Seeking very simple TIdCommandHandler conditional response example - delphi

Question

Delphi XE2, so Indy 10.
My client sends a command which is processed by a TIdCommandHandler of my TIdCmdTCPServer.
I want to be able to perform some logic and return either a success or fail response and check for that back at the client.
Can someone please point me at a few lines of code as an example? Thanks in advance.

Answer 1

Well, here's the simplest demo.
Add an IdCmdTCPServer to your form, and add one command, set its name in the Command property, I originally thought I should handle Response in OnCommand event like this:
procedure TForm1.IdCmdTCPServer1CommandHandlers0Command(ASender: TIdCommand);
begin
//ASender.Response.Add('Hello'); // wrong way
ASender.Reply.SetReply(0,'HELLO');
end;
Update Remy pointed out I shouldn't be using Response.
So you want to return success or failure, it's traditional to use a numeric result followed by the string value. Each string in the response strings list has an implied end-of-line transmitted back to the client:
procedure TForm1.IdCmdTCPServer1CommandHandlers0Command(ASender: TIdCommand);
begin
if DoSomething then
ASender.Reply.SetReply(0,'OK')
else
ASender.Reply.SetReply(999,'ERROR');
end;
The idea with the IdCommandHandler and a CmdTCPServer/Client is that you follow the "RFC" style of protocols, which are ANSI/ASCII text-based. An RFC-style internet protocol's reply is typically encoded over the wire as text with both a numeric and string value. ASender.Response could be used if you needed to take the content of a string list and return that as the response.
As for the client, a question here suggests that TIdCmdTcpClient is not the most natural way to build the client for this server. From their names, you'd have thought they were made for each other, but it's not exactly. For most simple TIdCMDTCPServers that you could build, you would find that a plain-vanilla TIdTCPClient is the simplest building block to start your client with.