如果服务暴露自己的不同步？ [英] Should services expose their asynchronicity?

问题描述

我正在写一个服务，将异步检索数据（$ HTTP或$资源）。我可以隐藏的事实，它是通过返回一个数组，最初是空的异步的，但最终会填充：

I'm writing a service that will retrieve data asynchronously ($http or $resource). I can hide the fact that it is asynchronous by returning an array that will initially be empty, but that will eventually get populated:

.factory('NewsfeedService1', ['$http', function($http) {
   var posts = [];
   var server_queried = false;
   return {
      posts: function() {
         if(!server_queried) {
            $http.get('json1.txt').success(
              function(data) {
                server_queried = true;
                angular.copy(data, posts);
            });
         }
         return posts;
      }
   };
}])
.controller('Ctrl1', ['$scope','NewsfeedService1',
function($scope, NewsfeedService1) {
    $scope.posts = NewsfeedService1.posts();
}])

或者，我可以通过返回的承诺揭露异步性：

Or I can expose the asynchronicity by returning a promise:

.factory('NewsfeedService2', ['$http', function($http) {
  var posts = [];
  var server_queried = false;
  var promise;
  return {
     posts_async: function() {
       if(!promise || !server_queried) {
         promise = $http.get('json2.txt').then(
           function(response) {
              server_queried = true;
              posts = response.data;
              return posts;
         });
       }
       return promise;
     }
  };
}])

.controller('Ctrl2', ['$scope','NewsfeedService2',
function($scope, NewsfeedService2) {
  NewsfeedService2.posts_async().then(
    function(posts) {
      $scope.posts = posts;
  });
  // or take advantage of the fact that $q promises are
  // recognized by Angular's templating engine:
  // (note that Peter and Pawel's AngularJS book recommends against this, p. 100)
  $scope.posts2 = NewsfeedService2.posts_async();
}]);

（ Plunker - 如果有人想玩弄上述两种实现）

(Plunker - if someone wants to play around with the above two implementations.)

露出的异步性会增加一个错误处理程序的则（）的方法，我可以处理控制器错误的一个潜在优势。不过，我很可能会再追和$ HTTP错误在处理应用程序范围的拦截。

One potential advantage of exposing the asychronicity would be that I can deal with errors in the controller by adding an error handler to the then() method. However, I'll likely be catching and dealing with $http errors in an application-wide interceptor.

所以，当要服务的异步性暴露？

So, when should a service's asynchronicity be exposed?

推荐答案

我的猜测是，你会发现人们对这个栅栏的两侧。的个人的，我觉得你应该总是暴露的库或函数的异步性（或更正确：我觉得你应该的从不掩饰的库或函数的异步性） 。主要的原因是透明度;例如，将这项工作？

My guess is that you'll find people on both sides of this fence. Personally, I feel that you should always expose the asynchronicity of a library or function (or more correctly: I feel that you should never hide the asynchronicity of a library or function). The main reason is transparency; for example, will this work?

app.controller('MyController', function(NewsfeedService) {
  $scope.posts = NewsfeedService.posts();
  doSomethingWithPosts($scope.posts); // <-- will this work?
});

如果你使用第一种方法（例如： $资源），它不会，即使 $ scope.posts 在技术上是一个数组。如果 doSomethingWithPosts 都有自己的异步操作，你可以用一个竞争条件结束。相反，你无论如何都要使用异步code：

If you're using the first method (e.g. $resource), it won't, even though $scope.posts is technically an array. If doSomethingWithPosts has its own asynchronous operations, you could end up with a race condition. Instead, you have to use asynchronous code anyway:

app.controller('MyController', function(NewsfeedService) {
  $scope.posts = NewsfeedService.posts(function() {
    doSomethingWithPosts($scope.posts);
  });
});

（当然，你可以回调接受帖子作为参数，但我仍然认为这是混乱和不规范。）

(Of course, you can make the callback accept the posts as an argument, but I still think it's confusing and non-standard.)

幸运的是，我们有承诺，承诺的目的就是要的再present 的操作的未来价值。此外，由于与承诺角的 $ Q 库创建可以绑定到的意见，有没有错：

Luckily, we have promises, and the very purpose of a promise is to represent the future value of an operation. Furthermore, since promises created with Angular's $q libraries can be bound to views, there's nothing wrong with this:

app.controller('MyController', function(NewsfeedService) {
  $scope.posts = NewsfeedService.posts();
  // $scope.posts is a promise, but when it resolves
  // the AngularJS view will work as intended.
});

[更新：你不能再直接绑定承诺的观点;你必须等待承诺解决，手动分配scope属性]

[Update: you can no longer bind promises directly to the view; you must wait for the promise to be resolved and assign a scope property manually.]

顺便说一句， Restangular ，一个流行的替代 $资源，使用承诺，AngularJS自己的 $资源将在1.2支持他们（它们可能已经支持他们最新的1.1.x版本的）。

As an aside, Restangular, a popular alternative to $resource, uses promises, and AngularJS' own $resource will be supporting them in 1.2 (they may already support them in the latest 1.1.x's).

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