import UIKit

/*
class Person {
let name: String //强引用
init(name: String) {
self.name = name
print("\(name) is being initialized")
} //引用计数销毁时
deinit {
print("\(name) is being deinitialized")
}
}
*/ 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 }
// 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 = unit4A
unit4A!.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 = nil
unit4A = 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: String
var card: CreditCard?
init(name: String) {
self.name = name
}
deinit { print("\(name) is being deinitialized") }
}
class CreditCard {
let number: UInt64
unowned let customer: Customer
init(number: UInt64, customer: Customer) {
self.number = number
self.customer = customer
}
deinit { print("Card #\(number) is being deinitialized") }
} override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
} }
05-06 11:43