Injecting $state (ui-router) into $http interceptor causes circular dependency

Question

What I'm trying to achieve

I would like to to transition to a certain state (login) in case an $http request returns a 401 error. I have therefore created an $http interceptor.

The problem

When I am trying to insert '$state' into the interceptor I get a circular dependency. Why and how do i fix it?

Code

//Inside Config function

    var interceptor = ['$location', '$q', '$state', function($location, $q, $state) {
        function success(response) {
            return response;
        }

        function error(response) {

            if(response.status === 401) {
                $state.transitionTo('public.login');
                return $q.reject(response);
            }
            else {
                return $q.reject(response);
            }
        }

        return function(promise) {
            return promise.then(success, error);
        }
    }];

    $httpProvider.responseInterceptors.push(interceptor);

Answer 1

The Fix

Use the $injector service to get a reference to the $state service.

var interceptor = ['$location', '$q', '$injector', function($location, $q, $injector) {
    function success(response) {
        return response;
    }

    function error(response) {

        if(response.status === 401) {
            $injector.get('$state').transitionTo('public.login');
            return $q.reject(response);
        }
        else {
            return $q.reject(response);
        }
    }

    return function(promise) {
        return promise.then(success, error);
    }
}];

$httpProvider.responseInterceptors.push(interceptor);

The Cause

angular-ui-router injects the $http service as a dependency into $TemplateFactory which then creates a circular reference to $http within the $httpProvider itself upon dispatching the interceptor.

The same circular dependency exception would be thrown if you attempt to inject the $http service directly into an interceptor like so.

var interceptor = ['$location', '$q', '$http', function($location, $q, $http) {

Separation of Concerns

Circular dependency exceptions can indicate that there is a mixing of concerns within your application which could cause stability issues. If you find yourself with this exception you should take the time to look at your architecture to ensure you avoid any dependencies that end up referencing themselves.

@Stephen Friedrich's answer

I agree with the answer below that using the $injector to directly get a reference to the desired service is not ideal and could be considered an anti pattern.

Emitting an event is a much more elegant and also decoupled solution.