深入探索Silverlight网络通信：从策略服务器到本地连接

### 深入探索Silverlight网络通信：从策略服务器到本地连接 #### 1. 策略服务器相关 策略服务器在Silverlight网络通信中扮演着重要角色。当策略服务器停止时，会执行如下代码： ```vb policyServer.Stop() Console.WriteLine("Policy server shut down.") End Sub ``` 策略连接类（PolicyConnection）的任务相对简单。创建时，它会存储对策略文件数据的引用。当调用`HandleRequest()`方法时，代码会访问新连接的网络流并尝试从中读取数据。如果一切正常，客户端会发送包含“<policy-file-request/>”文本的字符串。读取该字符串后，客户端会将策略数据写入该流并关闭连接。以下是完整代码： ```vb Public Class PolicyConnection Private client As TcpClient Private policy As Byte() Public Sub New(ByVal client As TcpClient, ByVal policy As Byte()) Me.client = client Me.policy = policy End Sub ' The request that the client sends. Private Shared policyRequestString As String = "<policy-file-request/>" Public Sub HandleRequest() Dim s As Stream = client.GetStream() ' Read the policy request string. ' This code doesn't actually check the content of the request string. ' Instead, it returns the policy for every request. Dim buffer As Byte() = New Byte(policyRequestString.Length - 1){} ' Only wait 5 seconds. That way, if you attempt to read the request string ' and it isn't there or it's incomplete, the client only waits for 5 ' seconds before timing out. client.ReceiveTimeout = 5000 s.Read(buffer, 0, buffer.Length) ' Send the policy. s.Write(policy, 0, policy.Length) ' Close the connection. client.Close() Console.WriteLine("Served policy file.") End Sub End Class ``` 不过，目前还无法测试策略服务器，因为Silverlight不允许显式请求策略文件，而是在尝试使用基于套接字的应用程序时自动请求。在构建基于套接字应用程序的客户端之前，需要先构建服务器。 #### 2. 消息服务器 消息服务器可以作为单独的应用程序创建，但将其与策略服务器放在同一个应用程序中会更整洁。因为策略服务器在单独的线程上进行监听和请求处理工作，所以消息服务器可以同时工作。 消息服务器和策略服务器类似，也分为两个类：`MessengerServer`用于监听请求并跟踪客户端，`MessengerConnection`用于处理单个客户端的交互。 消息服务器与策略服务器有以下不同点： - **监听端口**：Silverlight允许基于套接字的应用程序使用4502到4534之间的有限端口范围，消息服务器使用端口4530，代码如下： ```vb listener = New TcpListener(IPAddress.Any, 4530) ``` - **处理连接请求**：当消息服务器收到连接请求时，除了创建一个新类的实例（`MessengerConnection`）来处理通信外，还会将客户端添加到一个集合中，以便跟踪所有当前连接的用户。相关代码如下： ```vb Private clientNum As Integer Private clients As New List(Of MessengerConnection) clientNum += 1 Console.WriteLine("Messenger client #" & clientNum.ToString() & " connected.") ' Create a new object to handle this connection. Dim clientHandler As New MessengerConnection(client, "Client " & _ clientNum.ToString(), Me) clientHandler.Start() SyncLock clients clients.Add(clientHandler) End SyncLock ``` 由于可能有多个客户端同时连接，在添加客户端之前需要锁定`clients`集合，以避免线程错误。当消息服务器停止时，会遍历整个集合并确保每个客户端都断开连接： ```vb For Each client As MessengerConnection In clients client.Close() Next ``` 消息服务器目前还缺少消息传递功能，即一个客户端提交消息并将其传递给所有客户端的能力。要实现此功能，需要在`MessengerConnection`类中处理消息提交，在`MessengerServer`类中处理消息传递。 当`MessengerConnection`对象创建并调用其`Start()`方法时，它会开始监听任何数据： ```vb Public Sub Start() Try ' Listen for messages. client.Client.BeginReceive(message, 0, message.Length, SocketFlags.None, _ New AsyncCallback(AddressOf OnDataReceived), Nothing) Catch se As SocketException Console.WriteLine(se.Message) End Try End Sub ``` 当客户端发送数据时，`OnDataReceived()`回调会被触发。它会逐字节读取数据，直到获取客户端发送的所有信息，然后将数据传递给`MessengerServer.Deliver()`方法，并开始监听下一条消息： ```vb Public Sub OnDataReceived(ByVal asyn As IAsyncResult) Try Dim bytesRead As Integer = client.Client.EndReceive(asyn) If bytesRead > 0 Then ' Ask the server to send the message to all the clients. server.DeliverMessage(message, bytesRead) ' Listen for more messages. client.Client.BeginReceive(message, 0, message.Length, _ SocketFlags.None, New AsyncCallback(AddressOf OnDataReceived), _ Nothing) End If Catch err As Exception Console.WriteLine(err.Message) End Try End Sub ``` `MessengerServer.DeliverMessage()`方法会遍历客户端集合，并调用每个客户端的`ReceiveMessage()`方法来传递通信内容。为避免线程问题，该方法会先创建集合的快照副本，然后使用该副本传递消息： ```vb Public Sub DeliverMessage(ByVal message As Byte(), ByVal bytesRead As Integer) Console.WriteLine("Delivering message.") ' Duplicate the collection to prevent threading issues. Dim connectedClients As MessengerConnection() SyncLock clients connectedClients = clients.ToArray() End SyncLock For Each client As MessengerConnection In connectedClients Try client.ReceiveMessage(message, bytesRead) Catch ' Client is disconnected. ' Remove the client to avoid future attempts. SyncLock clients clients.Remove(client) End SyncLock client.Close() End Try Next End Sub ``` `MessengerConnection.ReceiveMessage()`方法会将消息数据写回网络流，以便客户端可以接收： ```vb Public Sub ReceiveMessage(ByVal data As Byte(), ByVal bytesRead As Integer) client.GetStream().Write(data, 0, bytesRead) End Sub ``` 最后，需要修改启动代码，使应用程序同时创建并启动策略服务器和消息服务器： ```vb Shared Sub Main(ByVal args As String()) ```