}

我尝试用Scrutor的装饰方法注册一个装饰器，但是在用IMediator.Send发送请求时不会调用装饰器。这可能是因为它是ICommandHandler装饰师，而不是IRequestHandler装饰师。

services.Decorate(typeof(ICommandHandler<>), typeof(UnitOfWorkCommandHandlerDecorator<>));

是否可以使用Microsoft，还是需要另一个DI容器？

Answer 1

据我所读，使用Microsofts不可能在开放泛型上实现一个开放泛型的装饰。但是你可以用一个开放的通用来装饰一个封闭的泛型。这就产生了这样的想法:扫描程序集以查找泛型，找到关闭的实现，然后装饰它。

services.Scan(scan =>
  scan.FromAssembliesOf(typeof(ICommandHandler<>))
    .AddClasses(classes => 
    classes.AssignableTo(typeof(ICommandHandler<>)).Where(_ => !_.IsGenericType))
        .AsImplementedInterfaces()
        .WithTransientLifetime());

我不能为来自GitHub票证：https://github.com/khellang/Scrutor/issues/68的代码取得任何荣誉。

Answer 2

正如我所预期的，当ICommandHandler从Microsoft请求IRequestHandler时，问题是如何装饰IRequestHandler。

我找到了两种方法来实现这个目标：

当ICommandHandler.请求

1. 时，让Microsoft DI返回一个修饰的IRequestHandler。

// Register all concrete classes implementing ICommandHandler as ICommandHandlers.
services.Scan(x => x.FromAssembliesOf(typeof(ICommand))
    .AddClasses(classes => classes.AssignableTo(typeof(ICommandHandler<>))
        .Where(t => !t.IsGenericType))
    .AsImplementedInterfaces(t => t.IsGenericType
        && t.GetGenericTypeDefinition() == typeof(ICommandHandler<>))
    .WithTransientLifetime());

// Decorate registered ICommandHandlers with open generic decorator.
services.Decorate(typeof(ICommandHandler<>), typeof(UnitOfWorkCommandHandlerDecorator<>));

// Get registered ICommandHandlers service descriptors from the container.
var registeredCommandHandlers = services
    .Where(x => x.ServiceType.IsGenericType
        && x.ServiceType.GetGenericTypeDefinition() == typeof(ICommandHandler<>))
    .ToArray();
foreach(var serviceDescriptor in registeredCommandHandlers)
{
    // Find the IRequestHandler type of registered ICommandHandler.
    var requestHandlerType = serviceDescriptor.ServiceType.GetInterfaces()
        .First(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(IRequestHandler<,>));
    // Register a decorated ICommandHandler to be returned for IRequestHandler.
    services.Add(new ServiceDescriptor(requestHandlerType,
        sp => sp.GetRequiredService(serviceDescriptor.ServiceType), serviceDescriptor.Lifetime));
}

// Invoke AddMediatR method to register IRequestHandlers that are not ICommandHandlers
// and others required by MediatR classes.
services.AddMediatR(typeof(TestCommandHandler));

IRequestHandler.

• 将MediatR更改为请求ICommandHandler而不是ICommandHandler

// Register all concrete classes implementing ICommandHandler as ICommandHandlers.
services.Scan(x => x.FromAssembliesOf(typeof(ICommand))
    .AddClasses(classes => classes.AssignableTo(typeof(ICommandHandler<,>))
        .Where(t => !t.IsGenericType))
    .AsImplementedInterfaces(t => t.IsGenericType
        && t.GetGenericTypeDefinition() == typeof(ICommandHandler<,>))
    .WithTransientLifetime());

// Decorate registered ICommandHandlers with open generic decorator.
services.Decorate(typeof(ICommandHandler<,>), typeof(UnitOfWorkCommandHandlerDecorator<,>));

// Invoke AddMediatR method to register IRequestHandlers that are not ICommandHandlers
// and others required by MediatR classes.
services.AddMediatR(typeof(TestCommandHandler));

// Register ServiceFactory delegate, which MediatR uses to resolve all services.
services.AddTransient<ServiceFactory>(sp =>
{
    return t =>
    {
        if (t.IsGenericType
            && t.GetGenericTypeDefinition() == typeof(IRequestHandler<,>))
        {
            var genericArguments = t.GetGenericArguments();
            var commandHandlerType = typeof(ICommandHandler<,>).MakeGenericType(genericArguments);
            // Return ICommandHandler or IRequestHandler if it is not registered.
            return sp.GetService(commandHandlerType)
                ?? sp.GetRequiredService(t);
        }
        return sp.GetRequiredService(t);
    };
});

就我个人而言，我更喜欢第一种方法；它更容易理解，而且在运行时没有那么多的开销。