ViewControllerA.swift

protocol Loader {}

class RemoteLoader: Loader {...}

class LocalLoader: Loader {...}

class ViewControllerA {
  private var loader: Loader?
  
  init(){
    loader = RemoteLoader()
  }
}

let viewControllerA = ViewControllerA()

class ViewControllerB {
  private var loader: Loader?
  
  init(loader: Loader){
    self.loader = loader
  }
}

let loader = RemoteLoader()
let viewControllerB = ViewControllerB(loader: loader)