AngularJS Dependency Injection
Dependency Injection is a software design in which components are given their dependencies instead of hard coding them within the component. It relieves a component from locating the dependency and makes dependencies configurable. It also helps in making components reusable, maintainable and testable.
AngularJS provides a supreme Dependency Injection mechanism. It provides following core components which can be injected into each other as dependencies.
Value
Factory
Service
Provider
Constant
value
Value is a simple JavaScript object, which is required to pass values to the controller duringconfig phase (config phase is when AngularJSbootstraps itself).
//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 is a function which is used to return value. It creates a value on demand whenever a service or a controller requires it. It generally uses a factory function to calculate and return the value.
//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) {
returnMathService.multiply(a,a);
}
});
...
Service
Service is a singleton JavaScript object containing a set of functions to perform certain tasks. Service is defined using service() function and it is then injected into the controllers.
//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) {
returnMathService.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
Provider is used by AngularJS internally to create services, factory, etc. during the configphase. The following script can be used to createMathService that we created earlier. Provider is a special factory method with get() method which is used to return the value/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
Constants are used to pass values at the configphase considering the fact that value cannot be used during the config phase.
mainApp.constant("configParam", "constant value");
Example
The following example shows the use of all the above-mentioned directives:
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" />
<button ng-click="square()">X<sup>2</sup></button>
<p>Result: {{result}}</p>
</div>
<scriptsrc="http://ajax.googleapis.com/ajax/libs/angularjs/1.2.15/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) {
returnMathService.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>
Output
Open testAngularJS.htm in a web browser and see the result.
AngularJS Dependency Injection
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