lib/visibility.core.js
function(event) {
            var state = self.state();

            for ( var i in self._callbacks ) {
                self._callbacks[i].call(self._doc, event, state);
            }
        }
function(event) {
            var state = self.state();

            for ( var i in self._callbacks ) {
                self._callbacks[i].call(self._doc, event, state);
            }
        }
function ( event ) { var state = self . state ( ) ; for ( var i in self . _callbacks ) { self . _callbacks [ i ] . call ( self . _doc , event , state ) ; } }
Listener for `visibilitychange` event.
Listener for visibilitychange event .