Swift: Unit testing for retain cycles

Vehicle_wheels_retain_cycles.swift

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)