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app.directive('scrollPosition', function($window) {
    return {
        scope: {
            scroll: '=scrollPosition'
        },
        link: function(scope, element, attrs) {
            var windowEl = angular.element($window);
            var handler = function() {
                scope.scroll = windowEl.scrollTop();
            };
            windowEl.on('scroll', scope.$apply.bind(scope, handler));
            handler();
        }
    };
});

<span scroll-position="scroll"></span>
<div ng-class="{show: scroll > 400, hide: scroll <= 400}">
  <h2>I'm here</h2>
</div>