BackendServiceProxy.swift

protocol BakendService {
  func sum(first: Int, second: Int) -> Int
  func someNumber() -> Int
}

class BakendServiceProxy: BakendService {
  
  // lazy building function provided from DI
  private let instanceBuilder: () -> BakendService
  
  // instance which would be created when the first call
  // to the service is made
  private lazy var instance: BakendService = instanceBuilder()
  
  init(instanceBuilder: @escaping () -> BakendService) {
    self.instanceBuilder = instanceBuilder
  }
  
  func sum(first: Int, second: Int) -> Int {
    instance.sum(first: first, second: second)
  }
  
  func someNumber() -> Int {
    instance.someNumber()
  }
}