WCF中使用观察者模式实现事件通知

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( " CreateLogServiceInstance. " );
}

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.Exceptione)
{
errorClints.Add(listener.Key);
Trace.WriteLine( " BROADEVENTERROR: " + e.Message);
}
}

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

#region ILogService成员

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

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

Trace.WriteLine( " SessionID: " + OperationContext.Current.SessionId);
Trace.WriteLine( " Registerlistener.ClientCount: " + listeners.Count.ToString());
}

public void UnregisterListener()
{
listeners.Remove(OperationContext.Current.SessionId);
Trace.WriteLine( " SessionID: " + OperationContext.Current.SessionId);
Trace.WriteLine( " Unregisterlistener.ClientCount: " + 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( " RECVLOGEVENT: " + logMsg);
}

#endregion
}

然后在程序中使用它：

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

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

Console.WriteLine( " Pressenterkeytoexit. " );
Console.ReadLine();

service.UnregisterListener();
}

需要注意的问题：
A. 因为客户也要监听端口，所以确保防火墙没有对它进行阻止。
B. 这里使用的是单实例的服务，所以需要进行多进程访问的保护，才能实际使用。