AngularJS 依赖注入

依赖注入是一种软件设计方式，组件在其中被赋予它们的依赖项，而不是在组件内部进行硬编码。它使组件从查找依赖关系中解脱出来，使依赖关系可配置。它还有助于使组件可复用、可维护和可测试。

AngularJS提供了一个出色的依赖注入机制。它提供以下核心组件，可以作为依赖项注入到彼此中：

Value
Factory
Service
Provider
Constant

Value

Value是一个简单的JavaScript对象，在配置阶段（AngularJS自启动阶段）期间需要将值传递给控制器。

//define a module
var mainApp = angular.module("mainApp", []);

//create a value object as "defaultInput" and pass it a data.
mainApp.value("defaultInput", 5);
...

//inject the value in the controller using its name "defaultInput"
mainApp.controller('CalcController', function(scope, CalcService, defaultInput) {scope.number = defaultInput;
   scope.result = CalcService.square(scope.number);

   scope.square = function() {scope.result = CalcService.square($scope.number);
   }
});

Factory

Factory是一个用来返回值的函数。它在服务或控制器需要时按需创建一个值。通常使用一个工厂函数来计算并返回该值。

//define a module
var mainApp = angular.module("mainApp", []);

//create a factory "MathService" which provides a method multiply to return multiplication of two numbers
mainApp.factory('MathService', function() {
   var factory = {};

   factory.multiply = function(a, b) {
      return a * b
   }
   return factory;
}); 

//inject the factory "MathService" in a service to utilize the multiply method of factory.
mainApp.service('CalcService', function(MathService) {
   this.square = function(a) {
      return MathService.multiply(a,a);
   }
});
...

Service

Service是一个单例的JavaScript对象，包含一组执行特定任务的函数。服务使用service()函数定义，并在控制器中注入。

//define a module
var mainApp = angular.module("mainApp", []);
...

//create a service which defines a method square to return square of a number.
mainApp.service('CalcService', function(MathService) {
   this.square = function(a) {
      return MathService.multiply(a,a); 
   }
});

//inject the service "CalcService" into the controller
mainApp.controller('CalcController', function(scope, CalcService, defaultInput) {scope.number = defaultInput;
   scope.result = CalcService.square(scope.number);

   scope.square = function() {scope.result = CalcService.square($scope.number);
   }
});

Provider

在配置阶段，AngularJS内部使用提供者来创建服务、工厂等。以下脚本可用于创建之前创建的MathService。提供者是一个特殊的工厂方法，带有get()方法，用于返回值/服务/工厂。

//define a module
var mainApp = angular.module("mainApp", []);
...

//create a service using provider which defines a method square to return square of a number.
mainApp.config(function(provide) {provide.provider('MathService', function() {
      this.$get = function() {
         var factory = {};  

         factory.multiply = function(a, b) {
            return a * b; 
         }
         return factory;
      };
   });
});

Constant

Constant在配置阶段使用，考虑到在配置阶段不能使用值。

mainApp.constant("configParam", "constant value");

示例

下面的示例展示了所有上述指令的使用方法：

testAngularJS.htm

<html>
   <head>
      <title>AngularJS Dependency Injection</title>
   </head>

   <body>
      <h2>AngularJS Sample Application</h2>

      <div ng-app = "mainApp" ng-controller = "CalcController">
         <p>Enter a number: <input type = "number" ng-model = "number" /></p>
         <button ng-click = "square()">X<sup>2</sup></button>
         <p>Result: {{result}}</p>
      </div>

      <script src = "https://ajax.googleapis.com/ajax/libs/angularjs/1.3.14/angular.min.js">
      </script>

      <script>
         var mainApp = angular.module("mainApp", []);

         mainApp.config(function(provide) {provide.provider('MathService', function() {
               this.get = function() {
                  var factory = {};

                  factory.multiply = function(a, b) {
                     return a * b;
                  }
                  return factory;
               };
            });
         });

         mainApp.value("defaultInput", 5);

         mainApp.factory('MathService', function() {
            var factory = {};

            factory.multiply = function(a, b) {
               return a * b;
            }
            return factory;
         });
         mainApp.service('CalcService', function(MathService) {
            this.square = function(a) {
               return MathService.multiply(a,a);
            }
         });
         mainApp.controller('CalcController', function(scope, CalcService, defaultInput) {
            scope.number = defaultInput;scope.result = CalcService.square(scope.number);scope.square = function() {
               scope.result = CalcService.square(scope.number);
            }
         });
      </script>

   </body>
</html>