TMyControl = class(TCustomControl)
  private
    procedure OnParentResizeHandler(Sender: TObject);
  protected
    procedure SetParent(NewParent: TWinControl); override;
  public ...
  end;


procedure TMyControl.OnParentResizeHandler(Sender: TObject);
begin
  Debugln('%s.OnParentResizeHandler PWith[%d]', [Name, Parent.ClientWidth]);
end;

procedure TMyControl.SetParent(NewParent: TWinControl);
var
  vOldParent: TWinControl;
begin
  vOldParent := Parent;
  inherited SetParent(NewParent);
  if Parent <> vOldParent then
  begin
    if assigned(vOldParent) then
      vOldParent.RemoveHandlerOnResize(@OnParentResizeHandler);
    if assigned(Parent) then
      Parent.AddHandlerOnResize(@OnParentResizeHandler, False);
  end;
end;