import UIKit/*
class Person {let name: String//强引用init(name: String) {self.name = nameprint("\(name) is being initialized")}//引用计数销毁时deinit {print("\(name) is being deinitialized")}
}
*/class Person {let name: Stringinit(name: String) { self.name = name }var apartment: Apartment?deinit { print("\(name) is being deinitialized") }
}
class Apartment {let unit: Stringinit(unit: String) { self.unit = unit }
//    var tenant: Person?
weak var tenant: Person? //弱引用deinit { print("Apartment \(unit) is being deinitialized") }
}class First_Demo2: UIViewController {//    var reference1: Person?
//    var reference2: Person?
//    var reference3: Person?
    var john: Person?var unit4A: Apartment?var johnnn: Customer?override func viewDidLoad() {super.viewDidLoad()self.navigationItem.title = "自动引用计数"self.view.backgroundColor = UIColor.white//        let person = Person(name:"wangyalu")//reference1 = Person(name: "John Appleseed")//John Appleseed is being initialized//在你清楚地表明不再使用这个 Person 实例,强引用被断开时,ARC 会销毁它:
//        reference1 = nil//互为强引用john = Person(name:"wang")unit4A = Apartment(unit:"4A")// Person 实例现在有了一个指向 Apartment 实例的强引 用,而 Apartment 实例也有了一个指向 Person 实例的强引用。因此,当你断开 john 和 unit4A 变量所持有的强 引用时,引用计数并不会降为 0 ,实例也不会被 ARC 销毁:
john!.apartment = unit4Aunit4A!.tenant = john//当你把这两个变量设为 nil 时,没有任何一个析构函数被调用。循环强引用会一直阻止 Person 和 Apartme nt 类实例的销毁,这就在你的应用程序中造成了内存泄漏。john = nilunit4A = nil//注意:/*在使用垃圾收 的系统里,弱指针有时用来实现简单的缓冲机制,因为没有强引用的对象只会在内存压力触发垃 圾收 时才被销毁。但是在 ARC 中,一旦值的最后一个强引用被移除,就会被立即销毁,这导致弱引用并不适 合上面的用途。*/johnnn = Customer(name: "John Appleseed")johnnn!.card = CreditCard(number: 1234_5678_9012_3456, customer: johnnn!)johnnn = nil// 打印 “John Appleseed is being deinitialized”// 打印 ”Card #1234567890123456 is being deinitialized”
    }/*Customer 实例持有对 CreditCard 实例的强引用,而 CreditCard 实例持有对 Customer 实例的无主引用。 由于 customer 的无主引用,当你断开 johnnn 变量持有的强引用时,再也没有指向 Customer 实例的强引用了:由于再也没有指向 Customer 实例的强引用,该实例被销毁了。其后,再也没有指向 CreditCard 实例的强引 用,该实例也随之被销毁了:*/class Customer {let name: Stringvar card: CreditCard?init(name: String) {self.name = name}deinit { print("\(name) is being deinitialized") }}class CreditCard {let number: UInt64unowned let customer: Customerinit(number: UInt64, customer: Customer) {self.number = numberself.customer = customer}deinit { print("Card #\(number) is being deinitialized") }}override func didReceiveMemoryWarning() {super.didReceiveMemoryWarning()// Dispose of any resources that can be recreated.
    }}