WCF中使用观察者模式实现事件通知

    看了一些WCF的例子，实现事件通知使用的是多播委托的特性，有点复杂，操作起来也不是很直观，看到一堆委托和事件我一般头就晕了。下面介绍一种使用观察者模式实现事件通知的简单方法。没别的，就是简单，简单最美。
    工程代码如下：https://files.cnblogs.com/dyj057/WcfEvent.rar
    1.定义接口

[ServiceContract(SessionMode = SessionMode.Required, CallbackContract = typeof(IWriteLogCallback))]
    public interface ILogService
    {
        [OperationContract(IsInitiating = true, IsTerminating = false)]
         void Write(string logMsg);

        [OperationContract(IsInitiating = true, IsTerminating = false)]
        void RegisterListener();

        [OperationContract(IsInitiating = false, IsTerminating = false)]
        void UnregisterListener();
    }

    [ServiceContract]
    public interface IWriteLogCallback
    {
        [OperationContract(IsOneWay = true)]
        void OnWriteLog(string logMsg);
    }

为了简单举了一个写日志的例子， Write(string logMsg)就是写入日志的方法，参数logMsg是需要写入的日志信息。当客户单没有调用RegisterListener()订阅事件的时候，是不会收到写日志的事件通知的，相应的要获得写日志的事件通知，就需要调用RegisterListener()方法。如果要取消订阅就调用UnregisterListener()方法。写日志的功能和事件的订阅功能是分开的。

2.服务实现

[ServiceBehavior(
            IncludeExceptionDetailInFaults = true,
            InstanceContextMode = InstanceContextMode.Single,
            ConcurrencyMode = ConcurrencyMode.Multiple)]
    class LogService:ILogService
    {
        public LogService()
        {
            Trace.WriteLine("Create LogService Instance.");
        }

        Dictionary<string, OperationContext> listeners = new Dictionary<string, OperationContext>();

        private void BroadCast(string logMsg)
        {
            List<string> errorClints = new List<string>();

            foreach (KeyValuePair<string, OperationContext> listener in listeners)
            {
                try
                {
                    listener.Value.GetCallbackChannel<IWriteLogCallback>().OnWriteLog(logMsg);
                }
                catch (System.Exception e)
                {
                    errorClints.Add(listener.Key);
                    Trace.WriteLine("BROAD EVENT ERROR:" + e.Message);
                }
            }

            foreach (string id in errorClints)
            {
                listeners.Remove(id);
            }
        }

        #region ILogService 成员

        public void Write(string logMsg)
        {
            Trace.WriteLine("Write LOG:"+logMsg);
            BroadCast(logMsg);
        }

        public void RegisterListener()
        {
            listeners.Add(OperationContext.Current.SessionId, OperationContext.Current);

            Trace.WriteLine("SessionID:" + OperationContext.Current.SessionId);
            Trace.WriteLine("Register listener. Client Count:" + listeners.Count.ToString());
        }

        public void UnregisterListener()
        {
            listeners.Remove(OperationContext.Current.SessionId);
            Trace.WriteLine("SessionID:" + OperationContext.Current.SessionId);
            Trace.WriteLine("Unregister listener. Client Count:" + listeners.Count.ToString());
        }

        #endregion
    }

    Dictionary<string, OperationContext> listeners包含了所有的事件订阅者。发布事件的时候，如果调用订阅者的回调函数失败，就把该订阅者从listeners移除。代码很简单，就不多说了。
    3.客户端访问
    定义回调的客户端：

class LogClient:IWriteLogCallback
    {
        #region IWriteLogCallback 成员

        public void OnWriteLog(string logMsg)
        {
            Trace.WriteLine("RECV LOG EVENT:" + logMsg);
        }

        #endregion
    }

然后在程序中使用它：

class Program
    {
        static void Main(string[] args)
        {
            Trace.Listeners.Add(new ConsoleTraceListener());
            LogClient client = new LogClient();
            ILogService service = DuplexChannelFactory<ILogService>.CreateChannel(client,
                new WSDualHttpBinding(), new EndpointAddress("http://localhost:8888/log"));

            //订阅消息
            service.RegisterListener();
            service.Write("Client start