新人，学习swift，但是发现有个析构的问题？

强引用循环，发现加了weak仍旧不起作用deinit中的打印没有运行，根本就没有进行析构，麻烦大佬看看，谢谢了
import Foundation

class Person {
    let name: String
    init(name: String) {
        self.name = name
    }
    var apartment: Apartment?
    deinit {
        print("\(name) is being deinitialized")
    }
}

class Apartment {
    let unit: String
    init(unit: String) {
        self.unit = unit
    }
    weak var tenant: Person?
    deinit {
        print("Apartment \(unit) is being deinitialized")
    }
}

var john: Person?
var unit4A: Apartment?

john = Person(name: "John Appleseed")
unit4A = Apartment(unit: "4A")

john!.apartment = unit4A
unit4A!.tenant = john

john = nil