MediatR 中介模式

使用MediatR完成基于内存级别的消息发布订阅

在微服务架构中领域驱动模型中处理领域事件的相关操作

在区分好领域模型后，就拿代码中来说嘛，用户领域中添加用户操作可能或存在跟用户相关的一些领域事件，在添加用户的时候会执行相关的领域事件

首先需要添加nuget包 MediatR

MediatR中有几个对象：IRequest，IRequestHandler，INotificationHandler

具体用法可以看下MediatR github

领域模型需要继承Entity,里面封装了 领域事件的相关操作

public abstract class Entity
    {
        int? _requestedHashCode;
        int _Id;
        public virtual int Id
        {
            get
            {
                return _Id;
            }
            protected set
            {
                _Id = value;
            }
        }

        private List<INotification> _domainEvents;
        public IReadOnlyCollection<INotification> DomainEvents => _domainEvents?.AsReadOnly();

        public void AddDomainEvent(INotification eventItem)
        {
            _domainEvents = _domainEvents ?? new List<INotification>();
            _domainEvents.Add(eventItem);
        }

        public void RemoveDomainEvent(INotification eventItem)
        {
            _domainEvents?.Remove(eventItem);
        }

        public void ClearDomainEvents()
        {
            _domainEvents?.Clear();
        }

        public bool IsTransient()
        {
            return this.Id == default(Int32);
        }

        public override bool Equals(object obj)
        {
            if (obj == null || !(obj is Entity))
                return false;

            if (Object.ReferenceEquals(this, obj))
                return true;

            if (this.GetType() != obj.GetType())
                return false;

            Entity item = (Entity)obj;

            if (item.IsTransient() || this.IsTransient())
                return false;
            else
                return item.Id == this.Id;
        }

        public override int GetHashCode()
        {
            if (!IsTransient())
            {
                if (!_requestedHashCode.HasValue)
                    _requestedHashCode = this.Id.GetHashCode() ^ ; 

                return _requestedHashCode.Value;
            }
            else
                return base.GetHashCode();

        }
        public static bool operator ==(Entity left, Entity right)
        {
            if (Object.Equals(left, null))
                return (Object.Equals(right, null)) ? true : false;
            else
                return left.Equals(right);
        }

        public static bool operator !=(Entity left, Entity right)
        {
            return !(left == right);
        }
    }

Entity

如：添加用户的时候添加一个默认角色相关的表处理

 public class TbUser : Entity, IAggregateRoot
    {

        public string UserName { get; set; }
        public string UserPasswrod { get; set; }
        public string UserPhone { get; set; }
        public string XXXXX { get; set; }
        [NotMapped]
        public List<TbUserRole> TbUserRoles { get; set; }
        public TbUser()
        {
            TbUserRoles = new List<TbUserRole>();
            AddDomainEvent(new UserCreatedEvent() { tbUser = this });
        }

        #region 领域职责  结合领域事件处理 黎又铭
        /*
        如：添加用户的时候添加默认角色  其实本身是可以直接在业务逻辑中写的
        有点类是SQL触发器，添加用户的时候构建件默认角色信息模型触发事件添加角色信息  

        这里有注意到 实体模型并没有强制处理表接口的主外键关系 只是做了模型对象映射
        */
        /// <summary>
        /// 添加用户职责  只构建职责 不处理  处理交友领取事件的Command CommandHandler处理
        /// </summary>
        public void AddDefalutRole(TbUserRole tbUserRole)
        {
            TbUserRoles.Add(tbUserRole);
            //添加默认角色的领域事件
            AddDomainEvent(new UserRoleCreatedEvent() { TbUserRole = tbUserRole });

        }

        #endregion

    }

User

先定义一个创建用户的命令操作： 这里的TbUser是领域时间模型，传递数据以及对领域模型事件操作

public class UserCreateCommand : IRequest<TbUser>
    {

        public TbUser tbUser{ get; set; }
    }

public class UserCreateCommandHandler : IRequestHandler<UserCreateCommand, TbUser>
    {

        private IUserRepository _userRepository;

        public UserCreateCommandHandler(IUserRepository userRepository)
        {
            _userRepository = userRepository;
        }

        public async Task<TbUser> Handle(UserCreateCommand request, CancellationToken cancellationToken)
        {

            await _userRepository.AddUserAsync(request.tbUser);
            await _userRepository.UnitOfWork.SaveEntitiesAsync();
            return request.tbUser;
        }
    }

这里还需要一个MediatR的扩展，发布领域事件

 static class MediatorExtension
    {
        /// <summary>
        /// 异步处理领域事件
        /// </summary>
        /// <param name="mediator"></param>
        /// <param name="ctx"></param>
        /// <returns></returns>
        public static async Task DispatchDomainEventsAsync(this IMediator mediator, UserDbContext ctx)
        {
            var domainEntities = ctx.ChangeTracker
                .Entries<Entity>()
                .Where(x => x.Entity.DomainEvents != null && x.Entity.DomainEvents.Any());

            var domainEvents = domainEntities
                .SelectMany(x => x.Entity.DomainEvents)
                .ToList();

            domainEntities.ToList()
                .ForEach(entity => entity.Entity.ClearDomainEvents());

            var tasks = domainEvents
                .Select(async (domainEvent) => {
                    await mediator.Publish(domainEvent);
                });

            await Task.WhenAll(tasks);
        }
    }

扩展

下面是订阅领域事件

 public class UserRoleNotification : INotificationHandler<UserRoleCreatedEvent>
    {
        private IUserRepository _userRepository;
        public UserRoleNotification(IUserRepository userRepository)
        {
            _userRepository = userRepository;
        }
        public async Task Handle(UserRoleCreatedEvent notification, CancellationToken cancellationToken)
        {

            await _userRepository.AddUserRoleAsync(notification.TbUserRole);
        }

在controller中的使用：

[HttpPost]
        [Route("adduser")]
        public async Task<IActionResult> AddUser([FromBody] TbUser tbUser)
        {
            tbUser.AddDefalutRole(new TbUserRole { RoleId = , UserId =  });
            var command = new UserCreateCommand() { tbUser = tbUser };
            var result = await _mediator.Send(command);
            return Ok(result);
        }

看下效果