AngularJS unit test controller with only private functions

I have following controller that has only private functions. I am struggling to test this controller. Shall I test if it is doing $emit, since the ImageService has been tested? How do I test $emit in this case? Or shall I test if it is calling the ImageService.fetchImageStacks method? In this case, how do I trigger init function?

(function (angular, global, undefined) {
'use strict';

var ImageController = {};

ImageController.$inject = [
    '$rootScope',
    '$log',
    'ImageService'
];

ImageController = function (
    $rootScope,
    $log,
    ImageService
) {

    var getImageStacks = function() {
        ImageService
            .fetchImageStacks()
            .success(function (result) {
                ImageService.setImageStacks(result);
                $rootScope.$emit('rootScope:imageStacksUpdated', result);
            })
            .error(function (){
                $log.error('Failed to get imageStackInfo file.');
            });
    };

    var init = function () {
        getImageStacks();
    };

    init();

    return {
        getImageStacks: getImageStacks
    };
}

angular.module('myApp')
    .controller('ImageController', ImageController);

})(angular, this);