swif-自动引用计数

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.

    }

}