DDD(六)【领域事件与事件总线】

谈谈它

终于有些眉目了，搜刮了很多牛人的资料，英文的，中文的，民国文的，终于小有成就了，同时也做了个ＤＥＭＯ，领域事件这东西好，但需要你明白它之后才会说好，而对于明白领域事件这件事来说，它的门槛有点高，居然花了我三天的时间才把它搞定，嗨！

占占给它的定义

领域事件：Domain Event,是针对某个业务来说的，或者说针对某个聚合的业务来说的，例如订单生成这种业务，它可以同时对应一种事件，比如叫做OrderGeneratorEvent，而你的零散业务可能随时会变，加一些业务，减一些业务，而对于订单生成这个事件来说，它是唯一不变的，而我们需要把这些由产生订单而发生变化的事情拿出来，而拿出来的这些业务就叫做＂领域事件＂．其中的领域指的就是订单生成这个聚合；而事件指的就是那些零散业务的统称．

它的主要几个抽象

在面向对象的编程世界里，做这种事情我们需要几个抽象：

领域对象事件标示：（标示接口，接口的一种，用来约束一批对象）IEvent

领域对象的处理方法，行为：（需要让那些零散的模块重写的方法，起个听起来熟悉的名字，叫它handle吧）IEventHandler＝＞Handle

事件总线：事件处理核心类，承载了事件的发布，订阅与取消订阅的逻辑，EventBus

某个领域对象：为了实现某个业务，而创建的实体类，它里面有事件所需要的数据，它继承了IEvent

某个领域对象的事件：它是一个事件处理类，它实现了IEventHandler，它所处理的事情需要在Handle里去完成

我的Demo的实现

一　结果图：

二　核心类：

IEvent接口，标示接口往往都是空的，呵呵

    /// 

    /// 事件实体基类
    /// 
    public interface IEvent
    {
    }

IEventHandler接口，只有一个行为方法Handle

    /// 

    /// 事件处理接口
    /// 
    /// 继承IEvent对象的事件源对象
    public interface IEventHandler<TEvent>
        where TEvent : IEvent
    {
        /// 

        /// 处理程序
        /// 
        /// 
        void Handle(TEvent evt);

    }

EventBus是实现事件的核心，在这版里，它支持异步事件机制,使用Task实现，所以它需要运行在.net4.5平台之上

 　 /// 

    /// 事件总线
    /// 发布与订阅处理逻辑
    /// 核心功能代码
    /// 
    public class EventBus
    {
        private EventBus() { }

        private static EventBus _eventBus = null;
        private readonly object sync = new object();
        /// 

        /// 对于事件数据的存储，目前采用内存字典
        /// 
        private static Dictionary<Type, List<object>> eventHandlers = new Dictionary<Type, List<object>>();
        /// 

        // checks if the two event handlers are equal. if the event handler is an action-delegated, just simply
        // compare the two with the object.Equals override (since it was overriden by comparing the two delegates. Otherwise,
        // the type of the event handler will be used because we don't need to register the same type of the event handler
        // more than once for each specific event.
        /// 
        private readonly Func<object, object, bool> eventHandlerEquals = (o1, o2) =>
         {
             var o1Type = o1.GetType();
             var o2Type = o2.GetType();
             if (o1Type.IsGenericType &&
                 o1Type.GetGenericTypeDefinition() == typeof(ActionDelegatedEventHandler<>) &&
                 o2Type.IsGenericType &&
                 o2Type.GetGenericTypeDefinition() == typeof(ActionDelegatedEventHandler<>))
                 return o1.Equals(o2);
             return o1Type == o2Type;
         };
        /// 

        /// 初始化空的事件总件
        /// 
        public static EventBus Instance
        {
            get
            {
                return _eventBus ?? (_eventBus = new EventBus());
            }
        }
        /// 

        /// 通过ＸＭＬ文件初始化事件总线，订阅信自在ＸＭＬ里配置
        /// 
        /// 
        public static EventBus InstanceForXml()
        {
            if (_eventBus == null)
            {
                XElement root = XElement.Load(System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "EventBus.xml"));
                foreach (var evt in root.Elements("Event"))
                {
                    List<object> handlers = new List<object>();

                    Type publishEventType = Type.GetType(evt.Element("PublishEvent").Value);
                    foreach (var subscritedEvt in evt.Elements("SubscribedEvents"))
                        foreach (var concreteEvt in subscritedEvt.Elements("SubscribedEvent"))
                            handlers.Add(Type.GetType(concreteEvt.Value));

                    eventHandlers[publishEventType] = handlers;
                }

                _eventBus = new EventBus();
            }
            return _eventBus;
        }

        #region 事件订阅&取消订阅，可以扩展
        /// 

        /// 订阅事件列表
        /// 
        /// 
        /// 
        public void Subscribe<TEvent>(IEventHandler<TEvent> eventHandler)
            where TEvent : class, IEvent
        {
            lock (sync)
            {
                var eventType = typeof(TEvent);
                if (eventHandlers.ContainsKey(eventType))
                {
                    var handlers = eventHandlers[eventType];
                    if (handlers != null)
                    {
                        if (!handlers.Exists(deh => eventHandlerEquals(deh, eventHandler)))
                            handlers.Add(eventHandler);
                    }
                    else
                    {
                        handlers = new List<object>();
                        handlers.Add(eventHandler);
                    }
                }
                else
                    eventHandlers.Add(eventType, new List<object> { eventHandler });
            }
        }
        /// 

        /// 订阅事件实体
        /// 
        /// 
        /// 
        public void Subscribe<TEvent>(Action<TEvent> eventHandlerFunc)
            where TEvent : class, IEvent
        {
            Subscribe<TEvent>(new ActionDelegatedEventHandler<TEvent>(eventHandlerFunc));
        }
        public void Subscribe<TEvent>(IEnumerable<IEventHandler<TEvent>> eventHandlers)
            where TEvent : class, IEvent
        {
            foreach (var eventHandler in eventHandlers)
                Subscribe<TEvent>(eventHandler);
        }
        /// 

        /// 取消订阅事件
        /// 
        /// 
        /// 
        public void Unsubscribe<TEvent>(IEventHandler<TEvent> eventHandler)
            where TEvent : class, IEvent
        {
            lock (sync)
            {
                var eventType = typeof(TEvent);
                if (eventHandlers.ContainsKey(eventType))
                {
                    var handlers = eventHandlers[eventType];
                    if (handlers != null
                        && handlers.Exists(deh => eventHandlerEquals(deh, eventHandler)))
                    {
                        var handlerToRemove = handlers.First(deh => eventHandlerEquals(deh, eventHandler));
                        handlers.Remove(handlerToRemove);
                    }
                }
            }
        }
        public void Unsubscribe<TEvent>(IEnumerable<IEventHandler<TEvent>> eventHandlers)
          where TEvent : class, IEvent
        {
            foreach (var eventHandler in eventHandlers)
                Unsubscribe<TEvent>(eventHandler);
        }
        public void Unsubscribe<TEvent>(Action<TEvent> eventHandlerFunc)
            where TEvent : class, IEvent
        {
            Unsubscribe<TEvent>(new ActionDelegatedEventHandler<TEvent>(eventHandlerFunc));
        }
        #endregion

        #region 事件发布
        /// 

        /// 发布事件，支持异步事件
        /// 
        /// 
        /// 
        public void Publish<TEvent>(TEvent evnt)
           where TEvent : class, IEvent
        {
            if (evnt == null)
                throw new ArgumentNullException("evnt");
            var eventType = evnt.GetType();
            if (eventHandlers.ContainsKey(eventType)
                && eventHandlers[eventType] != null
                && eventHandlers[eventType].Count > 0)
            {
                var handlers = eventHandlers[eventType];
                foreach (var handler in handlers)
                {
                    var eventHandler = handler as IEventHandler<TEvent>;
                    if (eventHandler.GetType().IsDefined(typeof(HandlesAsynchronouslyAttribute), false))
                    {
                        Task.Factory.StartNew((o) => eventHandler.Handle((TEvent)o), evnt);
                    }
                    else
                    {
                        eventHandler.Handle(evnt);
                    }
                }
            }
        }

        public void Publish<TEvent>(TEvent evnt, Action<TEvent, bool, Exception> callback, TimeSpan? timeout = null)
           where TEvent : class, IEvent
        {
            if (evnt == null)
                throw new ArgumentNullException("evnt");
            var eventType = evnt.GetType();
            if (eventHandlers.ContainsKey(eventType) &&
                eventHandlers[eventType] != null &&
                eventHandlers[eventType].Count > 0)
            {
                var handlers = eventHandlers[eventType];
                List<Task> tasks = new List<Task>();
                try
                {
                    foreach (var handler in handlers)
                    {
                        var eventHandler = handler as IEventHandler<TEvent>;
                        if (eventHandler.GetType().IsDefined(typeof(HandlesAsynchronouslyAttribute), false))
                        {
                            tasks.Add(Task.Factory.StartNew((o) => eventHandler.Handle((TEvent)o), evnt));
                        }
                        else
                        {
                            eventHandler.Handle(evnt);
                        }
                    }
                    if (tasks.Count > 0)
                    {
                        if (timeout == null)
                            Task.WaitAll(tasks.ToArray());
                        else
                            Task.WaitAll(tasks.ToArray(), timeout.Value);
                    }
                    callback(evnt, true, null);
                }
                catch (Exception ex)
                {
                    callback(evnt, false, ex);
                }
            }
            else
                callback(evnt, false, null);
        }

        #endregion

    }

一个具体的领域对象，它继承IEvent

    /// 

    /// 添加订单的事件
    /// 
    public class OrderGeneratorEvent : IEvent
    {
        public int OrderID { get; set; }
    }

一个为OrderGeneratorEvent工作的领域事件，它用来为客户发邮件

　  /// 

    /// 发邮件功能
    /// 
    public class OrderAddedEventHandler_SendEmail : IEventHandler<OrderGeneratorEvent>
    {
        public void Handle(OrderGeneratorEvent evt)
        {
            Console.WriteLine("Order_Number:{0},Send a Email.", evt.OrderID);
        }
    }

下面看一个主程序：

   static void Main(string[] args)
    {
      　　  EventBus.Instance.Subscribe(new OrderAddedEventHandler_SendEmail());
            var entity = new OrderGeneratorEvent { OrderID = 1 };
            Console.WriteLine("生成一个订单，单号为{0}", entity.OrderID);
            EventBus.Instance.Publish(entity);
            Console.ReadKey();
    }

下面是运行结果：