How to do it...

1. Create two classes, Person and Dog, with a relation between them, as shown in the following code snippet (this code snippet has a memory issue called reference cycle):

      class Dog{
        var name: String
        var owner: Person!
        init(name: String){
          self.name = name
        }
      }
      class Person{
        var name: String
        var id: Int
        var dogs = [Dog]()
        init(name: String, id: Int){
          self.name = name
          self.id = id
        }
      }
      let john = Person(name: "John", id: 1)
      let rex = Dog(name: "Rex")
      let rocky = Dog(name: "Rocky")
      john.dogs += [rex, rocky] // append dogs
      rex.owner = john
      rocky.owner = john

1. Update the reference type of owner property in the Dog class to break this cycle:

      class Dog{
        var name: String
        weak var owner: Person!
        init(name: String){
        self.name = name
      }
    }