DefaultHandler (Delphi)

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Description

This code example is a Delphi Forms Application that illustrates the usage of the DefaultHandler method.

Code

The following code listing contains the definition of a custom edit box class: TMyEdit. This is necessary to override the DefaultHandler method.

type
  TMyEdit = class(TEdit)
  public
    procedure DefaultHandler(var Message); override;
  end;

The DefaultHandler method in TMyEdit shows a message box every time a key is pressed into a TMyEdit instance:

procedure TMyEdit.DefaultHandler(var Message);
begin
  if TMessage(Message).Msg = WM_KEYUP then
    ShowMessage('No callback is associated with WM_KEYUP messages for LMyEdit instance');
  inherited;
end;

When the main form is created, an instance of TMyEdit is created and placed on it:

 procedure TForm1.FormCreate(Sender: TObject);
 var
   LMyEdit: TMyEdit;
 begin
   LMyEdit := TMyEdit.Create(Self);
   LMyEdit.Left := 6;
   LMyEdit.Top := 8;
   LMyEdit.Visible := True;
   LMyEdit.Parent := Self;
 end;

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