How to intercept user defined message twice?

Question

I'm using a component that internally has a KeyDown handler, which sends a user defined PostMessage(WM_GROUPUNGROUP), and also has a custom message handler to handle WM_GROUPUNGROUP.

I want my app to do something after this message handler has executed, without modifying the component code.

(How) Can this be done?

Answer 1

One way to achieve this is via the WindowProc property.

Simply supply your own window procedure by assigning to WindowProc on the instance you want to hook. You'll need to take a copy to the previous value of WindowProc so that you can make sure that the original handling is carried out.

Roughly it goes like this:

type
  TMyClass = class
  ....
    FOldWindowProc: TWndMethod;
    procedure NewWindowProc(var Message: TMessage);
  ....
  end;

To redirect the window procedure you do this:

FOldWindowProc := SomeControl.WindowProc;
SomeControl.WindowProc := NewWindowProc;

Then implement the new window procedure like this:

procedure TMyClass.NewWindowProc(var Message: TMessage);
begin
  FOldWindowProc(Message);
  if Message.Msg = WM_GROUPUNGROUP then 
    ....
end;

When you are done with the control, put the old window procedure back in place:

SomeControl.WindowProc := FOldWindowProc;

Another way to do it is to take advantage of the fact that the message is queued. You can add an Application.OnMessage handler, most likely by using a TApplicationEvents object. This will get a look at all queued messages. However, OnMessage fires before the message is dispatched to the control which sounds like it may be the wrong way round for you.