Communicating between your applications

Problem/Question/Abstract:

I want to perform communication between two my applications or between two instances of my application.

Answer:

You can perform communication between your application using Windows messages exchange mechanism. We can use HWND_BROADCAST value for first parameter for SendMessage function for suppressing finding of forms' in other applications HANDLE.
For using HWND_BROADCAST we should register our messages in Windows.

For performing this you could make the following:

(In example below we will inform about our form's top position)

1. Define type of your message structure, it could be something like this:

type
  TWMMYMessage = record
    Msg: Cardinal; // ( first is the message ID )
    Handle: HWND; // ( this is the wParam, Handle of sender)
    Info: LongInt; // ( this is lParam, pointer to our data)
    Result: LongInt;
  end;

2. Override your form's DefaultHandler method and add method for handling your message, like this

TForm1 = class(TForm)
  {... }
public
  { Public declarations }
   {... }
  procedure DefaultHandler(var Message); override;
  procedure WMMYMessage(var Msg: TWMMYMessage);
  {... }
end;

3. Declare message variable:

var
  WM_OURMESSAGE: DWORD;

4. Insert realisation of DefaultHandler and our message handler methods:

procedure TForm1.DefaultHandler(var Message);
var
  ee: TWMMYMessage;
begin
  with TMessage(Message) do
  begin
    if (Msg = WM_OURMESSAGE) then
    begin
      ee.Msg := Msg;
      ee.Handle := wParam;
      ee.Info := lParam;
      //      Checking if this message is not from us
      if ee.Handle <> Handle then
        WMMYMessage(ee);
    end
    else
      inherited DefaultHandler(Message);
  end;
end;

procedure TForm1.WMMYMessage(var Msg: TWMMYMessage);
begin
  Label1.Caption := Format('Our another form handle :%d', [Msg.Handle]);
  Label2.Caption := Format('Our another form top :%d', [Msg.Info]);
end;

5. Add registration of your message that you could handle the HWND_BROADCAST messages:

initialization
  WM_OURMESSAGE := RegisterWindowMessage('Our broadcast message');

6. Add the message sending somewhere:

procedure TForm1.Button1Click(Sender: TObject);
begin
  SendMessage(HWND_BROADCAST, WM_OURMESSAGE, Handle, Top);
end;

7. Compile and run two copies of your application and test it functionality.