Swift Injector

This framework is meant to be a minimal dependency injection framework. Dynamically register app dependencies to be used throughout your application. This framework is to help write swift tests easier. When using iOS singleton objects such as NotificationCenter or UserDefaults, there are hoops you have to jump through to make your code testable in XCTest. Since there is no way to dynamically mock a Swift object the normal routine is to pull these objects out to a class level and then introduce a mock for test. However, when writing extensions this is not so easy. Let's look at a couple examples

Normal implementation

import UIKit

class ViewController: UIViewController {
  
  viewDidLoad() {
    if UserDefaults.standard.bool(forKey: "first_time_user") {
      // Do Something
    }
  }
}

becomes

class ViewController: UIViewController {

  let userDefaults: UserDefaults = .standard
  
  viewDidLoad() {
    if userDefaults.bool(forKey: "first_time_user") {
      // Do Something
    }
  }
}

Injector Implementation

import UIKit
import Injector

class ViewController: UIViewController {

  let userDefaults: UserDefaults? = .resolve()
  
  viewDidLoad() {
    if userDefaults?.bool(forKey: "first_time_user") == true {
      // Do Something
    }
  }
}

In the previous examples the UserDefaults dependency was pulled out of the function and put at a class level. Now let's look at an example with an extension:

Extension Implementation

extension UIViewController {
  var isFirstTimeUser: Bool {
    return UserDefaults.standard.bool(forKey: "first_time_user")
  }
}

There's no way to pull the UserDefaults dependency to the class level because this is not allowed in extensions. So now let's see it with the Injector:

Injector Implementation

extension UIViewController {
  var isFirstTimeUser: Bool {
    let userDefaults: UserDefaults? = .resolve()
    return userDefaults?.bool(forKey: "first_time_user") ?? false
  }
}

Now in a test you can create a mock user defaults object and then use the injector to get your mock to the production code. You can see there is a small disadvantage here. The Injector tries to be as safe as it can. There are two resolve APIs.

.resolve() // returns an optional value
try .resolve() // returns a non-optional value but could generate an error

How do you use resolve? The Injector object requires any registered object to conform to the Resolvable protocol:

extension UserDefaults: Resolvable {}

Then the object can be registered on the Injector in the AppDelegate:

Injector.shared.register(UserDefaults.standard)

Injector API

Method Name	Description
register(_ objects: Resolvable...)	Adds a Resolvable object to the Injector singleton's list of resolvable objects.
unregister(_ obj: Resolvable)	Removes a Resolvable object from the Injector singleton's list of resolvable objects. The supplied object must be the exact instance originally registered with the Injector singleton.
unregisterAll()	Removes all Resolvable objects from the Injector singleton's list of resolvable objects.

License

MIT License

Copyright (c) 2018 Ericdowney

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