How to intercept and suppress a message for a TFrame's subcomponent?

Question

I need to intercept the WM_PASTE message for a TEdit component which is placed inside a TFrame's descendant class.

If a condition is not satisfied, I want to iniby the paste operation.

Is there a way to do this at the frame level? (I mean, without declaring a TEdit's descendant)

Answer 1

Is there a way to do this at the frame level? (I mean, without declaring a TEdit's descendant)

WM_PASTE is sent directly to the TEdit window, the TFrame never sees it, so you must subclass the TEdit directly in order to intercept the message. You can either:

• have the TFrame assign a handler to the TEdit's WindowProc property. This is a simple approach if you have only a few TEdits to subclass, but it gets more complicated the more TEdits you want to subclass:

  type
    TMyFrame = class(TFrame)
      Edit1: TEdit;
      ...
      procedure FrameCreate(Sender: TObject);
      ...
    private
      PrevWndProc: TWndMethod;
      procedure EditWndProc(var Message: TMessage);
      ...
    end;
  
  procedure TMyFrame.FrameCreate(Sender: TObject);
  begin
    PrevWndProc := Edit1.WindowProc;
    Edit1.WindowProc := EditWndProc;
    ...
  end;
  
  procedure TMyFrame.EditWndProc(var Message: TMessage);
  begin
    if Message.Msg = WM_PASTE then
    begin
      if SomeCondition then
        Exit;
    end;
    PrevWndProc(Message);
  end;
  

• write and install a new component that is derived from TEdit, similar to the TMemo example you presented.

• define an interposer class that is local to just the TFrame's unit, above the TFrame class declaration, which will intercept WM_PASTE for every TEdit on the Frame:

  type
    TEdit = class(Vcl.StdCtrls.TEdit)
      procedure WMPaste(var Message: TMessage); message WM_PASTE;
    end;
  
    TMyFrame = class(TFrame)
      Edit1: TEdit;
      Edit2: TEdit;
      ...
    end;
  
  procedure TEdit.WMPaste(var Message: TMessage);
  begin
    if not SomeCondition then
      inherited;
  end;