Swift: Unit testing for retain cycles

Complete_playground.swift

import Foundation
import XCTest

class Vehicle {
    private var wheels = [Wheel]()
    
    func add(_ wheel : Wheel) {
        wheels.append(wheel)
    }
}

class Wheel {
    private var vehicle : Vehicle
    
    required init(vehicle : Vehicle) {
        self.vehicle = vehicle
    }
}

let car = Vehicle()
let wheel1 = Wheel(vehicle: car)
let wheel2 = Wheel(vehicle: car)
let wheel3 = Wheel(vehicle: car)
let wheel4 = Wheel(vehicle: car)

car.add(wheel1)
car.add(wheel2)
car.add(wheel3)
car.add(wheel4)

class testsForVehicle: XCTestCase {
    
    override func setUp() {
        super.setUp()
    }
    
    func testVehicleIsNotRetained() {
        var sut: Vehicle? = Vehicle()
        let wheel1 = Wheel(vehicle: sut!)
        sut!.add(wheel1)
        
        weak var weakSut = sut
        //After this line, the weak reference should also be nil
        sut = nil
        XCTAssertNil(weakSut)
        
    }
}

testsForVehicle.defaultTestSuite.run()