WCF后续之旅(2): 如何对Channel Layer进行扩展——创建自定义Channel
在上一篇文章中，我们通过一个直接借助BasicHttpBinding对象实现Client和Server端进行通信的例子，对WCF channel layer进行了一个大致上的介绍。由此引出了一些列通信相关的概念和对象，比如Channel，Outputchannel， Input channel，Request channel, Reply Channel，Duplex channel，Channel Shape，Channel manager，Channel factory, Channel listener,Binding element 等。通过这些元素，我们很容易地实现对WCF channel layer进行扩展。
对channellayer进行扩展一般适用于当你的需求通过现有的Binding，或者channel不能实现，而需要自定义一些channel来实现你所需的功能。不如现在的WCF系统定义的Channel中没有实现对Message body的压缩功能。你可以就需要将此功能定义到一个customchannel中，然后将其注入到channel stack中。一般来说，仅仅创建custom channel是不够的，因为在runtime，channel是通过Channel manager进行创建的，所以你需要创建对应的Channel factory（如何对发送方进行扩展）或者Channel listener（如果对接受方进行扩展）。而Channel factory和channel listener最终又是通过Binding element进行创建的，所以你还需要创建相应的Binding element。（Binding element=〉Channel factory&Channel listener=>Channel）
在本章节中，我们将继续讨论WCF channel layer。我们将通过如何创建和应用custom channel来介绍channel layer一些知识。
1、ICommunicationObject 和 CommunicationObject
我们知道WCF channel layer的绝大部分对象，比如Channel，Channelfactory，Channellistener，从功能上讲都是用于通信（Communication）的对象，对传统的communicationobject，比如socket，他们往往都具有通过状态和状态转化规则（状态机：Statemachine）。这些状态包括Creating、Created、Opening、Opened、Closing、Closed等等。为了统一管理这些状态和状态之间的转化，WCF定义个一个特殊的Interface：ICommunicationObject
public interface ICommunicationObject
{
// Events event EventHandler Closed; event EventHandler Closing; event EventHandler Faulted; event EventHandler Opened; event EventHandler Opening; // Methods void Abort(); IAsyncResult BeginClose(AsyncCallback callback, object state); IAsyncResult BeginClose(TimeSpan timeout, AsyncCallback callback, object state); IAsyncResult BeginOpen(AsyncCallback callback, object state); IAsyncResult BeginOpen(TimeSpan timeout, AsyncCallback callback, object state); void Close(); void Close(TimeSpan timeout); void EndClose(IAsyncResult result); void EndOpen(IAsyncResult result); void Open(); void Open(TimeSpan timeout); // Properties
CommunicationState State { get; }
}
ICommunicationObject定义了3种成员：
- Property：State， 得到当前的状态，返回值是一个CommunicationState 枚举。
- Method：同步、异步Open和Close方法。
- Event：通过注册这些状态相关的Event，当时对象转化到对应的状态时执行相应操作。
WCF定义了一个abstract class: CommunicationObject直接实现了该Interface。CommunicationObject的实现统一的State machine。WCF channel layer的很多的class都直接或者间接的继承了这个class。你也可以让你的class继承该class。当你让你自己的class继承CommunicationObject的时候，在override 掉base相应的method的时候，强烈建议你先调用base对应的方法，CommunicationObject会帮你进行相应的State转换和触发相应的事件。
2. Channel 和Channel Shape
在上一篇文章中，我们讨论过了。在不同的消息交换模式（MEP）中，发送方和接受方的Channel扮演的角色是不相同的。我们并把这种不同MEP中消息交互双方Channel的结构差异表述为Channel shape。我们有四种不同的Channel shape：Datagram、Request/reply、Duplex和P2P。不同Channel shape中Channel的结构性差性通过实现不同的Channel interface来体现。
对于Datagram channel shape，采用了One-way的MEP。发送方的channel 必须实现IOutputChannel interface。该Interface的方法成员主要集中在用于发送message的Send方法（同步/异步）：
public interface IOutputChannel : IChannel, ICommunicationObject{ // Methods IAsyncResult BeginSend(Message message, AsyncCallback callback, object state); IAsyncResult BeginSend(Message message, TimeSpan timeout, AsyncCallback callback, object state); void EndSend(IAsyncResult result); void Send(Message message); void Send(Message message, TimeSpan timeout); // Properties EndpointAddress RemoteAddress { get; } Uri Via { get; }}
与之相应是IInputChannel inteface，该Interface用于Datagram channel shape中接收方的channel定义。其主要方法成员主要集中在用于接收Message的Receive方法（同步/异步）：
public
interface
IInputChannel : IChannel, ICommunicationObject
{ // Methods IAsyncResult BeginReceive(AsyncCallback callback, object state); IAsyncResult BeginReceive(TimeSpan timeout, AsyncCallback callback, object state); IAsyncResult BeginTryReceive(TimeSpan timeout, AsyncCallback callback, object state); IAsyncResult BeginWaitForMessage(TimeSpan timeout, AsyncCallback callback, object state); Message EndReceive(IAsyncResult result); bool EndTryReceive(IAsyncResult result, out Message message); bool EndWaitForMessage(IAsyncResult result); Message Receive(); Message Receive(TimeSpan timeout); bool TryReceive(TimeSpan timeout, out Message message); bool WaitForMessage(TimeSpan timeout); // Properties EndpointAddress LocalAddress { get; }}
注：无论对于同步或者异步方法，一般由两个重载，一个接收一个TimeSpan 作为参数，表是Send或者Receive允许的时间范围。而另一个没有该参数的方式，不不是建议你使用一个无限的TimeSpan，而是使用一个可配置的默认时间段（实际上是Binding对象对应的属性）
不同于Datagram channel shape，Request/request channel shape下交互双方的Channel具有不同的行为。发送方的Channel实现IRequestChannel。该interface的方面成员主要集中在一些用于向接收方进行请求的Request方法（同步/异步）：上面。
public interface IRequestChannel : IChannel, ICommunicationObject{ // Methods IAsyncResult BeginRequest(Message message, AsyncCallback callback, object state); IAsyncResult BeginRequest(Message message, TimeSpan timeout, AsyncCallback callback, object state); Message EndRequest(IAsyncResult result); Message Request(Message message); Message Request(Message message, TimeSpan timeout); // Properties EndpointAddress RemoteAddress { get; } Uri Via { get; }}
同理，对于接收方的IReplyChannel则主要定义了一些用于Reply的方法：
public interface IReplyChannel : IChannel, ICommunicationObject{ // Methods IAsyncResult BeginReceiveRequest(AsyncCallback callback, object state); IAsyncResult BeginReceiveRequest(TimeSpan timeout, AsyncCallback callback, object state); IAsyncResult BeginTryReceiveRequest(TimeSpan timeout, AsyncCallback callback, object state); IAsyncResult BeginWaitForRequest(TimeSpan timeout, AsyncCallback callback, object state); RequestContext EndReceiveRequest(IAsyncResult result); bool EndTryReceiveRequest(IAsyncResult result, out RequestContext context); bool EndWaitForRequest(IAsyncResult result); RequestContext ReceiveRequest(); RequestContext ReceiveRequest(TimeSpan timeout); bool TryReceiveRequest(TimeSpan timeout, out RequestContext context); bool WaitForRequest(TimeSpan timeout); // Properties EndpointAddress LocalAddress { get; }}
而对与Duplex和P2P，消息交互双方使用相同的Channel：Duplex channel。本质上讲，DuplexChannel = OutputChannel + IntputChannel。这一点从IDuplexChannel的定义上就可以看出来：
public interface IDuplexChannel : IInputChannel, IOutputChannel, IChannel, ICommunicationObject{}
3、创建Custom Channel
为了让大家对WCF channel layer有一个深刻的认识，以及掌握如何有效地对其进行扩展。我在整篇文章中穿插介绍一个具体的Sample：创建一个自定义的channel，以及相关的辅助对象，比如Channel factory、Channel listener和Binding element。
这个Sample将基于我们最为常用的Request/Reply channel shape。所以我们需要创建两个Channel，一个是用于发送方的实现了IRequestChannel的Channel，而另一个则是实现了IReplyChannel的用于接收方的Channel。
为了简单起见，在我定义的channel的每个方法仅仅打印出相应的方法名称而已（这样做不但简单，还有的一个好处，那就是当我最后将其应用到具体的Messaging场景中，可以根据控制台打印出来的文字清楚地看清当我们的Channel应用到具体的场景中后先后执行了那些方法）。
我们先来看看实现了IRequestChannel的MyRequestChannel的定义：
namespace Artech.ChannleStackExplore.Channels{ public class MyRequestChannel :ChannelBase, IRequestChannel { private IRequestChannel InnerChannel {get;set;} public MyRequestChannel(ChannelManagerBase channleManager, IRequestChannel innerChannel) : base(channleManager) { this.InnerChannel = innerChannel;
} 
ChannelBase Members IRequestChannel Members }}
这里唯一需要注意的一点是：在实际的运行环境中，我们的channel仅仅了位于Channel stack的某个环节。该channel和其他的一些channel组成一个管道，这个管道里流淌是Message。所以当一个Channel执行了它相应的操作的时候，需要将message传到下一个channel作进一步处理。所有我们的Channel需要下一个Channel的应用，这个Channel就是我们的InnerChannel字段，该成员在构造函数中指定。
private IRequestChannel InnerChannel{get;set;} public MyRequestChannel(ChannelManagerBase channleManager, IRequestChannel innerChannel) : base(channleManager){ this.InnerChannel = innerChannel;}
所以，对于每一个方法，在实现了本Channel的功能之后，只需要调用InnerChannel 的对应的方法即可。
我们再来看看实现了IReplyChannel的MyReplyChannel， 它用于接收方：
namespace Artech.ChannleStackExplore.Channels{ public class MyReplyChannel: ChannelBase, IReplyChannel { private IReplyChannel InnerChannel { get; set; } public MyReplyChannel(ChannelManagerBase channelManager, IReplyChannel innerChannel):base(channelManager) { this.InnerChannel = innerChannel; } ChannelBase Members IReplyChannel Members }}
MyReplyChannel的定义方式和MyRequestChannel完全一样，我们就不用再多说什么了。
4. 创建Custom Channel Factory & Channel Listener
通过上一篇文章的介绍，我们知道Channel是通过Channel Manager来创建并管理的，在发送方的Channel Manager被称为Channel Factory。
对于Channel factory，除了定义了两个Interface之外（IChannelFactory 和IChannelFactory<TChannel>）
public interface IChannelFactory : ICommunicationObject { // Methods T GetProperty<T>() where T : class; } public interface IChannelFactory<TChannel> : IChannelFactory, ICommunicationObject{ // Methods TChannel CreateChannel(EndpointAddress to); TChannel CreateChannel(EndpointAddress to, Uri via);}
还定义了两个Base class：ChannelFactoryBase 和ChannelFactoryBase<TChannel>（限于篇幅，在这里就不多作介绍了）。
为了简单起见，我们上我们的Channel factory继承自ChannelFactoryBase<TChannel>：
namespace Artech.ChannleStackExplore.Channels{ public class MyChannelFactory<TChannel> : ChannelFactoryBase<TChannel> { private IChannelFactory<TChannel> InnerChannelFactory { get; set; } public MyChannelFactory(BindingContext context) { this.InnerChannelFactory = context.BuildInnerChannelFactory<TChannel>(); } protected override TChannel OnCreateChannel(EndpointAddress address, Uri via) { Console.WriteLine("MyChannelFactory<TChannel>.OnClose()"); TChannel innerChannel = this.InnerChannelFactory.CreateChannel(address, via); return (TChannel)(object)(new MyRequestChannel(this, innerChannel as IRequestChannel)); } protected override IAsyncResult OnBeginOpen(TimeSpan timeout, AsyncCallback callback, object state) { Console.WriteLine("MyChannelFactory<TChannel>.OnBeginOpen()"); return this.InnerChannelFactory.BeginOpen(timeout, callback, state); } protected override void OnEndOpen(IAsyncResult result) { Console.WriteLine("MyChannelFactory<TChannel>.OnEndOpen()"); this.InnerChannelFactory.EndOpen(result); } protected override void OnOpen(TimeSpan timeout) { Console.WriteLine("MyChannelFactory<TChannel>.OnOpen()"); this.InnerChannelFactory.Open(); } }}
我们说过，和Channel stack一样，Channel factory仍然是一个stack，原因很简单，一个个的Channel需要相应的channel factory来创建。同Channel一样，当channel factory创建了自己的channel之后需要将接力棒交到下一个Channel factory。不过不通于Channel的是，下一个Channel factory不时在构造函数直接指定的，而是通过构造函数中的BindingContext 对象的BuildInnerChannelFactory()创建。
private IChannelFactory<TChannel> InnerChannelFactory{ get; set; } public MyChannelFactory(BindingContext context){ this.InnerChannelFactory = context.BuildInnerChannelFactory<TChannel>();}
注：BindingContext 的两个最重要的方法就是BuildInnerChannelFactory和BuildInnerChannelListener。前者创建Inner channel factory后者创建Inner Channel listener。
熟悉了ChannelFactory的定义，大家很自然的想得到ChannelListner的定义（不过ChannelListner的成员比ChannelFactory 要多些）：
namespace Artech.ChannleStackExplore.Channels{ public class MyChannelListener<TChannel> : ChannelListenerBase<TChannel> where TChannel : class, IChannel { private IChannelListener<TChannel> InnerChannelListener { get; set; } public MyChannelListener(BindingContext context) { this.InnerChannelListener = context.BuildInnerChannelListener<TChannel>(); } protected override TChannel OnAcceptChannel(TimeSpan timeout) { Console.WriteLine("MyChannelListener<TChannel>.OnAcceptChannel()"); TChannel innerChannel = this.InnerChannelListener.AcceptChannel(timeout); return new MyReplyChannel(this, innerChannel as IReplyChannel) as TChannel; } protected override IAsyncResult OnBeginAcceptChannel(TimeSpan timeout, AsyncCallback callback, object state) { Console.WriteLine("MyChannelListener<TChannel>.OnBeginAcceptChannel()"); return this.InnerChannelListener.BeginAcceptChannel(timeout, callback, state); } protected override TChannel OnEndAcceptChannel(IAsyncResult result) { Console.WriteLine("MyChannelListener<TChannel>.OnEndAcceptChannel()"); TChannel innerChannel = this.InnerChannelListener.EndAcceptChannel(result); return new MyReplyChannel(this, innerChannel as IReplyChannel) as TChannel; } protected override IAsyncResult OnBeginWaitForChannel(TimeSpan timeout, AsyncCallback callback, object state) { Console.WriteLine("MyChannelListener<TChannel>.OnBeginWaitForChannel()"); return this.InnerChannelListener.BeginWaitForChannel(timeout, callback, state); } protected override bool OnEndWaitForChannel(IAsyncResult result) { Console.WriteLine("MyChannelListener<TChannel>.OnEndWaitForChannel()"); return this.InnerChannelListener.EndWaitForChannel(result); } protected override bool OnWaitForChannel(TimeSpan timeout) { Console.WriteLine("MyChannelListener<TChannel>.OnWaitForChannel()"); return this.InnerChannelListener.WaitForChannel(timeout); } public override Uri Uri { get { Console.WriteLine("MyChannelListener<TChannel>.Uri"); return this.InnerChannelListener.Uri; } } protected override void OnAbort() { Console.WriteLine("MyChannelListener<TChannel>.OnAbort()"); this.InnerChannelListener.Abort(); } protected override IAsyncResult OnBeginClose(TimeSpan timeout, AsyncCallback callback, object state) { Console.WriteLine("MyChannelListener<TChannel>.OnBeginClose()"); return this.InnerChannelListener.BeginClose(timeout, callback, state); } protected override IAsyncResult OnBeginOpen(TimeSpan timeout, AsyncCallback callback, object state) { Console.WriteLine("MyChannelListener<TChannel>.OnBeginOpen()"); return this.InnerChannelListener.BeginOpen(timeout, callback, state); } protected override void OnClose(TimeSpan timeout) { Console.WriteLine("MyChannelListener<TChannel>.OnClose()"); this.InnerChannelListener.Close(timeout); } protected override void OnEndClose(IAsyncResult result) { Console.WriteLine("MyChannelListener<TChannel>.OnEndClose()"); this.InnerChannelListener.EndClose(result); } protected override void OnEndOpen(IAsyncResult result) { Console.WriteLine("MyChannelListener<TChannel>.OnEndOpen()"); this.InnerChannelListener.EndOpen(result); } protected override void OnOpen(TimeSpan timeout) { Console.WriteLine("MyChannelListener<TChannel>.OnOpen()"); this.InnerChannelListener.Open(timeout); } }}
5. 创建Custom Binding Element
我们知道Binding是Service mode layer进入Channel layer的中介，而Binding由一系列的Binding element组成。我们上面创建的Channel factory和Channel listener需要最终通过对应的BindingElement应用到Binding中才能最终发挥作用。我们就来创建这个BindingElement：MyBindingElement。
namespace Artech.ChannleStackExplore.Channels{ public class MyBindingElement:BindingElement { public override BindingElement Clone() { return new MyBindingElement(); } public override T GetProperty<T>(BindingContext context) { return context.GetInnerProperty<T>(); } public override IChannelFactory<TChannel> BuildChannelFactory<TChannel>(BindingContext context) { Console.WriteLine("MyBindingElement.BuildChannelFactory()"); return new MyChannelFactory<TChannel>(context) as IChannelFactory<TChannel>; } public override IChannelListener<TChannel> BuildChannelListener<TChannel>(BindingContext context) { Console.WriteLine("MyBindingElement.BuildChannelListener()"); return new MyChannelListener<TChannel>(context) as IChannelListener<TChannel>; } }}
够简单吧，直接调用MyChannelFactory和MyChannelListener的构造函数就可以了。
6. 创建Custom Binding
我们进入了最后的阶段，创建一个Custom Binding。MyBinding继承Binding class。 在CreateBindingElements方法中将我们的Binding element（MyBindingElement），连同其他必须的Binding element添加到BindingElementCollection 中。
namespace Artech.ChannleStackExplore.Channels{ public class MyBinding:Binding { public override BindingElementCollection CreateBindingElements() { BindingElementCollection elemens = new BindingElementCollection(); elemens.Add(new TextMessageEncodingBindingElement()); elemens.Add(new MyBindingElement()); elemens.Add(new HttpTransportBindingElement()); return elemens.Clone(); } public override string Scheme { get { return "http"; } } }}
注：对BindElement的组装可以通过configuration来实现。
7. 使用Custom Binding
我们上面所做的一切都汇集到我们的Custombinding：MyBinding。既然我们为之写了那么多代码，我们一定要通过某种方式测试一下它时候具有我们需要的功能。我们通过MyBinding创建一个Messaging via Binding的应用。不熟悉的朋友可以转到上一篇去熟悉一下。
Server端的代码：
namespace Server{ class Program { static void Main(string[] args) { MyBinding binding = new MyBinding(); IChannelListener<IReplyChannel> channelListener= binding.BuildChannelListener<IReplyChannel>(new Uri("http://127.0.0.1:8888/messagingviabinding")); channelListener.Open(); while (true) { IReplyChannel channel= channelListener.AcceptChannel(TimeSpan.MaxValue); channel.Open(); RequestContext context = channel.ReceiveRequest(TimeSpan.MaxValue); Console.WriteLine("Receive a request message:\n{0}", context.RequestMessage); Message replyMessage = Message.CreateMessage(MessageVersion.Soap12WSAddressing10, "http://artech.messagingviabinding", "This is a mannualy created reply message for the purpose of testing"); context.Reply(replyMessage); channel.Close(); } } }}
Client端的代码：
namespace Client{ class Program { static void Main(string[] args) { MyBinding binding = new MyBinding(); IChannelFactory<IRequestChannel> channelFactory = binding.BuildChannelFactory<IRequestChannel>(); channelFactory.Open(); IRequestChannel channel = channelFactory.CreateChannel(new EndpointAddress("http://127.0.0.1:8888/messagingviabinding")); channel.Open(); Message requestMessage = Message.CreateMessage(MessageVersion.Soap12WSAddressing10, "http://artech.messagingviabinding", "This is a mannualy created reply message for the purpose of testing"); Message replyMessage = channel.Request(requestMessage); Console.WriteLine("Receive a reply message:\n{0}", replyMessage); channel.Close(); channelFactory.Close(); Console.Read(); } }}
运行的结果将会是这样:
Server:
Client:
通过上面的输出结果，你很直观的了解到了整个程序执行过程中，我们的定义在Channel，Channel factory，Channel listener和Binding element的方法是如何被依次执行的。