事件匯流排的設計與實現

事件匯流排（在有些框架中也稱時間聚合器，如Prism的EventAggregater）是訂閱-發佈模式的一種實現，類似觀察者模式，相對於觀察者模式，事件匯流排更靈活，它是一種集中處理事件的模式，允許不同組件之間通信，降低耦合度。
事件匯流排的三要素：事件源（事件的定義）、發佈事件（觸發事件）、訂閱事件（事件的應答實現）。
實現事件匯流排，主要工作是事件基類、參數基類、事件自動註冊的定義，基於需求場景的業務型事件，只需要按照預定義的規則增加上述三要素即可，大大降低業務複雜度，且可實現一對一、一對多，多對一，同步應答、非同步應答等方式。
1、定義事件基類（抽象類）

    public abstract class MyEventBase
    {
        public string EventName { get; set; }
    }

2、事件參數的基類（抽象類）

    public abstract class MyEventArgs<T>:EventArgs
    {
        public T Argument { get; set; }
    }

3、事件的基類（無返回值的事件基類，因無返回值，可非同步調用，也可同步調用，建議非同步調用）

   public class MyEvent<T>: MyEventBase where T : EventArgs
   {
       protected static readonly object lockObject = new object();
       protected readonly List<Action<object, T>> subscribers = new List<Action<object, T>>();

       public void Subscribe(Action<object, T> eventHandler)
       {
           lock (lockObject)
           {
               if (!subscribers.Contains(eventHandler))
               {
                   subscribers.Add(eventHandler);
               }
           }
       }

       public void Unsubscribe(Action<object, T> eventHandler)
       {
           lock (lockObject)
           {
               if (subscribers.Contains(eventHandler))
               {
                   subscribers.Remove(eventHandler);
               }
           }
       }

       public virtual void Publish(object sender, T eventArgs)
       {
           lock (lockObject)
           {
               for (int i = 0; i < subscribers.Count; i++)
               {
                   subscribers[i](sender, eventArgs);
               }
           }
       }
   }

4、事件的基類（有返回值事件的基類，因需要獲取返回值，一般採用同步方式調用）

    public class MyEventWithResult<T1, T2> : MyEventBase where T1 : EventArgs
    {
        protected static readonly object lockObject = new object();
        protected readonly List<Func<object, T1, T2>> subscribers = new List<Func<object, T1, T2>>();

        public void Subscribe(Func<object, T1, T2> eventHandler)
        {
            lock (lockObject)
            {
                if (!subscribers.Contains(eventHandler))
                {
                    subscribers.Add(eventHandler);
                }
            }
        }

        public void Unsubscribe(Func<object, T1, T2> eventHandler)
        {
            lock (lockObject)
            {
                if (subscribers.Contains(eventHandler))
                {
                    subscribers.Remove(eventHandler);
                }
            }
        }

        public virtual T2 Publish(object sender, T1 eventArgs)
        {
            T2 result = default;
            lock (lockObject)
            {
                for (int i = 0; i < subscribers.Count; i++)
                {
                    result = subscribers[i](sender, eventArgs);
                }
            }
            return result;
        }
    }

5、事件中心，負責自動裝載事件

    public class MyEventAggregator
    {
        private static MyEventAggregator _default;
        private static readonly object _locker = new object();
        private readonly Dictionary<Type, MyEventBase> _eventAggregator = new Dictionary<Type, MyEventBase>();

        /// <summary>
        /// 預設事件匯流排實例，建議只使用此實例
        /// </summary>
        public static MyEventAggregator Default
        {
            get
            {
                if (_default == null)
                {
                    lock (_locker)
                    {
                        // 如果類的實例不存在則創建，否則直接返回
                        if (_default == null)
                        {
                            _default = new MyEventAggregator();
                        }
                    }
                }
                return _default;
            }
        }

        /// <summary>
        /// 構造函數，自動載入EventBase的派生類實現
        /// </summary>
        public MyEventAggregator()
        {
            Type type = typeof(MyEventBase);
            Assembly assembly = Assembly.GetAssembly(type);
            //註冊時需排除的基類
            Type typeMyEvent = typeof(MyEvent<>);
            Type typeMyEventWithResult = typeof(MyEventWithResult<,>);

            List<Type> typeList2 = assembly.GetTypes()
                .Where(t => t != type && t!=typeAnalysisEvent && t != typeMyEventWithResult && type.IsAssignableFrom(t))
                .ToList();
            foreach (var item in typeList2)
            {
                MyEventBase eventBase = (MyEventBase)assembly.CreateInstance(item.FullName);
                _eventAggregator.Add(item, eventBase);
            }
        }

        /// <summary>
        /// 獲取事件實例
        /// </summary>
        /// <typeparam name="TEvent">事件類型</typeparam>
        /// <returns></returns>
        public TEvent GetEvent<TEvent>() where TEvent : MyEventBase
        {
            return (TEvent)_eventAggregator[typeof(TEvent)];
        }

        /// <summary>
        /// 添加事件類型
        /// </summary>
        /// <typeparam name="TEvent"></typeparam>
        public void AddEvent<TEvent>() where TEvent : MyEventBase, new()
        {
            lock (_locker)
            {
                Type type = typeof(TEvent);
                if (!_eventAggregator.ContainsKey(type))
                {
                    _eventAggregator.Add(type, new TEvent());
                }
            }
        }

        /// <summary>
        /// 移除事件類型
        /// </summary>
        /// <typeparam name="TEvent"></typeparam>
        public void RemoveEvent<TEvent>() where TEvent : MyEventBase, new()
        {
            lock (_locker)
            {
                Type type = typeof(TEvent);
                if (_eventAggregator.ContainsKey(type))
                {
                    _eventAggregator.Remove(type);
                }
            }
        }
    }
}

6、具體事件的定義和調用示例
定義

    /// <summary>
    /// 添加日誌 事件
    /// </summary>
    public class WriteLogEvent : MyEvent<WriteLogEventArgs>
    {
        public override void Publish(object sender, WriteLogEventArgs eventArgs)
        {
            for (int i = 0; i < subscribers.Count; i++)
            {
                var action = subscribers[i];
                Task.Run(() => action(sender, eventArgs));
            }
        }
    }

    public class WriteLogEventArgs : MyEventArgs<WriteLogEventArgs2> { };

    public class WriteLogEventArgs2
    {
        /// <summary>
        /// 日誌內容
        /// </summary>
        public string Msg { get; set; }

        /// <summary>
        /// 操作類型
        /// </summary>
        public OperationType OperationType { get; set; }

        /// <summary>
        /// 備註
        /// </summary>
        public string Remark { get; set; }

        /// <summary>
        /// 目標類型
        /// </summary>
        public LogTargetType LogTargetType { get; set; }

        /// <summary>
        /// 目標值
        /// </summary>
        public string TargetValue { get; set; }
    }

發佈事件

MyEventAggregator.Default.GetEvent<WriteLogEvent>().Publish(this, new WriteLogEventArgs
{
    Argument = new WriteLogEventArgs2
    {
        Msg = logMsg,
        OperationType = OperationType.Save,
        LogTargetType = LogTargetType.OriginalData,
        TargetValue = targetValue,
        Remark = _MPIComment
    }
});

訂閱事件

MyEventAggregator.Default.GetEvent<WriteLogEvent>().Subscribe(WriteLogEventHandler);

private void WriteLogEventHandler(object sender, WriteLogEventArgs e)
{
    -----business logic
    -----
}

本文來自博客園，作者：GIS民工，轉載請註明原文鏈接：https://www.cnblogs.com/kook2007/p/18379352