Dependenct Injection Intro

An application is composed of many objects that collaborate with each other. Objects usually depend on other objects to perform some task. When an object is responsible for referencing its own dependencies it leads to a highly coupled, hard-to-test and hard-to-change code.

Dependency injection is a software design pattern that implements inversion of control for resolving dependencies. An injection is passing of dependency to a dependent object that would use it. This allows a separation of client’s dependencies from the client’s behaviour, which allows the application to be loosely coupled.

Not to be confused with the above definition - a dependency injection simply means giving an object its instance variables.

It’s that simple, but it provides a lot of benefits:

• easier to test your code (using automated tests like unit and UI tests)
• when used in tandem with protocol-oriented programming it makes it easy to change the implementation of a certain class - easier to refactor
• it makes the code more modular and reusable

There are three most commonly used ways Dependency Injection (DI) can be implemented in an application:

1. Initializer injection
2. Property injection
3. Using third party DI frameworks (like Swinject, Cleanse, Dip or Typhoon)

There is an interesting article with links to more articles about Dependency Injection so check it out if you want to dig deeper into DI and Inversion of Control principle.

Let’s show how to use DI with View Controllers - an every day task for an average iOS developer.

Example Without DI

We’ll have two View Controllers: LoginViewController and TimelineViewController. LoginViewController is used to login and upon successful loign, it will switch to the TimelineViewController. Both view controllers are dependent on the FirebaseNetworkService.

LoginViewController

class LoginViewController: UIViewController {

    var networkService = FirebaseNetworkService()
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

TimelineViewController

class TimelineViewController: UIViewController {

    var networkService = FirebaseNetworkService()
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    @IBAction func logoutButtonPressed(_ sender: UIButton) {
        networkService.logutCurrentUser()
    }
}