自动引用计数

先陪妹子看电影，贴点代码占个坑。

弱引用

可以在前面加上 weak 表明这是一个弱引用。弱引用不会保持住引用的实例，并且不会阻止 ARC 销毁被引用的实例。

可以看下下面这个例子中如何通过弱引用避免循环引用的问题：

class Person {
    let name: String
    var apartment: Apartment?

    init(name: String) {
        self.name = name
    }
    deinit {
        println("\(name) is being deinitialized")
    }
}

class Apartment {
    let number: Int
    weak var tenant: Person?

    init(number: Int) {
        self.number = number
    }
    deinit {
        println("Apartment #\(number) is being deinitialized")
    }
}
var why: Person?
var number604: Apartment?

why = Person(name: "WHY")
number604 = Apartment(number: 604)

why!.apartment = number604
number604!.tenant = why
println("nil 1")
why = nil
println("nil 2")
number604 = nil

无主引用

可以在前面加上 unowned 表明这是一个无主引用。和弱引用类似，无主引用不会牢牢保持住引用的实例。和弱引用不同的是，无主引用是永远有值的。因此，无主引用总是被定义为非可选类型。

下面这个例子：

class Customer {
    let name: String
    var card: CreditCard?

    init(name: String) {
        self.name = name
    }
    deinit {
        println("\(name) is being deinitialized")
    }
}

class CreditCard {
    let number: Int
    unowned let customer: Customer

    init(number: Int, customer: Customer) {
        self.number = number
        self.customer = customer
    }
    deinit {
        println("Card #\(number) is being deinitialized")
    }
}
var john: Customer?
john = Customer(name: "WHY")
john!.card = CreditCard(number: 1234_5678_9012_3456, customer: john!)
john = nil

References

• Automatic Reference Counting

[Swift]Day14：自动引用计数