HTTP SSE （Server-Sent Events）流式服务器推送技术

SSE（Server-Sent Events）是一种基于HTTP协议的服务器推送技术，用于实现服务器向客户端实时发送事件的功能。在SSE中，服务器可以发送不同类型的消息给客户端。

以下是SSE可能出现的消息类型：

事件消息（Event Message）：服务器发送的一般事件消息，可以包含自定义的事件类型和数据。

注释消息（Comment Message）：服务器发送的注释消息，用于向客户端传递一些额外的信息，但不会触发任何事件处理。

重连消息（Reconnect Message）：服务器发送的重连消息，用于告知客户端重新连接服务器。

下面是一个完整的HTTP请求返回结果示例，展示了SSE的使用：

 HTTP/1.1 200 OK Content-Type: text/event-stream Cache-Control: no-cache Connection: keep-alive  id: msgid event: myEvent data: {"message": "Hello, SSE!"}  id: msgid event: myEvent data: {"message": "Another event"}  : This is a comment  id: msgid event: reconnect retry: 5000  

在这个示例中，服务器返回的HTTP响应头中指定了Content-Type为text/event-stream，表示这是一个SSE的响应。Cache-Control设置为no-cache，确保每次请求都能从服务器获取最新的数据。Connection设置为keep-alive，保持长连接。

接下来，服务器通过多个事件消息和注释消息向客户端发送数据。每个消息以event字段指定事件类型，以data字段包含消息的数据。在示例中，服务器发送了两个事件消息，分别是myEvent类型的消息，包含不同的数据。还发送了一个注释消息，以冒号开头，用于传递额外的信息。

最后，服务器发送了一个重连消息，以event字段指定为reconnect类型，同时通过retry字段指定了重连的时间间隔为5000毫秒。

客户端可以通过监听SSE事件，实时接收服务器发送的消息，并进行相应的处理。

下面是一个使用C#解析SSE示例，展示了SSE的解析过程：

  namespace System.Net {     using System;     using System.Collections.Generic;     using System.IO;     using System.Text;      /// 
     /// 解析SSE 。     /// 
     public class EventStreamHandler     {         /// 
         /// 新的换行符。         /// 
         private readonly string newLine;         /// 
         /// 消息体与消息体之间的空白行的数量。         /// 
         private const int emptySplitLineCount = 1;         /// 
         /// 最后1个消息体与流结束符的空白行的数量。         /// 
         private readonly static int emptySplitLineCountEnd = 1;          private string idSource;         private string eventSource;         private string dataSource;         // 是不是可能有未知项（消息体与消息体之间的未知项）。         private bool possibleUnknownItem;         /// 
         /// 当前行。         /// 
         private string line;         /// 
         /// 是不是处理了当前行 。         /// 
         private bool isHandledLine;         /// 
         /// 消息体和消息体中间的未知行。         /// 
         private readonly List unknownLines = new List(16);         /// 
         /// 消息体列表。         /// 
         private readonly List addedItems = new List(64);          public EventStreamHandler(string newLine)         {             this.newLine = newLine;         }          /// 
         /// 消息体列表。         /// 
         public List AddedItems         {             get { return addedItems; }         }          protected string DataSource { get { return dataSource; } }          protected bool IsHandledLine { get { return isHandledLine; } }          /// 
         /// 测试代码。         /// 
         [System.Diagnostics.Conditional("DEBUG")]         public static void Test()         {             var builder = new StringBuilder(64);             builder.AppendLine("id: msgid");             builder.AppendLine("event: myEvent");             builder.AppendLine("data: Hello, SSE!");             builder.AppendLine();             builder.AppendLine("id: msgid");             builder.AppendLine("event: myEvent");             builder.AppendLine("data: Another event");             builder.AppendLine();             builder.AppendLine(": This is a comment");             builder.AppendLine();             builder.AppendLine("id: msgid");             builder.AppendLine("event: reconnect");             builder.AppendLine("retry: 5000");             builder.AppendLine();             builder.AppendLine("id: msgid end");             builder.AppendLine("event: myEvent end");             builder.AppendLine("data: Another event end");             builder.AppendLine();             var bytes = Encoding.UTF8.GetBytes(builder.ToString());             var stream = new MemoryStream(bytes);             var reader = new StreamReader(stream, Encoding.UTF8);             var handler = new EventStreamHandler("\r\n");             handler.HandleStreamReader(reader);             var messageItems = handler.AddedItems;             System.Diagnostics.Trace.WriteLine($"Message items count {messageItems.Count}");         }          /// 
         /// 解析SSE 。         /// 
         ///          public void HandleStreamReader(StreamReader streamReader)         {             while (!streamReader.EndOfStream)             {                 line = streamReader.ReadLine();                 isHandledLine = false;                 if (!string.IsNullOrEmpty(line))                 {                     if (line.StartsWith(EventStreamEventArgs.IdBegin))                     {                         HandleForIdBegin();                     }                     else if (line.StartsWith(EventStreamEventArgs.EventBegin))                     {                         HandleForEventBegin();                     }                     else if (line.StartsWith(EventStreamEventArgs.DataBegin))                     {                         HandleForDataBegin();                     }                     else if (line.StartsWith(EventStreamEventArgs.RetryBegin))                     {                         HandleForRetryBegin();                     }                 }                 if (!isHandledLine)                 {                     unknownLines.Add(line);                 }             }              HandleUnknownItemWhenEnd();         }          #region 解析各种消息。          private void HandleForIdBegin()         {             // 当前行是不是用来传输 id: 信息的行。             var isIdLine = HandleCheckIdLine();             if (isIdLine)             {                 idSource = line;                 isHandledLine = true;             }         }          private bool HandleCheckIdLine()         {             // 当前行是不是用来传输 id: 信息的行。             bool isIdLine = false;              // 如果是第1次遇到 idSource 。移除前面遇到的未知行。             if (addedItems.Count < 1)             {                 ClearIfNotEmpty(unknownLines);                 isIdLine = true;             }             // 如果遇到了，上1个消息体结束的特征。             else if (MatchEmptySplitLine(unknownLines, emptySplitLineCount))             {                 HandleUnknownItemWhenMiddle();                  isIdLine = true;             }              return isIdLine;         }          private void HandleForEventBegin()         {             ClearIfNotEmpty(unknownLines);              eventSource = line;             isHandledLine = true;         }          private void HandleForDataBegin()         {             ClearIfNotEmpty(unknownLines);              dataSource = line;             isHandledLine = true;              AddItem(addedItems, idSource, eventSource, dataSource);             possibleUnknownItem = true;         }          private void HandleForRetryBegin()         {             ClearIfNotEmpty(unknownLines);              isHandledLine = true;             possibleUnknownItem = true;         }          private void HandleUnknownItemWhenMiddle()         {             if (possibleUnknownItem)             {                 // 如果遇到消息体与消息体之间，插入了，额外的未知内容时。                 AddUnknownItem(addedItems, emptySplitLineCount, unknownLines);                 possibleUnknownItem = false;                 ClearIfNotEmpty(unknownLines);             }         }          private void HandleUnknownItemWhenEnd()         {             if (possibleUnknownItem)             {                 var emptySplitLineCountEndNew = (emptySplitLineCountEnd == 0) ? 0 :                     Math.Min(emptySplitLineCountEnd, GetEndEmptyLineCount(unknownLines));                 AddUnknownItem(addedItems, emptySplitLineCountEndNew, unknownLines);                 possibleUnknownItem = false;                 ClearIfNotEmpty(unknownLines);             }         }          #endregion          #region 帮助函数。          private static void ClearIfNotEmpty(List unknownLines)         {             if (unknownLines.Count > 0)             {                 unknownLines.Clear();             }         }          /// 
         /// 是不是匹配了，消息体与消息体之间的分割特征。         /// 
         ///          ///          ///          private static bool MatchEmptySplitLine(List lines, int emptySplitLineCount)         {             bool isMatch = lines.Count >= emptySplitLineCount;             if (isMatch && emptySplitLineCount > 0)             {                 int minIndex = lines.Count - emptySplitLineCount;                 for (int index = lines.Count - 1; index >= minIndex; index--)                 {                     if (!string.IsNullOrEmpty(lines[index]))                     {                         isMatch = false;                         break;                     }                 }             }             return isMatch;         }          /// 
         /// 末尾连续的空白行数。         /// 
         ///          ///          private static int GetEndEmptyLineCount(List lines)         {             int count = 0;             for (int index = lines.Count - 1; index >= 0 && string.IsNullOrEmpty(lines[index]); index--)             {                 count++;             }             return count;         }          private string GetUnknownValue(int splitLineCount, List unknownLines)         {             string followingValue = string.Empty;             if (unknownLines.Count > splitLineCount)             {                 var addLine = unknownLines.Count - splitLineCount;                 StringBuilder builder = new StringBuilder(64);                 for (int index = 0; index < addLine; index++)                 {                     var unknownLine = unknownLines[index];                     if (!string.IsNullOrEmpty(unknownLine))                     {                         builder.Append(unknownLine);                     }                     builder.Append(newLine);                     followingValue = builder.ToString();                 }             }             return followingValue;         }          private void AddUnknownItem(List addedItems, string unknownValue)         {             var item = new EventStreamEventArgs(unknownValue);             AddItem(addedItems, item);         }          private void AddUnknownItem(List addedItems, int splitLineCount, List unknownLines)         {             var unknownValue = GetUnknownValue(splitLineCount, unknownLines);             if (!string.IsNullOrEmpty(unknownValue))             {                 AddUnknownItem(addedItems, unknownValue);             }         }          private void AddItem(List addedItems, string idSource, string eventSource, string dataSource)         {             var item = new EventStreamEventArgs(idSource, eventSource, dataSource);             AddItem(addedItems, item);         }          private void AddItem(List addedItems, EventStreamEventArgs item)         {             addedItems.Add(item);         }          #endregion     }      /// 
     /// 一个消息体。     /// 
     public class EventStreamEventArgs : EventArgs     {         internal const string IdBegin = "id: ";         internal const string EventBegin = "event: ";         internal const string DataBegin = "data: ";         /// 
         /// 重连消息（Reconnect Message）         /// 服务器发送的重连消息，用于告知客户端重新连接服务器。         /// 示例：         /// event: reconnect         /// retry: 5000         /// 
         internal const string RetryBegin = "retry: ";         [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)]         private readonly string idSource;         [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)]         private readonly string eventSource;         [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)]         private readonly string dataSource;         [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)]         private string idValue;         [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)]         private string eventValue;         [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)]         private string dataValue;          public EventStreamEventArgs(string idSource, string eventSource, string dataSource)         {             this.idSource = idSource;             this.eventSource = eventSource;             this.dataSource = dataSource;         }          public EventStreamEventArgs(string unknownValue)         {             this.UnknownValue = unknownValue;         }          public string IdValue         {             get             {                 if (idValue == null) idValue = GetValue(idSource, IdBegin);                 return idValue;             }         }          public string EventValue         {             get             {                 if (eventValue == null) eventValue = GetValue(eventSource, EventBegin);                 return eventValue;             }         }          public string DataValue         {             get             {                 if (dataValue == null) dataValue = GetValue(dataSource, DataBegin);                 return dataValue;             }         }          /// 
         /// 上1个  中的  后面跟随的未知内容。         /// 
         public string UnknownValue         {             get;             private set;         }          private string GetValue(string source, string begin)         {             string value;             if (source != null && source.Length > begin.Length)             {                 value = source.Substring(begin.Length, source.Length - begin.Length);             }             else             {                 value = string.Empty;             }             return value;         }     } }