Non-Singleton Services in AngularJS

问题:

AngularJS clearly states in its documentation that Services are Singletons:

AngularJS services are singletons

Counterintuitively, module.factory also returns a Singleton instance.

Given that there are plenty of use-cases for non-singleton services, what is the best way to implement the factory method to return instances of a Service, so that each time an ExampleService dependency is declared, it is satisfied by a different instance of ExampleService?

回答1:

I don't think we should ever have a factory return a newable function as this begins to break down dependency injection and the library will behave awkwardly, especially for third parties. In short, I am not sure there are any legitimate use cases for non-singleton sevices.

A better way to accomplish the same thing is to use the factory as an API to return a collection of objects with getter and setter methods attached to them. Here is some pseudo-code showing how using that kind of service might work:

.controller( 'MainCtrl', function ( $scope, widgetService ) {
  $scope.onSearchFormSubmission = function () {
    widgetService.findById( $scope.searchById ).then(function ( widget ) {
      // this is a returned object, complete with all the getter/setters
      $scope.widget = widget;
    });
  };

  $scope.onWidgetSave = function () {
    // this method persists the widget object
    $scope.widget.$save();
  };
});

This is just pseudo-code for looking up a widget by ID and then being able to save changes made to the record.

Here's some pseudo-code for the service:

.factory( 'widgetService', function ( $http ) {

  function Widget( json ) {
    angular.extend( this, json );
  }

  Widget.prototype = {
    $save: function () {
      // TODO: strip irrelevant fields
      var scrubbedObject = //...
      return $http.put( '/widgets/'+this.id, scrubbedObject );
    }
  };

  function getWidgetById ( id ) {
    return $http( '/widgets/'+id ).then(function ( json ) {
      return new Widget( json );
    });
  }


  // the public widget API
  return {
    // ...
    findById: getWidgetById
    // ...
  };
});

Though not included in this example, these kinds of flexible services could also easily manage state.

---

I don't have time right now, but if it will be helpful I can put together a simple Plunker later to demonstrate.

回答2:

I'm not entirely sure what use case you are trying to satisfy. But it is possible to have a factory return instances of an object. You should be able to modify this to suit your needs.

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


ExampleApplication.factory('InstancedService', function(){

    function Instance(name, type){
        this.name = name;
        this.type = type;
    }

    return {
        Instance: Instance
    }

});


ExampleApplication.controller('InstanceController', function($scope, InstancedService){
       var instanceA = new InstancedService.Instance('A','string'),
           instanceB = new InstancedService.Instance('B','object');

           console.log(angular.equals(instanceA, instanceB));

});

JsFiddle

Updated

Consider the following request for non-singleton services. In which Brian Ford notes:

The idea that all services are singletons does not stop you from writing singleton factories that can instantiate new objects.

and his example of returning instances from factories:

myApp.factory('myService', function () {
  var MyThing = function () {};
  MyThing.prototype.foo = function () {};
  return {
    getInstance: function () {
      return new MyThing();
    }
  };
});

I would also argue his example is superior due to the fact that you do not have to use the new keyword in your controller. It is encapsulated within the getInstance method of the service.

回答3:

Another way is to copy service object with angular.extend().

app.factory('Person', function(){
  return {
    greet: function() { return "Hello, I'm " + this.name; },
    copy: function(name) { return angular.extend({name: name}, this); }
  };
});

and then, for example, in your controller

app.controller('MainCtrl', function ($scope, Person) {
  michael = Person.copy('Michael');
  peter = Person.copy('Peter');

  michael.greet(); // Hello I'm Michael
  peter.greet(); // Hello I'm Peter
});

Here is a plunk.

回答4:

I know this post has already been answered but I still think there would be some legitimate scenarios that you need to have non-singleton service. Let's say there are some reusable business logic which can be shared between several controllers. In this scenario the best place to put the logic would be a service, but what if we need to keep some state in our reusable logic? Then we need non-singleton service so can be shared across different controllers in app. This is how I would implement these services:

angular.module('app', [])
    .factory('nonSingletonService', function(){

        var instance = function (name, type){
            this.name = name;
            this.type = type;
            return this;
        }

        return instance;
    })
    .controller('myController', ['$scope', 'nonSingletonService', function($scope, nonSingletonService){
       var instanceA = new nonSingletonService('A','string');
       var instanceB = new nonSingletonService('B','object');

       console.log(angular.equals(instanceA, instanceB));

    }]);

回答5:

Here's my example of a non singleton service, It's from a ORM im working on. In the example I show a Base Model (ModelFactory) which I want services('users','documents') to inherit and potential extend.

In my ORM ModelFactory injects other services to provide extra functionality(query,persistence,schema mapping) which is sandboxed using the module system.

In the example both user and document service have the same functionality but have their own independent scopes.

/*
    A class which which we want to have multiple instances of, 
    it has two attrs schema, and classname
 */
var ModelFactory;

ModelFactory = function($injector) {
  this.schema = {};
  this.className = "";
};

Model.prototype.klass = function() {
  return {
    className: this.className,
    schema: this.schema
  };
};

Model.prototype.register = function(className, schema) {
  this.className = className;
  this.schema = schema;
};

angular.module('model', []).factory('ModelFactory', [
  '$injector', function($injector) {
    return function() {
      return $injector.instantiate(ModelFactory);
    };
  }
]);


/*
    Creating multiple instances of ModelFactory
 */

angular.module('models', []).service('userService', [
  'ModelFactory', function(modelFactory) {
    var instance;
    instance = new modelFactory();
    instance.register("User", {
      name: 'String',
      username: 'String',
      password: 'String',
      email: 'String'
    });
    return instance;
  }
]).service('documentService', [
  'ModelFactory', function(modelFactory) {
    var instance;
    instance = new modelFactory();
    instance.register("Document", {
      name: 'String',
      format: 'String',
      fileSize: 'String'
    });
    return instance;
  }
]);


/*
    Example Usage
 */

angular.module('controllers', []).controller('exampleController', [
  '$scope', 'userService', 'documentService', function($scope, userService, documentService) {
    userService.klass();

    /*
        returns 
        {
            className: "User"
            schema: {
                name : 'String'
                username : 'String'
                password: 'String'
                email: 'String'     
            }
        }
     */
    return documentService.klass();

    /*
        returns 
        {
            className: "User"
            schema: {
                name : 'String'
                format : 'String'
                formatileSize: 'String' 
            }
        }
     */
  }
]);

回答6:

angular only gives a singleton service/factory option. one way around it is to have a factory service that will build a new instance for you inside your controller or other consumer instances. the only thing that is injected is the class that creates new instances. this is a good place to inject other dependencies or to initialize your new object to the specification of the user (adding services or config)

namespace admin.factories {
  'use strict';

  export interface IModelFactory {
    build($log: ng.ILogService, connection: string, collection: string, service: admin.services.ICollectionService): IModel;
  }

  class ModelFactory implements IModelFactory {
 // any injection of services can happen here on the factory constructor...
 // I didnt implement a constructor but you can have it contain a $log for example and save the injection from the build funtion.

    build($log: ng.ILogService, connection: string, collection: string, service: admin.services.ICollectionService): IModel {
      return new Model($log, connection, collection, service);
    }
  }

  export interface IModel {
    // query(connection: string, collection: string): ng.IPromise<any>;
  }

  class Model implements IModel {

    constructor(
      private $log: ng.ILogService,
      private connection: string,
      private collection: string,
      service: admin.services.ICollectionService) {
    };

  }

  angular.module('admin')
    .service('admin.services.ModelFactory', ModelFactory);

}

then in your consumer instance you need the factory service and call the build method on the factory to get a new instance when you need it

  class CollectionController  {
    public model: admin.factories.IModel;

    static $inject = ['$log', '$routeParams', 'admin.services.Collection', 'admin.services.ModelFactory'];
    constructor(
      private $log: ng.ILogService,
      $routeParams: ICollectionParams,
      private service: admin.services.ICollectionService,
      factory: admin.factories.IModelFactory) {

      this.connection = $routeParams.connection;
      this.collection = $routeParams.collection;

      this.model = factory.build(this.$log, this.connection, this.collection, this.service);
    }

  }

you can see it provides opperatiunity to inject some specific services that are not available in the factory step. you can always have injection happen on the factory instance to be used by all Model instances.

Note I had to strip off some code so I might made some context errors... if you need a code sample that works let me know.

I believe that NG2 will have the option to inject a new instance of your service in the right place in your DOM so you dont need to build your own factory implementation. will have to wait and see :)

回答7:

I believe there is good reason to create a new instance of an object within a service. We should keep an open mind as well rather than just say we ought never do such a thing, but the singleton was made that way for a reason. Controllers are created and destroyed often within the lifecycle of the app, but the services must be persistent.

I can think of a use case where you have a work flow of some kind, like accepting a payment and you have multiple properties set, but must now change their payment type because the customer's credit card failed and they need to provide a different form of payment. Of course, this does have a lot to do with the way you create your app. You could reset all properties for the payment object, or you could create a new instance of an object within the service. But, you would not want a new instance of the service, nor would you want to refresh the page.

I believe a solution is providing an object within the service that you can create a new instance of and set. But, just to be clear, the single instance of the service is important because a controller may be created and destroyed many times, but the services need persistence. What you are looking for may not be a direct method within Angular, but an object pattern that you can manage inside your service.

As an example, I have a made a reset button. (This is not tested, its really just a quick idea of a use case for creating a new object within a service.

app.controller("PaymentController", ['$scope','PaymentService',function($scope, PaymentService) {
    $scope.utility = {
        reset: PaymentService.payment.reset()
    };
}]);
app.factory("PaymentService", ['$http', function ($http) {
    var paymentURL = "https://www.paymentserviceprovider.com/servicename/token/"
    function PaymentObject(){
        // this.user = new User();
        /** Credit Card*/
        // this.paymentMethod = ""; 
        //...
    }
    var payment = {
        options: ["Cash", "Check", "Existing Credit Card", "New Credit Card"],
        paymentMethod: new PaymentObject(),
        getService: function(success, fail){
            var request = $http({
                    method: "get",
                    url: paymentURL
                }
            );
            return ( request.then(success, fail) );

        }
        //...
    }
    return {
        payment: {
            reset: function(){
                payment.paymentMethod = new PaymentObject();
            },
            request: function(success, fail){
                return payment.getService(success, fail)
            }
        }
    }
}]);

回答8:

Here's another approach to the problem that I was quite satisfied with, specifically when used in combination with Closure Compiler with advanced optimizations enabled:

var MyFactory = function(arg1, arg2) {
    this.arg1 = arg1;
    this.arg2 = arg2;
};

MyFactory.prototype.foo = function() {
    console.log(this.arg1, this.arg2);

    // You have static access to other injected services/factories.
    console.log(MyFactory.OtherService1.foo());
    console.log(MyFactory.OtherService2.foo());
};

MyFactory.factory = function(OtherService1, OtherService2) {
    MyFactory.OtherService1_ = OtherService1;
    MyFactory.OtherService2_ = OtherService2;
    return MyFactory;
};

MyFactory.create = function(arg1, arg2) {
    return new MyFactory(arg1, arg2);
};

// Using MyFactory.
MyCtrl = function(MyFactory) {
    var instance = MyFactory.create('bar1', 'bar2');
    instance.foo();

    // Outputs "bar1", "bar2" to console, plus whatever static services do.
};

angular.module('app', [])
    .factory('MyFactory', MyFactory)
    .controller('MyCtrl', MyCtrl);