Swift: Unit testing for retain cycles

retain_cycle_unit_test.swift

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)
    }
}