How to detect modifier key change in a control which doesn't have focus?

Question

Background:

I\'m working on a control derived from TCustomControl class which can get focus and which has some inner elements inside. Those inner elem

Answer 1

Remy's answer is likely your solution, but in case you're trying to do this without the restriction of encapsulating it into a control and found yourself here:

You could handle this with a three step process, as I've shown below.

The key things here are:

1. Set the control's cursor, not the screen's cursor
2. Use the form's KeyPreview property
3. Find the control under the cursor

I've used a button to illustrate the process. Be sure to set your form's KeyPreview to True.

procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word;
  Shift: TShiftState);
var
  myControl: TControl;
begin
  // If they pressed CTRL while over the control
  if ssCtrl in Shift then
  begin
    myControl := ControlAtPos(ScreenToClient(Mouse.CursorPos), False, True);
    // is handles nil just fine
    if (myControl is TButton) then
    begin
      myControl.Cursor := crSizeAll;
    end;
  end;
end;

procedure TForm1.FormKeyUp(Sender: TObject; var Key: Word; Shift: TShiftState);
var
  myControl: TControl;
begin
  // If they released CTRL while over the control
  if not(ssCtrl in Shift) then
  begin
    myControl := ControlAtPos(ScreenToClient(Mouse.CursorPos), False, True);
    if (myControl is TButton) then
    begin
      myControl.Cursor := crDefault;
    end;
  end;
end;

procedure TForm1.Button1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  // If they move over the button, consider current CTRL key state
  if ssCtrl in Shift then
  begin
    Button1.Cursor := crSizeAll;
  end
  else
  begin
    Button1.Cursor := crDefault;
  end;
end;