AngularJS: Patience for an asynchronous request

Question

AngularJS: Patience for an asynchronous request

I'm having trouble understanding the concept of promises in AngularJS.

Here is a provider I have:

var packingProvider = angular.module('packingProvider',[]);

    packingProvider.provider('packingProvider',function(){
        return{
        $get: function($http){
            return{
            getPackings: function(){
                $http.post('../sys/core/fetchPacking.php').then(function(promise){
                    var packings = promise.data;
                    return packings;
                });
            }
            }
        }
    }
    });

This provides a method called getPackings(), which will return an object

If I were to use this in my main application to retrieve the data, the call would be asynchronous, causing an issue where I need to 'wait' for the data:

var packings = packingProvider.getPackings();

    console.log(packings); // undefined

Is there a way to achieve this without restructuring the process within my main controller?

javascript angularjs synchronization

Answer 1

Answer №1

The result you are expecting from the then method is not actually returned at that moment; it actually gets returned later on.

In the line

var packings = packingProvider.getPackings();

, the reason why the return value is undefined is because the promise generated by $http is asynchronous. This means that when you call $http.post, it starts but does not finish before your function returns. In JavaScript, functions without a return statement default to returning undefined. The completion of the post call will happen later and then execute return packings;, which doesn't go anywhere.

It would be better for the getPackings method to directly return the promise coming from $http.post. This way, any code utilizing this method can directly invoke then on the promise and handle the value as needed. For example, in a controller, you could assign this promise directly to $scope for use in the view. You can check out this post explaining this concept further:

By the way, your service declaration seems quite verbose; have you considered simplifying it like so?

var module = angular.module('packing', []);

module.service('packing', function($http) {
  return {
    getPackings: function() {
      return $http.post('../sys/core/fetchPacking.php');
    }
  };
});

I'm still getting the hang of AngularJS, but I don't see much benefit in all that extra typing. ( =

Answer 2

The result you are expecting from the then method is not actually returned at that moment; it actually gets returned later on.

In the line

var packings = packingProvider.getPackings();

, the reason why the return value is undefined is because the promise generated by $http is asynchronous. This means that when you call $http.post, it starts but does not finish before your function returns. In JavaScript, functions without a return statement default to returning undefined. The completion of the post call will happen later and then execute return packings;, which doesn't go anywhere.

It would be better for the getPackings method to directly return the promise coming from $http.post. This way, any code utilizing this method can directly invoke then on the promise and handle the value as needed. For example, in a controller, you could assign this promise directly to $scope for use in the view. You can check out this post explaining this concept further:

By the way, your service declaration seems quite verbose; have you considered simplifying it like so?

var module = angular.module('packing', []);

module.service('packing', function($http) {
  return {
    getPackings: function() {
      return $http.post('../sys/core/fetchPacking.php');
    }
  };
});

I'm still getting the hang of AngularJS, but I don't see much benefit in all that extra typing. ( =

Answer 3

Answer №2

Give it a shot

var packingProvider = angular.module('packingProvider',[]);

packingProvider.provider('packingProvider',function(){
    return{
       $get: function($http){
           return{
              getPackings: function(){
                  return $http.post('../sys/core/fetchPacking.php').then(function(response){
                      return response.data; // packings
                  });
              }
           }
       }
   }
});

packingProvider.getPackings().then(function(packings){
    console.log(packings);
});

Main point to note: ensure you return the promise object from the getPackings function. Making the call synchronous is not recommended. If you want to use "packings" for two-way binding in your controller, assign the promise object to $scope.packings:

$scope.packings = packingProvider.getPackings();

LATEST UPDATE

After version 1.2.0-rc.3, promise unwrapping has been deprecated (https://github.com/angular/angular.js/blob/master/CHANGELOG.md#120-rc3-ferocious-twitch-2013-10-14). Therefore, the aforementioned code line will no longer result in two-way binding.

Answer 4

Give it a shot

var packingProvider = angular.module('packingProvider',[]);

packingProvider.provider('packingProvider',function(){
    return{
       $get: function($http){
           return{
              getPackings: function(){
                  return $http.post('../sys/core/fetchPacking.php').then(function(response){
                      return response.data; // packings
                  });
              }
           }
       }
   }
});

packingProvider.getPackings().then(function(packings){
    console.log(packings);
});

Main point to note: ensure you return the promise object from the getPackings function. Making the call synchronous is not recommended. If you want to use "packings" for two-way binding in your controller, assign the promise object to $scope.packings:

$scope.packings = packingProvider.getPackings();

LATEST UPDATE

After version 1.2.0-rc.3, promise unwrapping has been deprecated (https://github.com/angular/angular.js/blob/master/CHANGELOG.md#120-rc3-ferocious-twitch-2013-10-14). Therefore, the aforementioned code line will no longer result in two-way binding.

Answer 5

Answer №3

There are numerous resources out there to help guide you in utilizing Deferreds/Promises with jQuery. Consider doing a search for more information on this topic as it may assist you in overcoming any challenges you encounter. As far as I'm aware, AngularJS promises function similarly to jQuery promises.

A typical usage of a jQuery promise looks something like this:

var getPackings = function() { return $.get('../sys/core/fetchPacking.php'); };
var packings;
$.when(getPackings()).then(function(data){ packings = data; console.log(packings) });

It's worth noting that jQuery ajax calls have methods such as .done, .success, etc., which can be used instead of the generic Deferreds' .when().then() functions.

In the example above using jQuery, you can observe that setting and outputting the data in the .then() function is necessary since the completion of asynchronous tasks cannot be guaranteed elsewhere. Ideally, you would call the next processing function within the .then() function, like this:

$.when(myAsyncFunc()).then(myAppCanContinue(dataReturnedByAsyncFunc));

The principles applied here with jQuery are likely to be analogous in Angular. If you grasp the concept explained above, approaching this task in Angular should become clearer. Additionally, refer to this article for straightforward examples:

To ensure your code functions correctly, consider implementing something along these lines:

packingProvider.provider('packingProvider',function(){
    return{
       $get: function($http){
           return{
              getPackings: function(){
                  return $http.post('../sys/core/fetchPacking.php');
              }
           }
       }
   }
});
var packings = packingProvider.getPackings();

packings.then(function(data) { console.log(data)});

Answer 6

There are numerous resources out there to help guide you in utilizing Deferreds/Promises with jQuery. Consider doing a search for more information on this topic as it may assist you in overcoming any challenges you encounter. As far as I'm aware, AngularJS promises function similarly to jQuery promises.

A typical usage of a jQuery promise looks something like this:

var getPackings = function() { return $.get('../sys/core/fetchPacking.php'); };
var packings;
$.when(getPackings()).then(function(data){ packings = data; console.log(packings) });

It's worth noting that jQuery ajax calls have methods such as .done, .success, etc., which can be used instead of the generic Deferreds' .when().then() functions.

In the example above using jQuery, you can observe that setting and outputting the data in the .then() function is necessary since the completion of asynchronous tasks cannot be guaranteed elsewhere. Ideally, you would call the next processing function within the .then() function, like this:

$.when(myAsyncFunc()).then(myAppCanContinue(dataReturnedByAsyncFunc));

The principles applied here with jQuery are likely to be analogous in Angular. If you grasp the concept explained above, approaching this task in Angular should become clearer. Additionally, refer to this article for straightforward examples:

To ensure your code functions correctly, consider implementing something along these lines:

packingProvider.provider('packingProvider',function(){
    return{
       $get: function($http){
           return{
              getPackings: function(){
                  return $http.post('../sys/core/fetchPacking.php');
              }
           }
       }
   }
});
var packings = packingProvider.getPackings();

packings.then(function(data) { console.log(data)});

AngularJS: Patience for an asynchronous request

Answer №1

Answer №2

Answer №3

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