Catch WM_COPYDATA from Delphi component - delphi

Question

I'm trying to write a component, to send string messages between applications by WM_COPYDATA.
I'd like trap the WM_COPYDATA, but this doesn't work:
TMyMessage = class(TComponent)
private
{ Private declarations }
…
protected
{ Protected declarations }
…
procedure WMCopyData(var Msg : TMessage); message WM_COPYDATA;
…
end;
Searching Google a lot, found some reference using wndproc. I tried it, but it isn't working either.
TMyMessage = class(TComponent)
…
protected
{ Protected declarations }
…
procedure WMCopyData(var Msg : TMessage); message WM_COPYDATA;
procedure WndProc(var Msg: TMessage);
…
end;
…
procedure TMyMessage.WndProc(var Msg: TMessage);
begin
//inherited;
if Msg.Msg = WM_COPYDATA then
WMCopyData(Msg);
end;
Please help, what is wrong?

Answer 1

What you have so far is fine, but you need to arrange for messages to be delivered to your component in the first place. That requires a window handle. Call AllocateHWnd and pass it your component's WndProc method. It will return a window handle, which you should destroy as your component is destroyed.
constructor TMyMessage.Create(AOwner: TComponent);
begin
inhreited;
FHandle := AllocateHWnd(WndProc);
end;
destructor TMyMessage.Destroy;
begin
DeallocateHWnd(FHandle);
inherited;
end;
Rather than testing for each message directly, you can let TObject do that for you. That's what the Dispatch method is for. Pass it a TMessage record, and it will find and call the corresponding message-handler method for you. If there is no such handler, it will call DefaultHandler instead. Override that can call DefWindowProc.
procedure TMyMessage.WndProc(var Message);
begin
Dispatch(Message);
end;
procedure TMyMessage.DefaultHandler(var Message);
begin
TMessage(Message).Result := DefWindowProc(Self.Handle, TMessage(Message).Msg,
TMessage(Message).WParam, TMessage(Message).LParam);
end;

Answer 2

Your problem is that TComponent is not a windowed component. WM_COPYDATA is a windows message and is delivered via a window procedure. Hence you need a window handle. Use AllocateHwnd to get hold of one of these.
type
TMyComponent = class(TComponent)
private
FWindowHandle: HWND;
procedure WndProc(var Msg: TMessage);
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
end;
constructor TMyComponent.Create(AOwner: TComponent);
begin
inherited;
FWindowHandle := AllocateHwnd(WndProc);
end;
destructor TMyComponent.Destroy;
begin
DeallocateHwnd(FWindowHandle);
inherited;
end;
procedure TMyComponent.WndProc(var Msg: TMessage);
begin
if Msg.Msg=WM_COPYDATA then
//do domething
else
Msg.Result := DefWindowProc(FWindowHandle, Msg.Msg, Msg.WParam, Msg.LParam);
end;
Whatever is sending the messages will need to find a way to get hold of the window handle.

Answer 3

I did it this way:
My web modules which are running in a thread need to send strings to a memo on the main form.
FReceiverFromWS is a THandle
On create:
procedure TWebModuleWebServices.WebModuleCreate(Sender: TObject);
begin
FReceiverFromWS := FindWindow(PChar('TFormWebServices'),PChar(cFormCaption + FormWebServices.Instance)); // Search by class name and caption of receiving form
// ==> you could to that without form caption, but I need to distinguish between running services
if FReceiverFromWS = 0 then
begin
Assert(False,'CopyData receiver NOT found!'); // Probably TFormWebServices not yet created
Exit;
end;
end;
To send messages:
procedure TWebModuleWebServices.SendAMessage(Msg: String);
// Windows will guarantee that the data sent in the COPYDATASTRUCT will exist until after the WM_COPYDATA message
// has been carried out. As such, we must use SendMessage() to send a WM_COPYDATA message. We cannot use PostMessage().
var
lCopyDataStruct: TCopyDataStruct;
begin
lCopyDataStruct.dwData := 0;
lCopyDataStruct.cbData := 1 + Length(Msg);
lCopyDataStruct.lpData := PChar(Msg);
SendMessage(FReceiverFromWS, WM_COPYDATA, wParam(FReceiverFromWS), lParam(#lCopyDataStruct));
end;
In the main form, public method
procedure WMCopyData(var Msg : TWMCopyData) ; message WM_COPYDATA;
is:
procedure TFormWebServices.WMCopyData(var Msg: TWMCopyData);
var
i : integer;
s : string;
begin
i := Msg.CopyDataStruct.dwData;
case i of
0: begin // Message to display
s := String(PChar(Msg.CopyDataStruct.lpData));
AddMemoLine(s);
end;
1: begin // Statistical data
s := String(PChar(Msg.CopyDataStruct.lpData));
FrmWebServiceStats.CollectStats(s);
end;
end;
end;
(As you can see, I actually use dwData to signal the kind of message and handle these differently)