[sugar] Notify active event firing

[Benjamin M. Schwartz]

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Arjun Sarwal wrote:
> I need to turn off some drawing code when the user switches to Home View
> (why waste so much of CPU power in the meanwhile)
> How can I do that ?

I just figured this out while writing Clock.  In my activity's __init__, I do:

        self.add_events(gtk.gdk.VISIBILITY_NOTIFY_MASK)
        self.connect("visibility-notify-event", self._visible_cb)

then later,

    def _visible_cb(self, widget, event):
        if event.state == gtk.gdk.VISIBILITY_FULLY_OBSCURED:
            self._innards.pause()
        else:
            self._innards.resume()

It works perfectly.
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