Injecting $scope into an angular service function()

Question

I have a Service:

angular.module('cfd')
  .service('StudentService', [ '$http',
    function ($http) {
    // get some data via the $http
    var path = 'data/people/students.json';
    var students = $http.get(path).then(function (resp) {
      return resp.data;
    });     
    //save method create a new student if not already exists
    //else update the existing object
    this.save = function (student) {
      if (student.id == null) {
        //if this is new student, add it in students array
        $scope.students.push(student);
      } else {
        //for existing student, find this student using id
        //and update it.
        for (i in students) {
          if (students[i].id == student.id) {
            students[i] = student;
          }
        }
      }
    };

But when I call save(), I don't have access to the $scope, and get ReferenceError: $scope is not defined. So the logical step (for me), is to provide save() with the $scope, and thus I must also provide/inject it to the service. So if I do that like so:

  .service('StudentService', [ '$http', '$scope',
                      function ($http, $scope) {

I get the following error:

Error: [$injector:unpr] Unknown provider: $scopeProvider <- $scope <- StudentService

The link in the error (wow that is neat!) lets me know it is injector related, and might have to do with order of declaration of the js files. I have tried reordering them in the index.html, but I think it is something more simple, such as the way I am injecting them.

Using Angular-UI and Angular-UI-Router

Answer 1

The $scope that you see being injected into controllers is not some service (like the rest of the injectable stuff), but is a Scope object. Many scope objects can be created (usually prototypically inheriting from a parent scope). The root of all scopes is the $rootScope and you can create a new child-scope using the $new() method of any scope (including the $rootScope).

The purpose of a Scope is to "glue together" the presentation and the business logic of your app. It does not make much sense to pass a $scope into a service.

Services are singleton objects used (among other things) to share data (e.g. among several controllers) and generally encapsulate reusable pieces of code (since they can be injected and offer their "services" in any part of your app that needs them: controllers, directives, filters, other services etc).

I am sure, various approaches would work for you. One is this:
Since the StudentService is in charge of dealing with student data, you can have the StudentService keep an array of students and let it "share" it with whoever might be interested (e.g. your $scope). This makes even more sense, if there are other views/controllers/filters/services that need to have access to that info (if there aren't any right now, don't be surprised if they start popping up soon).
Every time a new student is added (using the service's save() method), the service's own array of students will be updated and every other object sharing that array will get automatically updated as well.

Based on the approach described above, your code could look like this:

angular.
  module('cfd', []).

  factory('StudentService', ['$http', '$q', function ($http, $q) {
    var path = 'data/people/students.json';
    var students = [];

    // In the real app, instead of just updating the students array
    // (which will be probably already done from the controller)
    // this method should send the student data to the server and
    // wait for a response.
    // This method returns a promise to emulate what would happen 
    // when actually communicating with the server.
    var save = function (student) {
      if (student.id === null) {
        students.push(student);
      } else {
        for (var i = 0; i < students.length; i++) {
          if (students[i].id === student.id) {
            students[i] = student;
            break;
          }
        }
      }

      return $q.resolve(student);
    };

    // Populate the students array with students from the server.
    $http.get(path).then(function (response) {
      response.data.forEach(function (student) {
        students.push(student);
      });
    });

    return {
      students: students,
      save: save
    };     
  }]).

  controller('someCtrl', ['$scope', 'StudentService', 
    function ($scope, StudentService) {
      $scope.students = StudentService.students;
      $scope.saveStudent = function (student) {
        // Do some $scope-specific stuff...

        // Do the actual saving using the StudentService.
        // Once the operation is completed, the $scope's `students`
        // array will be automatically updated, since it references
        // the StudentService's `students` array.
        StudentService.save(student).then(function () {
          // Do some more $scope-specific stuff, 
          // e.g. show a notification.
        }, function (err) {
          // Handle the error.
        });
      };
    }
]);

---

_{One thing you should be careful about when using this approach is to never re-assign the service's array, because then any other components (e.g. scopes) will be still referencing the original array and your app will break.
E.g. to clear the array in StudentService:}

/* DON'T DO THAT   */  
var clear = function () { students = []; }

/* DO THIS INSTEAD */  
var clear = function () { students.splice(0, students.length); }