HTTP SSE （Server-Sent Events）流式服务器推送技术
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变乱的功能。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大概出现的消息类型：
变乱消息（Event Message）：服务器发送的一般变乱消息，可以包含自定义的变乱类型和数据。
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，但不会触发任何变乱处理。
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。
下面是一个完备的HTTP请求返回结果示例，展示了SSE的利用：
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相应头中指定了Content-Type为text/event-stream，表示这是一个SSE的相应。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，服务器通过多个变乱消息和注释消息向客户端发送数据。每个消息以event字段指定变乱类型，以data字段包含消息的数据。在示例中，服务器发送了两个变乱消息，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变乱，实时吸取服务器发送的消息，并进行相应的处理。
下面是一个利用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;
/// <summary>
/// 解析SSE 。
/// </summary>
public class EventStreamHandler
{
/// <summary>
/// 新的换行符。
/// </summary>
private readonly string newLine;
/// <summary>
/// 消息体与消息体之间的空白行的数量。
/// </summary>
private const int emptySplitLineCount = 1;
/// <summary>
/// 最后1个消息体与流结束符的空白行的数量。
/// </summary>
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;
/// <summary>
/// 当前行。
/// </summary>
private string line;
/// <summary>
/// 是不是处理了当前行 <see cref="line"/>。
/// </summary>
private bool isHandledLine;
/// <summary>
/// 消息体和消息体中间的未知行。
/// </summary>
private readonly List<string> unknownLines = new List<string>(16);
/// <summary>
/// 消息体列表。
/// </summary>
private readonly List<EventStreamEventArgs> addedItems = new List<EventStreamEventArgs>(64);
public EventStreamHandler(string newLine)
{
this.newLine = newLine;
}
/// <summary>
/// 消息体列表。
/// </summary>
public List<EventStreamEventArgs> AddedItems
{
get { return addedItems; }
}
protected string DataSource { get { return dataSource; } }
protected bool IsHandledLine { get { return isHandledLine; } }
/// <summary>
/// 测试代码。
/// </summary>
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}");
}
/// <summary>
/// 解析SSE 。
/// </summary>
/// <param name="streamReader"></param>
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<string> unknownLines)
{
if (unknownLines.Count > 0)
{
unknownLines.Clear();
}
}
/// <summary>
/// 是不是匹配了，消息体与消息体之间的分割特征。
/// </summary>
/// <param name="lines"></param>
/// <param name="emptySplitLineCount"></param>
/// <returns></returns>
private static bool MatchEmptySplitLine(List<string> 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))
{
isMatch = false;
break;
}
}
}
return isMatch;
}
/// <summary>
/// 末尾连续的空白行数。
/// </summary>
/// <param name="lines"></param>
/// <returns></returns>
private static int GetEndEmptyLineCount(List<string> lines)
{
int count = 0;
for (int index = lines.Count - 1; index >= 0 && string.IsNullOrEmpty(lines); index--)
{
count++;
}
return count;
}
private string GetUnknownValue(int splitLineCount, List<string> 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++)
{
builder.Append(newLine);
var unknownLine = unknownLines;
if (!string.IsNullOrEmpty(unknownLine))
{
builder.Append(unknownLine);
}
}
followingValue = builder.ToString();
}
return followingValue;
}
private void AddUnknownItem(List<EventStreamEventArgs> addedItems, string unknownValue)
{
var item = new EventStreamEventArgs(unknownValue);
AddItem(addedItems, item);
}
private void AddUnknownItem(List<EventStreamEventArgs> addedItems, int splitLineCount, List<string> unknownLines)
{
var unknownValue = GetUnknownValue(splitLineCount, unknownLines);
if (!string.IsNullOrEmpty(unknownValue))
{
AddUnknownItem(addedItems, unknownValue);
}
}
private void AddItem(List<EventStreamEventArgs> addedItems, string idSource, string eventSource, string dataSource)
{
var item = new EventStreamEventArgs(idSource, eventSource, dataSource);
AddItem(addedItems, item);
}
private void AddItem(List<EventStreamEventArgs> addedItems, EventStreamEventArgs item)
{
addedItems.Add(item);
}
#endregion
}
/// <summary>
/// 一个消息体。
/// </summary>
public class EventStreamEventArgs : EventArgs
{
internal const string IdBegin = "id: ";
internal const string EventBegin = "event: ";
internal const string DataBegin = "data: ";
/// <summary>
/// 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
/// </summary>
internal const string RetryBegin = "retry: ";
private readonly string idSource;
private readonly string eventSource;
private readonly string dataSource;
private string idValue;
private string eventValue;
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;
}
}
/// <summary>
/// 上1个 <see cref="EventStreamEventArgs"/> 中的 <see cref="DataValue"/> 后面跟随的未知内容。
/// </summary>
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;
}
}
}
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