Sounds like hyperscript. Look at this hypothetical PyGtk example:

  i = 1
  def click(event):
    i += 1

  g.Window(
    g.Frame(title=“Hello”,
      g.VBox(
        spacing=10,
        g.Label(
          “i = %d” % i,
          expand=True
        ),
        g.Button(“click me”,
          click=click,
        )
      )
    )
  )

Desktop frameworks could do this for decades instead of Glide XMLs or manual setup, but they didn’t.