Separate auth and sync logic

sep_auth_sync.js

app.controller('TestCtrl', ['$scope', 'getAuth', 'getAllPosts', function($scope, Auth, getAllPosts) {
  Auth.$onAuth(function(user) {
    if( user ) {
      $scope.posts = getAllPosts();
    }
    else {
      purgePosts();
    }
  });
  
  // clean up posts when the controller is unloaded
  $scope.$on('$destroy', purgePosts);
  
  function purgePosts() {
    if( $scope.posts ) { 
      $scope.posts.$destroy(); 
    }
  }
}];

app.factory('getAllPosts',['$firebase', 'ref', function ($firebase, ref) {
  return function() {
    // always return a new instance so the list can be renewed
    return $firebase(ref('posts')).$asArray(); 
  };
}]);

app.factory('Auth', ['$firebaseAuth', 'ref' ,
  // auth remains a singleton
  return $firebaseAuth(ref());
]);

// a simple abstraction for creating Firebase references
app.factory('ref', ['FIREBASE_URL', '$window', function(URL, $window) {
  // takes any number of strings representing paths to join
  return function(path) {
    var ref = new $window.Firebase(URL);
    if( arguments.length ) {
      ref = ref.child( Array.prototype.slice.call(arguments, 0).join('/') );
    }
    return ref;
  }
}]);