Dependency Injection Finally Makes Senses For Me (in Angular 1)

I started learning about Dependency Injection(DI) with Angular 1 and frankly speaking, it was not a very pleasant experiences for me at the start.

Angular uses all kinds of jargons in one short article: service, provider, then $provider, $provider.service(), $provider.factory(), with some actually referring to the one of program paradigms and some are Angular’s directives, I was totally confused at that time.

Now it slows makes sense to me, and to quote from Laravel’s official documents:

Dependency injection is a fancy phrase that essentially means this: class dependencies are “injected” into the class via the constructor or, in some cases, “setter” methods.

And the more I looked into how Laravel handles DI, the more I see how Angular is truly a MVC framework.

Laravel has two concepts around DI: service container and service provider. Basically in service provider we ‘register’ (or ‘define’) a ‘service’, and use service container to do App::bind or App::singleton, and later Laraval can use type hinting to do DI.

Angular follows the same concept here: we use $provide to define a bunch of ‘service’, or use its shortcuts(module.provider, module.service or module.factory); later we inject all these dependency with either an array in the callback:

`someModule.controller('MyController', ['$scope', 'dep1', 'dep2', function($scope, dep1, dep2) {...})` 

or use

`MyController.$inject = ['$scope', 'dep1', 'dep2'];`. 

One thing to notice is that all ‘service’ in Angular are singletons.

Similarly, in Spring, the IoC container handles dependencies for beans. In Spring’s official documents, it describes IoC, or Inversion of Control, as Dependency Injection, as an inversion of Service locator. This confuses me for a little bit, until I saw the following:

Service Locator is a kind of dependency injection pattern. The Service Locator pattern does not describe how to instantiate the services. It describes a way to register services and locate them. A Service Locator should be able to locate a service without knowing its concrete type. For example, it might use a string key. This allows you to replace the concrete implementation of the dependency without modifying the classes. source

So for Spring, the inversion is that instead of bean(or module) using Service Locator to do ‘dependency injection’, there is a container, in this case called IoC container, as service container in Laravel, to do DI.