Function callback with GtkSwitch

Hi all-

I’m trying to create a function callback for GtkSwitch. What I current have does not work:

using Gtk
win = GtkWindow()
switch = Gtk.GtkSwitchLeaf()
f(x) = println("Swiched!")
signal_connect(f, switch, :activate)
push!(win, switch)
showall(win)

Does anyone know why it does not work?