//**********************************************************************************
//说明: 阻塞线程下为什么不触发OnRead和OnWrite事件
//作者: licwing          时间: 2001-5-18
//Email: rurality@21cn.com
//**********************************************************************************
1.首先先分析TServerSocket的继承关系
  TAbstractSocket->TCustomSocket->TCustomServerSocket->TServerSocket
属性 ServerType 是在 TCustomServerSocket 里面定义的
  property ServerType: TServerType read GetServerType write SetServerType;
同时在TCustomServerSocket里定义了TServerWinSocket,用来处理客户连接
TServerWinSocket的继承关系如下:
  TCustomWinSocket->TServerWinSocket
也就是说我们在TServerSocket里面触发的OnRead,OnWrite事件事实上是由TServerSocket
的父类TCustomServerSocket中的TServerWinSocket触发的。

2.现在我们分析在TCustomServerSocket里定义的TServerWinSocket客户连接方法
procedure TServerWinSocket.Accept(Socket: TSocket);
var
  ClientSocket: TServerClientWinSocket;
  ClientWinSocket: TSocket;
  Addr: TSockAddrIn;
  Len: Integer;
  OldOpenType, NewOpenType: Integer;
begin
  Len := SizeOf(OldOpenType);
  if getsockopt(INVALID_SOCKET, SOL_SOCKET, SO_OPENTYPE, PChar(@OldOpenType),
    Len) = 0 then
  //从这里开始是处理多线程的
  try
    if FServerType = stThreadBlocking then
    begin
      //  SO_SYNCHRONOUS_NONALERT = $20 在winsock单元里面定义 
      //{$EXTERNALSYM SO_SYNCHRONOUS_NONALERT}
      NewOpenType := SO_SYNCHRONOUS_NONALERT;  
      setsockopt(INVALID_SOCKET, SOL_SOCKET, SO_OPENTYPE, PChar(@NewOpenType), Len);
    end;
    Len := SizeOf(Addr);
    ClientWinSocket := WinSock.accept(Socket, @Addr, @Len);
    if ClientWinSocket <> INVALID_SOCKET then
    begin
      ClientSocket := GetClientSocket(ClientWinSocket);
      if Assigned(FOnSocketEvent) then
        FOnSocketEvent(Self, ClientSocket, seAccept);
      if FServerType = stThreadBlocking then
      begin
        //********多线程事件响应关键部分*******
//TAsyncStyle = (asRead, asWrite, asOOB, asAccept, asConnect, asClose);
//asRead The socket receives notification that the connection is ready for reading.
        //asWrite The socket receives notification that the connection is ready for writing.
        ClientSocket.ASyncStyles := [];
//*************************************
        GetServerThread(ClientSocket);
      end;
    end;
  finally
    Len := SizeOf(OldOpenType);
    setsockopt(INVALID_SOCKET, SOL_SOCKET, SO_OPENTYPE, PChar(@OldOpenType), Len);
  end;
end;

3.ASyncStyles属性只有在TCustomWinSocke类里面定义的，再看SetAsyncStyles过程
procedure TCustomWinSocket.DoSetAsyncStyles;
var
  Msg: Integer;
  Wnd: HWnd;
  Blocking: Longint;
begin
  Msg := 0;
  Wnd := 0;
  if FAsyncStyles <> [] then  //非阻塞模式
  begin
    Msg := CM_SOCKETMESSAGE;  //由消息CM_SOCKETMESSAGE触发的过程是过程
                              //TCustomWinSocket.CMSocketMessage(var Message: TCMSocketMessage);
    Wnd := Handle;
  end;
  WSAAsyncSelect(FSocket, Wnd, Msg, Longint(Byte(FAsyncStyles))); //非阻塞模式处理
  if FASyncStyles = [] then   //阻塞模式
  begin
    Blocking := 0;
    ioctlsocket(FSocket, FIONBIO, Blocking);//阻塞模式处理
  end;
end;

4.再看由消息CM_SOCKETMESSAGE触发的过程
procedure TCustomWinSocket.CMSocketMessage(var Message: TCMSocketMessage);
begin
  with Message do
    if CheckError then
      case SelectEvent of
        FD_CONNECT: Connect(Socket);
        FD_CLOSE: Disconnect(Socket);
        FD_READ: Read(Socket);              //触发Read事件
        FD_WRITE: Write(Socket);            //触发Write事件
        FD_ACCEPT: Accept(Socket);
      end;
end;

现在应该明白为什么在阻塞模式下不触发OnRead和OnWrite事件了吧？