Wrong behavior with angularjs service?

Question

i have write one service like that :

pinholeAdminServices.factory('ServiceChannel', ['Channel', '$httpWrapper' , '$route', '$q' , function (Channel, $httpWrapper, $route, $q) {
    'use strict';
    return {
        'MultiChannelsLoader': function (params) {
            params = params || $route.current.params;
            var delay = $q.defer();
            Channel.query(params, function (response) {
                delay.resolve(response);
            }, function (error) {
                delay.reject(error);
            });
            return delay.promise;
        }
    }
}]);

and after that i have change the place of var delay = $q.defer(); in the top of service like that :

Services.factory('ServiceChannel', ['Channel', '$httpWrapper' , '$route', '$q' , function (Channel, $httpWrapper, $route, $q) {
    'use strict';
   var delay = $q.defer();
    return {
        'MultiChannelsLoader': function (params) {
            params = params || $route.current.params;
            Channel.query(params, function (response) {
                delay.resolve(response);
            }, function (error) {
                delay.reject(error);
            });
            return delay.promise;
        }
    }
}]);

and the application behavior became totally wrong. can anyone explain what's the difference between the two part's of code

Answer 1

This creates one deferred for every ServiceChannel usage.

This means, that when you call MultiChannelsLoader it will use the same promise each time.

A promise is an abstraction over a one time calculation. After it is resolved a promise can not change its state.

var d = $q.defer();
d.resolve("Foo"); // d will forever be remained in fulfilled state, with "Foo" 

Imagine that resolve is implemented as:

resolve: function(){
    if(pending){ // not resolved yet
        pending = undefined; // change state to not pending.
    }
}

Oh wait, it actually is. The promise specification is also crystal clear about this:

When fulfilled, a promise: must not transition to any other state. When rejected, a promise: must not transition to any other state.

(reference)