angularjs directive to directive communication (directive's controllers)

gistfile1.txt

app.directive("child", function() {
    return {
        ...
        require: ["^parent", "child"],
        link: function(scope, elem, attrs, ctrls) {
            var parentController = ctrls[0],
                childController = ctrls[1];

            childController.setParent(parentController);
        },
        controller: ["$scope", function($scope) {
            var parentController;

            this.setParent = function(parent) {
                parentController = parent;
            };

            // use parentController...
        }]
    };
});

http://stackoverflow.com/questions/25075762/how-do-you-call-a-parent-directive-controller-from-a-child-controller-not-link