一起来学习.net core程序使用中介者模式:MediatR插件

       中介者模式是一种常见的设计模式,旨再降低程序的耦合性,因为传统的三层模式层层之间需要显示的调用,必须上层依赖下层,耦合性很高,为了解耦,将所有的指令单独放在一个位置处理,其他位置均通过这个位置来间接的调用,从而减少耦合,具体的可以参考中介者模式,建议先了解下DDD里面的事件总线和命令分发。

        实现中介者模式有很多方式,例如MediatR就是一种很好用的插件,作者的介绍是这样说的“.NET中的简单中介实现,没有依赖关系的进程内消息传递。通过C#泛型方差支持请求/响应,命令,查询,通知和事件,同步和异步与智能调度。”。这里推荐一款插件Dnspy可以查看dll的源码,这样就可以知道我们所使用的的插件的原理了。

         这里先建好一个.Net Core程序,。然后程序入口处注入MeditaR

  services.AddMediatR(typeof(Startup));

//注册发布订阅中介处理

   services.AddScoped<IMediatorHandler,MediatorHandler>();

   services.AddScoped<IRequestHandler<CreateUserCommand, Unit>, UserCommandHandler>();

      对于中介,肯定要先做一个中介处理器,因为程序是面向接口的,所以先定义一个接口,用来表示中介处理器

    /// <summary>
    /// 中介处理接口
    /// </summary>
   public interface IMediatorHandler
    {
        /// <summary>
        /// 发送领域事件
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="command"></param>
        /// <returns></returns>
        Task SendCommand<T>(T command) where T :Command;
    }

     然后就是它的实现类

    public class MediatorHandler: IMediatorHandler
    {
        private readonly IMediator _mediator;
        public InMemoryBus(IMediator mediator)
        {
            _mediator = mediator;
        }
        /// <summary>
        /// 发送邻域事件
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="command"></param>
        /// <returns></returns>
        public Task SendCommand<T>(T command) where T : Command
        {
            try
            {
              return _mediator.Send(command);
            }
            catch (AggregateException ex)
            {
                throw ex;
            }
        }
      
    }

这里我们用到了IMediator,我们可以根据Dnspy这个工具查看到IMediator 的Send的方法的实现,

// MediatR.Mediator
// Token: 0x06000018 RID: 24 RVA: 0x00002100 File Offset: 0x00000300
public Task<TResponse> Send<TResponse>(IRequest<TResponse> request, CancellationToken cancellationToken = default(CancellationToken))
{
    if (request == null)
    {
        throw new ArgumentNullException("request");
    }
    Type requestType = request.GetType();
    return ((RequestHandlerWrapper<TResponse>)Mediator._requestHandlers.GetOrAdd(requestType, (Type t) => Activator.CreateInstance(typeof(RequestHandlerWrapperImpl<, >).MakeGenericType(new Type[]
    {
        requestType,
        typeof(TResponse)
    })))).Handle(request, cancellationToken, this._serviceFactory);
}

    首先我们看到它的参数IRequest<TResponse> request,所以我们可以知道Task SendCommand<T>(T command),这个方法的T一定是继层自IRequest,所以需要我们写一个基类继层于IRequest

 
 /// <summary>
    /// 领域命令基类
    /// </summary>
   public class Command:IRequest
    {
    }
 /// <summary>
    /// 创建用户领域命令
    /// </summary>
    public class CreateUserCommand: Command
    {
        public CreateUserCommand(User user)
        {
            User = user;
        }
        public User User { get; private set; }
    }

    因为项目需要添加功能,好比注册一个用户,所以有一个添加用户的Service代码,然后在构造函数里注入IMediatorHandler

 public class UserService :IUserService
    {
        private readonly IUserRepository _userRepository;
        private readonly IMediatorHandler _mediatorHandler;
        public UserService(IUserRepository userRepository, IMediatorHandler mediatorHandler)
        {
            _userRepository = userRepository;
            _mediatorHandler = mediatorHandler;
        }
        public void Insert(User user)
        {
            var command = new CreateUserCommand(user);
            try
            {
                Task task = _mediatorHandler.SendCommand(command);
                Task.WaitAll(task);
            }
            catch (AggregateException)
            {
                throw new FrameworkException("程序内部错误");
            }
        }

       
    }
  public class UserCommandHandler : IRequestHandler<CreateUserCommand, Unit>
    {
        private readonly IUserRepository _userRepository;
        private readonly IMediatorHandler _mediatorHandler;
        public UserCommandHandler(IUserRepository userRepository, IMediatorHandler mediatorHandler)
        {
            _userRepository = userRepository;
            _mediatorHandler = mediatorHandler;
        }
        public Task<Unit> Handle(CreateUserCommand command, CancellationToken cancellationToken)
        {

            _userRepository.Insert(command.User);
            return Task.FromResult(new Unit());
        }
    }

 



       到这里可以说已经完成了,可以执行了,这样的例子网上也有很多。但是还是需要去了解它的源码,但是自己目前自己只是读懂了源码的20%左右,根据我的理解就是我们在调用IMediator的Send方法时,线程安全集合_requestHandlers会把我们的请求添加到内存里面,具体的就是CreateInstance一个RequestHandlerWrapperImpl对象,但是通过MakeGenericType把它的类型变成了我们请求的类型和返回的类型,然后通过(RequestHandlerWrapper<TResponse>)将Mediator转换类型调用Handler,继续执行RequestHandlerWrapper的实现类RequestHandlerWrapperImpl,然后通过某种方式调用了IRequestHandler,然后找到IRequestHandler的实现类UserCommandHandler,从而完成添加功能。

   无奈自己才疏学浅,并没有理解作者的意思。我现在还是有很多疑问。

,以下是我自认为比较重要的的位置了:

 1 ,这个类我删了很多代码,只留了我们用的到的 Send方法MediatorHandler类里面用到的

public class Mediator : IMediator
    {
        // Token: 0x06000017 RID: 23 RVA: 0x000020F1 File Offset: 0x000002F1
        public Mediator(ServiceFactory serviceFactory)
        {
            this._serviceFactory = serviceFactory;
        }

        // Token: 0x06000018 RID: 24 RVA: 0x00002100 File Offset: 0x00000300
        public Task<TResponse> Send<TResponse>(IRequest<TResponse> request, CancellationToken cancellationToken = default(CancellationToken))
        {
            if (request == null)
            {
                throw new ArgumentNullException("request");
            }
            Type requestType = request.GetType();
            return ((RequestHandlerWrapper<TResponse>)Mediator._requestHandlers.GetOrAdd(requestType, (Type t) => Activator.CreateInstance(typeof(RequestHandlerWrapperImpl<, >).MakeGenericType(new Type[]
            {
                requestType,
                typeof(TResponse)
            })))).Handle(request, cancellationToken, this._serviceFactory);
        }
      // Token: 0x04000001 RID: 1
        private readonly ServiceFactory _serviceFactory;

        // Token: 0x04000002 RID: 2
        private static readonly ConcurrentDictionary<Type, object> _requestHandlers = new ConcurrentDictionary<Type, object>();

    }

   2,

public interface IPipelineBehavior<in TRequest, TResponse>
    {
        // Token: 0x0600000C RID: 12
        Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next);
    }

 3

 internal abstract class RequestHandlerWrapper<TResponse> : RequestHandlerBase
    {
        public abstract Task<TResponse> Handle(IRequest<TResponse> request, CancellationToken cancellationToken,
            ServiceFactory serviceFactory);
    }

    internal class RequestHandlerWrapperImpl<TRequest, TResponse> : RequestHandlerWrapper<TResponse>
        where TRequest : IRequest<TResponse>
    {
        public override Task<TResponse> Handle(IRequest<TResponse> request, CancellationToken cancellationToken,
            ServiceFactory serviceFactory)
        {
            Task<TResponse> Handler() => GetHandler<IRequestHandler<TRequest, TResponse>>(serviceFactory).Handle((TRequest) request, cancellationToken);

            return serviceFactory
                .GetInstances<IPipelineBehavior<TRequest, TResponse>>()
                .Reverse()
                .Aggregate((RequestHandlerDelegate<TResponse>) Handler, (next, pipeline) => () => pipeline.Handle((TRequest)request, cancellationToken, next))();
        }
    }

   4,

public abstract class RequestHandler<TRequest> : IRequestHandler<TRequest>, IRequestHandler<TRequest, Unit> where TRequest : IRequest
    {
        // Token: 0x06000014 RID: 20 RVA: 0x000020DB File Offset: 0x000002DB
        Task<Unit> IRequestHandler<TRequest, Unit>.Handle(TRequest request, CancellationToken cancellationToken)
        {
            this.Handle(request);
            return Unit.Task;
        }

        // Token: 0x06000015 RID: 21
        protected abstract void Handle(TRequest request);
    }

4

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