angularjs问题，为什么是两个succss处理？

[iriscole]

看到一段angularjs代码，两个succss处理，有必要么？原因是什么？

getCityIdByName: function (cityName) {
var deferred = $q.defer();
var promise = deferred.promise;
$http.get('api/home/getCityIdByName',{
params: {
cityName: cityName
}
}).success(function (message) {
deferred.resolve(message.data);
}).error(function (error) {
deferred.reject(error);
});
promise.success = function (fn) {
promise.then(fn);
return promise;
};
promise.error = function (fn) {
promise.then(null, fn);
return promise;
};
return promise;
},