AngularJS Services Part 1: Provider

Posted by Ilija Eftimov on February 25, 2015

I started using AngularJS couple of months ago, when we got a new client that wanted us to help with building an app written in Angular.

After couple of months of using it, I found myself struggling with uber-phat controllers and I started thinking of solutions about extracting knowledge out of the controller into separate entities. Also, having the ability to easily inject those entities back into the controller is a must.

When I found out about services it really reminded me of service/utility classes that I was used to write in Ruby. Interestingly, the people at Google added 5 types of services in Angular - to fit all your needs.

  • Provider
  • Constant
  • Value
  • Factory
  • Service

Problem is, this was quite confusing to me, and I guess for any dev that just got started with AngularJS. So I am writing this blog post that’s part of my learning and understanding of AngularJS services, and also, to help other fellow AngularJS developers to understand services.

Overview

Taken from AngularJS’ documentation on services:

An Angular service is a singleton object created by a service factory. These service factories are functions which, in turn, are created by a service provider. The service providers are constructor functions. When instantiated they must contain a property called $get, which holds the service factory function.

When you request a service, the $injector is responsible for finding the correct service provider, instantiating it and then calling its $get service factory function to get the instance of the service.

Mildly confusing. Okay, so, what is important:

  1. Every service in Angular is a singleton.
  2. Every service in Angular has a factory.
  3. These factories are functions, that are created by the service providers.
  4. A servide provider contains the constructor function of the factory. This constructor function is always called $get.

What happens when you inject your newly created service to (let’s say) a controller:

  1. the AngularJS injector finds the provider and instantiates it
  2. then it calls the $get function on the provider instance
  3. $get returns a service instance
  4. Voila! The service is injected in your controller!

Example (using Provider)

Let’s take a look at a tiny example. As we all know, the internet is made for cats. So, say we have a Cat provider.

(function(){
	"use strict";

	angular.module("app", [])

	.provider('cat', function(){

		this.$get = function() {
			var name = 'Tom';
			var color = 'Black';
			var age = "1";

			return {
				name: name,
				color: color
			};
		};
	});
})();

As you can see, this provider has a $get function that will return an instance of the service, which will be an object literal, containing two properties - the name and the color of the cat.

We can now inject this service in a controller and use it in a template.

(function(){
	angular.module('app')
		.controller('CatsController', CatsController);

	function CatsController(cat){ // here we inject the cat service in our controller
		this.name = cat.name;
		this.color = cat.color;
	}
})();
<!-- ...snip.. -->
<body ng-app='app'>
	<div ng-controller="CatsController as cats">
		The cat's name is: .
		The cat's color is: .
	</div>
</body>
<!-- ...snip.. -->

This is a very simple example of how to create services using the provider built-in service.

Provider

So we saw how we can easily use provider. Let’s see what else provider provides (no pun intended!).

Every provider we create (i.e. see first code snippet) is a module that we can inject. What’s cool about Angular is that we can configure any injectables before they are being injected (i.e. in a controller). Basically, the whole point of provider is to enable the developer to configure the service provider before it creates the service, therefore making services much more flexible.

Take this for example:

(function(){
	"use strict";

	angular.module("app", [])

	.provider('cat', function(){
		var self = this;
		var bornOn = new Date("June 18, 2012 22:03:00");
		var today = new Date();

		self.age = 1;

		self.$get = function() {
			var name = 'Tom';
			var color = 'Black';

			return {
				name: name,
				color: color,
				age: self.age,
			};
		};

		self.setAge = function(value){
			if(typeof value === 'undefined'){
				self.calculateAge();
			} else {
				self.age = value;
			}
		};

		self.calculateAge = function(){
		  var oneDayInMs = 1000*60*60*24;
		  var differenceInMs = today.getTime() - bornOn.getTime();
		  var daysDifference = Math.round(differenceInMs/oneDayInMs);
		  self.age = Math.round(daysDifference/365);
		};
	})

	.config(function(catProvider){
		catProvider.setAge();
	});
})();

Let me walk you through this (bear with me!).

This is a Cat service provider, which contains the $get factory function that will return the actual service object. The object, as you can see on lines 17-21, will contain the name, color and age properties. The name and color are hardcoded, meaning, they never change.

I mean, who dyes their cat’s fur or changes that little cute creature’s name? Right?

But, like any other cat, it ages. So we have a hardcoded date of birth and a setAge method. Take note that the setAge method, as well as the calculateAge are bound/in the scope of the provider, not the factory function.

The cool thing happens on lines 40-42. The .config block is get executed during the provider registrations and configuration phase. This means that we can do any custom configuration to the provider before we actually use the service object that the factory method will return.

Take note that on line 40, we pass in the catProvider to the config block, because it’s the provider that we are configuring, not the service. The service will be an object that the $get function will return.

In our example, the config block calls setAge which then calls calculateAge which dynamically calculates the age of the cat.

If one wants to set the age of the cat manually, he can pass a number as a parameter to the setAge function.

Also, if one completely removes the configuration block, the age of the cat will be set to 1. I know it doesn’t really make sense, but for this example’s sake lets leave it like that.

Provider (and config) in the wild

Take for example Restangular. For those who haven’t heard of it Restnagular is basically a AngularJS service that provides a wrapper on top of $http and is made to handle Restful Resources easily.

In the documentation, in the section How to configure them globally the author wrote a very cool snippet showing how one can configure the RestangularProvider before any Restangular service is created.

For example:

app.config(function(RestangularProvider) {
  RestangularProvider.setBaseUrl('/api/v1');
});

Here, we tell the RestangularProvider that every remote resource that we try to fetch (i.e. /users ) should have a base URL of /api/v1. So, when Restangular fetches the users, it will hit /api/v1/users.

This is just a small and tiny example of the flexibillity that AngularJS Providers gives us the developers.


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