依赖注入（Dependency Injection）

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2023-12-01

依赖注入是一种软件设计，其中组件被赋予其依赖性，而不是在组件内对它们进行硬编码。它减轻了组件定位依赖关系并使依赖关系可配置。它还有助于使组件可重用，可维护和可测试。

AngularJS提供了一种最高的依赖注入机制。它提供了以下核心组件，这些组件可以作为依赖项互相注入。

Value
Factory
Service
Provider
Constant

Value

Value是一个简单的JavaScript对象，在配置阶段需要将值传递给控制器（配置阶段是AngularJS引导自身时）。

//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是一个用于返回值的函数。只要服务或控制器需要，它就会按需创建值。它通常使用工厂函数来计算和返回值。

//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是一个单独的JavaScript对象，包含一组用于执行某些任务的函数。使用service（）函数定义服务，然后将其注入控制器。

//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

AngularJS内部使用Provider在配置阶段创建服务，工厂等。以下脚本可用于创建我们之前创建的MathService。 Provider是一个特殊的工厂方法，使用get（）方法返回值/ service/factory。

//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

考虑到在配置阶段不能使用值这一事实，常量用于在配置阶段传递值。

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

例子 (Example)

以下示例显示了使用上述所有指令 -

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>