Indy Query Using Delphi - sockets

Question

I have two TIdTCPClient components (TCPClient1 and TCPClient2) that share the same OnStatus property (see below).
The components are identified by their Tag property. How do I identify the calling TCPClient in the shared OnStatus event?
procedure TForm2.TCPClientStatus(ASender: TObject; const AStatus: TIdStatus;
const AStatusText: string);
begin
end;

Answer 1

In all event handlers, in every component, the Sender argument is always the object instance which is triggering the event.
To get a hand on that object, you can do a simple cast or a safe cast. In the case of the TIdTCPClient you use and the Tag property you want to check, it would become:
TIdTCPClient(Sender).Tag
or
(Sender as TIdTCPClient).Tag
The second form is safer because if, for any reason, Sender is not of the assumed type, you'll get an exception at runtime that you can manage/detect using a try/except construct.
You can even share the same event handler between different kinds of components. I don't know Indy enough to give a related example, so I'll use the OnClick event for a TButton and a TEdit. You can use the is operator to check the type:
if Sender is TButton then
// Do something using TButton(Sender).SomeProperty
else if Sender is TEdit then
// Do something using TEdit(Sender).SomeProperty

Answer 2

The ASender parameter points to the object that is firing the event. For a TIdTCPClient, that sender can be either:
the TIdTCPClient object itself
the client's IOHandler object
In the latter case, the TIdTCPClient.OnStatus event is hooked up to the IOHandler object only if that object is created implicitly within the client's Connect() method when the IOHandler property is nil. In that scenario, the TIdTCPClient is set as the Owner of that new object. On the other hand, if you explicitly assign your own TIdIOHandler-derived component to the IOHandler property before calling Connect(), the TIdTCPClient.OnStatus event will not be hooked up to the IOHandler component automatically, so you will have to assign a handler to that component's own OnStatus event separately, if needed.
So, inside of your TIdTCPClient.OnStatus event handler, assuming an implicit IOHandler object is being used, you can get back to the TIdTCPClient object like this:
procedure TForm2.TCPClientStatus(ASender: TObject; const AStatus: TIdStatus;
const AStatusText: string);
var
Client: TIdTCPClient;
IO: TIdIOHandler;
begin
Client := nil;
if ASender is TIdTCPClient then begin
Client := TIdTCPClient(ASender);
end
else if ASender is TIdIOHandler then begin
IO := TIdIOHandler(ASender);
if IO.Owner is TIdTCPClient then
Client := TIdTCPClient(IO.Owner);
end;
if Client <> nil then begin
// use Client.Tag as needed...
end;
end;