Swift Dependency Injection - ELI5

dependency-injection-eli5.swift

/// ❌ Without dependency injection

public final class Kid {
  
  private let fridge = Refridgerator()
  
  func eatLunch() {
    let food = fridge.fetchFood()
    let drink = fridge.fetchDrink() /// POTENTIAL ERROR: Might be expired!
    consume([food, drink])
  }
  
  private func consume(_ items: [Consumable]) {}
  
}

/// ✅ With dependency injection

protocol DrinkService {
  func fetchDrink()
}
protocol FoodService {
  func fetchFood()
}

public final class Kid {
  
  private let foodService: FoodService
  private let drinkService: DrinkService
  
  init(foodService: FoodService, drinkService: DrinkService) {
    self.foodService = foodService
    self.drinkService = drinkService
  }
  
  func eatFood() {
    /// Someone else will make sure you have something when you sit down to eat
    let food = foodService.fetchFood()
    let drink = drinkService.fetchDrink()
    consume([food, drink])
  }
}