gistfile1.js

  $scope.$watch(function () {
    return SessionService.currentUser;
  }, function (user) {
    $scope.user = user;
  }, true);

mySolution.js

angular.service('MyService', function() {

    return {

        value: 'default',
        handlers: [],

        getProperty : function(onChangeCallback) {
            handlers.push(onChangeCallback);
            return value;
        },

        setProperty : function(newVal) {
            value = newVal;
            handlers.forEach(function(e) { e(newVal); });
        }
    };
});