MocksPlayground.swift

//: Mocks Playground

import UIKit

struct User {
}

struct PushNotificationController {
    let registrar: PushNotificationRegistrar
    init(registrar: PushNotificationRegistrar) {
        self.registrar = registrar
    }
    
    var user: User? {
        didSet {
            if let _ = user {
                registrar.registerUserNotificationSettings(UIUserNotificationSettings())
            }
        }
    }
}

protocol PushNotificationRegistrar {
    func registerUserNotificationSettings(notificationSettings: UIUserNotificationSettings)
}

extension UIApplication: PushNotificationRegistrar { }

class FauxRegistrar: PushNotificationRegistrar {
    var registered = false
    func registerUserNotificationSettings(notificationSettings: UIUserNotificationSettings) {
        registered = true
    }
}

var registrar = FauxRegistrar()
var controller = PushNotificationController(registrar: registrar)
controller.user = User()

registrar.registered

Use this way, should I add a protocol for every system api? I afraid that too much code will be increased.

@cleexiang, you only need to test public method, in your boundary (the one that interested you much). Not every single system api need testing.