Delphi XE and Trapping Arrow Key with OnKeyDown

Question

I want my form to handle the arrow keys, and I can do it -- as long as there is no button on the form. Why is this?

Answer 1

Only the object that has the focus can receive a keyboard event.

To let the form have access to the arrow keys event, declare a MsgHandler in the public part of the form. In the form create constructor, assign the Application.OnMessage to this MsgHandler.

The code below intercepts the arrow keys only if they are coming from a TButton descendant. More controls can be added as needed.

procedure TForm1.FormCreate(Sender: TObject);
begin
  Application.OnMessage := Self.MsgHandler;
end;

procedure TForm1.MsgHandler(var Msg: TMsg; var Handled: Boolean);
var
  ActiveControl: TWinControl;
  key : word;
begin
  if (Msg.message = WM_KEYDOWN) then
    begin
      ActiveControl := Screen.ActiveControl;
      // if the active control inherits from TButton, intercept the key.
      // add other controls as fit your needs 
      if not ActiveControl.InheritsFrom(TButton)
        then Exit;

      key := Msg.wParam;
      Handled := true;
      case Key of // intercept the wanted keys
        VK_DOWN : ; // doStuff
        VK_UP : ; // doStuff
        VK_LEFT : ; // doStuff
        VK_RIGHT : ; // doStuff
        else Handled := false;
      end;
   end;
end;