[javascript] How to require a controller in an angularjs directive

Can anyone tell me how to include a controller from one directive in another angularJS directive. for example I have the following code

var app = angular.module('shop', []).
config(['$routeProvider', function ($routeProvider) {
    $routeProvider.when('/', {
        templateUrl: '/js/partials/home.html'
    })
        .when('/products', {
        controller: 'ProductsController',
        templateUrl: '/js/partials/products.html'
    })
        .when('/products/:productId', {
        controller: 'ProductController',
        templateUrl: '/js/partials/product.html'
    });
}]);

app.directive('mainCtrl', function () {
    return {
        controller: function ($scope) {}
    };
});

app.directive('addProduct', function () {
    return {
        restrict: 'C',
        require: '^mainCtrl',
        link: function (scope, lElement, attrs, mainCtrl) {
            //console.log(cartController);
        }
    };
});

By all account I should be able to access the controller in the addProduct directive but I am not. Is there a better way of doing this?

There is a good stackoverflow answer here by Mark Rajcok:

AngularJS directive controllers requiring parent directive controllers?

with a link to this very clear jsFiddle: http://jsfiddle.net/mrajcok/StXFK/

<div ng-controller="MyCtrl">
    <div screen>
        <div component>
            <div widget>
                <button ng-click="widgetIt()">Woo Hoo</button>
            </div>
        </div>
    </div>
</div>

JavaScript

var myApp = angular.module('myApp',[])

.directive('screen', function() {
    return {
        scope: true,
        controller: function() {
            this.doSomethingScreeny = function() {
                alert("screeny!");
            }
        }
    }
})

.directive('component', function() {
    return {
        scope: true,
        require: '^screen',
        controller: function($scope) {
            this.componentFunction = function() {
                $scope.screenCtrl.doSomethingScreeny();
            }
        },
        link: function(scope, element, attrs, screenCtrl) {
            scope.screenCtrl = screenCtrl
        }
    }
})

.directive('widget', function() {
    return {
        scope: true,
        require: "^component",
        link: function(scope, element, attrs, componentCtrl) {
            scope.widgetIt = function() {
                componentCtrl.componentFunction();
            };
        }
    }
})


//myApp.directive('myDirective', function() {});
//myApp.factory('myService', function() {});

function MyCtrl($scope) {
    $scope.name = 'Superhero';
}