初学者浅度剖析eShopOnContainers 里面用到的MediatR .

一.介绍

   简单了解下开源项目 MedatR, eShopOnContainers, MediatR作者Jimmy Bogard :

Simple mediator implementation in .NET
In-process messaging with no dependencies.
Supports request/response, commands, queries, notifications and events, synchronous and async with intelligent dispatching via C# generic variance.
.NET中的简单中介实现 进程内没有依赖关系消息传递。支持以同步或异步的形式进行请求/响应，命令，查询，通知和事件的消息传递，并通过C#泛型支持消息的智能调度。

  MediatR实现Pipeline ,通过Autofac 注入Log,FluentValidation ,来实现管道里记录日志,管道里验证实体数据.

二.MediatR的使用

   1.安装NuGet包:

Install-Package MediatR.Extensions.Microsoft.DependencyInjectionFixed

Install-Package Autofac.Extensions.DependencyInjection

   2.修改Startup.cs文件

public IServiceProvider ConfigureServices(IServiceCollection services)
{
      services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
      var container = new ContainerBuilder();
      container.Populate(services);
      container.RegisterModule(new MediatorModule());
      return new AutofacServiceProvider(container.Build());
}

   3.添加MediatorModule.cs文件

  添加 builder.RegisterGeneric(typeof(xxx<,>)).As(typeof(IPipelineBehavior<,>)); 按你添加的顺序执行管道

public class MediatorModule : Autofac.Module
    {
        protected override void Load(ContainerBuilder builder)
        {
            builder.RegisterAssemblyTypes(typeof(IMediator).GetTypeInfo().Assembly)
                .AsImplementedInterfaces();
             
            // Register all the Command classes (they implement IRequestHandler) in assembly holding the Commands
            builder.RegisterAssemblyTypes(typeof(CreateOrderCommand).GetTypeInfo().Assembly)
                .AsClosedTypesOf(typeof(IRequestHandler<,>));
//             Register the DomainEventHandler classes (they implement INotificationHandler<>) in assembly holding the Domain Events
//            builder.RegisterAssemblyTypes(typeof(ValuesController.Pings).GetTypeInfo().Assembly)
//                .AsClosedTypesOf(typeof(INotificationHandler<>));

            // Register the Command's Validators (Validators based on FluentValidation library)
            builder
                .RegisterAssemblyTypes(typeof(CreateOrderCommandValidator).GetTypeInfo().Assembly)
                .Where(t => t.IsClosedTypeOf(typeof(IValidator<>)))
                .AsImplementedInterfaces();


            builder.Register<ServiceFactory>(context =>
            {
                var componentContext = context.Resolve<IComponentContext>();
                return t => { object o; return componentContext.TryResolve(t, out o) ? o : null; };
            });

            builder.RegisterGeneric(typeof(LoggingBehavior<,>)).As(typeof(IPipelineBehavior<,>));
            builder.RegisterGeneric(typeof(ValidatorBehavior<,>)).As(typeof(IPipelineBehavior<,>));
//            builder.RegisterGeneric(typeof(TransactionBehaviour<,>)).As(typeof(IPipelineBehavior<,>));

        }
    }

　4.添加LoggingBehavior.cs文件

public class LoggingBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
 {
        private readonly ILogger<LoggingBehavior<TRequest, TResponse>> _logger;
        public LoggingBehavior(ILogger<LoggingBehavior<TRequest, TResponse>> logger) => _logger = logger;

        public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
        {
            _logger.LogInformation("----- Handling command {CommandName} ({@Command})", request.GetGenericTypeName(), request);
            var response = await next();
            _logger.LogInformation("----- Command {CommandName} handled - response: {@Response}", request.GetGenericTypeName(), response);

            return response;
        }
 }

　5.添加ValidatorBehavior.cs文件

public class ValidatorBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    {
        private readonly ILogger<ValidatorBehavior<TRequest, TResponse>> _logger;
        private readonly IValidator<TRequest>[] _validators;

        public ValidatorBehavior(IValidator<TRequest>[] validators, ILogger<ValidatorBehavior<TRequest, TResponse>> logger)
        {
            _validators = validators;
            _logger = logger;
        }

        public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
        {
            var typeName = request.GetGenericTypeName();

            _logger.LogInformation("----- Validating command {CommandType}", typeName);

            var failures = _validators
                .Select(v => v.Validate(request))
                .SelectMany(result => result.Errors)
                .Where(error => error != null)
                .ToList();

            if (failures.Any())
            {
                _logger.LogWarning("Validation errors - {CommandType} - Command: {@Command} - Errors: {@ValidationErrors}", typeName, request, failures);

                throw new OrderingDomainException(
                    $"Command Validation Errors for type {typeof(TRequest).Name}", new ValidationException("Validation exception", failures));
            }

            return await next();
        }
    }

  6.添加CreateOrderCommandValidator.cs

public class CreateOrderCommandValidator : AbstractValidator<CreateOrderCommand>
    {
        public CreateOrderCommandValidator()
        {
            RuleFor(command => command.City).NotEmpty();
            RuleFor(command => command.Street).NotEmpty();
            RuleFor(command => command.State).NotEmpty();
            RuleFor(command => command.Country).NotEmpty();
            RuleFor(command => command.ZipCode).NotEmpty();
            RuleFor(command => command.CardNumber).NotEmpty().Length(12, 19); 
            RuleFor(command => command.CardHolderName).NotEmpty();
            RuleFor(command => command.CardExpiration).NotEmpty().Must(BeValidExpirationDate).WithMessage("Please specify a valid card expiration date"); 
            RuleFor(command => command.CardSecurityNumber).NotEmpty().Length(3); 
            RuleFor(command => command.CardTypeId).NotEmpty();
            RuleFor(command => command.OrderItems).Must(ContainOrderItems).WithMessage("No order items found"); 
        }

        private bool BeValidExpirationDate(DateTime dateTime)
        {
            return dateTime >= DateTime.UtcNow;
        }

        private bool ContainOrderItems(IEnumerable<CreateOrderCommand.OrderItemDTO> orderItems)
        {
            return orderItems.Any();
        }
    }