ARC 弱 var Swift（不是闭包）答案

【问题标题】：ARC weak var Swift (not closures)ARC 弱 var Swift（不是闭包）
【发布时间】：2017-06-23 01:55:18
【问题描述】：

我正在努力更好地理解 ARC 并正在使用 Apples Documentation

通过第一个示例，我没有得到 Apple 声明的预期结果； “因为弱引用不会对其引用的实例保持强控制，所以当弱引用仍在引用它时，该实例可能会被释放。因此，ARC 自动将弱引用设置为 nil 当实例它指的是被释放。”

我在 XCode 8.3.2 中使用游乐场

import UIKit

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 }
    weak var tenant: Person?
    deinit { print("Apartment \(unit) is being deinitialized") }
}


var john: Person?
var unit4A: Apartment?

john = Person(name: "John Appleseed")
unit4A = Apartment(unit: "4A")

john!.apartment = unit4A
unit4A!.tenant = john

john = nil //This prints "John Appleseed is being deinitialized" (as expected)
unit4a?.tenant?.name //This shows "John Appleseed" (expected nil)
unit4a = nil //Prints "Unit4a is being deinitialized" (as expected)

我知道这会阻止强引用循环，以便可以取消初始化两者，但我不明白为什么 unit4a 保留对租户的引用？

【问题讨论】：

标签： swift automatic-ref-counting weak-references swift-playground

【解决方案1】：

您被右侧的游乐场输出所欺骗，该输出与您的打印的时间顺序不同（如在 deinits 中）。如果您将独立的“回声”替换为实际的打印调用，您将在控制台中看到，租户在您打印 unit4a?.tenant?.name 之后而不是之前被释放。这是因为弱对象的释放不会立即发生，而是在下一个运行循环中发生。导入 UIKit 导入 XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

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 }
    weak var tenant: Person?
    deinit { print("Apartment \(unit) is being deinitialized") }
}


var john: Person?
var unit4a: Apartment?

john = Person(name: "John Appleseed")
unit4a = Apartment(unit: "4A")

john!.apartment = unit4a
unit4a!.tenant = john

john = nil
print(unit4a?.tenant?.name)
unit4a = nil

输出：

Optional("John Appleseed")
John Appleseed is being deinitialized
Apartment 4A is being deinitialized

但是，如果您将最后一位更改为：

john = nil

DispatchQueue.main.asyncAfter(deadline: .now()) {
    print(unit4a?.tenant?.name) //This shows "John Appleseed" (expected nil)
    unit4a = nil //Prints "Unit4a is being deinitialized" (as expected)
}

输出是你所期望的：

John Appleseed is being deinitialized
nil
Apartment 4A is being deinitialized

在大多数情况下，您不会关心对象是否立即释放，但如果您这样做，请检查自动释放池

【讨论】：