Different way to test reference cycle

1.swift

import XCTest

final class Person {
    var dogs: [Dog] = [] {
        didSet { dogs.forEach { $0.onBark = { [weak self] in self?.feed() } } }
    }

    func feed() { }
}

final class Dog {
    weak var owner: Person?
    var onBark: (() -> Void)?
}

final class PersonAndDogsTests: XCTestCase {

    func testWriteYourTestsHere() {
        var person = Person() as Optional
        let dog = Dog()

        dog.owner = person
        person?.dogs.append(dog)
        person = nil

        XCTAssertNil(dog.owner)
    }

}

2.swift

    
import XCTest

final class Person {
    var dogs: [Dog] = [] {
        didSet { dogs.forEach { $0.onBark = { [weak self] in self?.feed() } } }
    }

    func feed() { }
}

final class Dog {
    weak var owner: Person?
    var onBark: (() -> Void)?
}

final class PersonAndDogsTests: XCTestCase {

    func testWriteYourTestsHere() {
        var person = Person()
        let dog = Dog()

        dog.owner = person
        person.dogs.append(dog)

        XCTAssertTrue(isKnownUniquelyReferenced(&person))
    }

}

3.swift

import XCTest

final class Person {
    var dogs: [Dog] = [] {
        didSet {
            dogs.forEach { $0.onBark = weakSelfMethod(self, Person.feed) }
        }
    }

    func feed() { }
}

final class Dog {
    var owner: Person?
    var onBark: (() -> Void)?
}

func weakSelfMethod<T: AnyObject>(_ valueForSelf: T, _ unbindedMethod: @escaping (T) -> () -> Void) -> (() -> Void) {
    return { [weak valueForSelf] in
        if let valueForSelf = valueForSelf {
            unbindedMethod(valueForSelf)()
        }
    }
}

final class PersonAndDogsTests: XCTestCase {

    func testWriteYourTestsHere() {
        var dog: Dog? = Dog()
        var person: Person? = Person()
        weak var weakPerson = person
        weak var weakDog = dog

        person?.dogs = [dog!]
        dog?.onBark?()
        person = nil
        dog = nil

        XCTAssertNil(weakPerson)
        XCTAssertNil(weakDog)
    }

}

4.swift

import XCTest

final class Person {
    var dogs: [WeakRef<Dog>] = [] {
        didSet { dogs.forEach { $0.value?.onBark = feed } }
    }

    func feed() { }
}

final class Dog {
    weak var owner: Person?
    var onBark: (() -> Void)?
}

class WeakRef<T> where T: AnyObject {
    private(set) weak var value: T?
    init(value: T?) {
        self.value = value
    }
}

final class PersonAndDogsTests: XCTestCase {

    func testWriteYourTestsHere() {
        var dog: Dog? = Dog()
        var person: Person? = Person()
        weak var weakPerson = person
        weak var weakDog = dog

        person?.dogs = [WeakRef<Dog>(value: dog!)]
        dog?.onBark?()
        person = nil
        dog = nil

        XCTAssertNil(weakPerson)
        XCTAssertNil(weakDog)
    }
}