swift学习arc

1.一个类，只要指向改类的强引用的个数为0的时候，这个类才销毁

例如代码：

例1：
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 }
    weakvar 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

unit4A =nil
//这个时候 unit4A还没销毁

此时为

john =nil
//这个时候 john先销毁 john再销毁

例2：

若执行顺序为
john =nil
//这个时候先销毁

unit4A =nil
//这个时候 unit4A销毁

例子2
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
    unownedlet customer:Customer
    init(number:UInt64, customer:Customer) {
        self.number = number
        self.customer = customer
    }
    deinit {print("Card #\(number) is being deinitialized") }
}

var john:Customer?

//例子3 
//john = Customer(name: "John Appleseed")
//
//var card = CreditCard(number: 1234_5678_9012_3456, customer:john!)
//john!.card = card
//
// john = nil
////打印John Appleseed is being deinitialized 只有john被销毁，因为card还有一个抢引用
////例子4


john =Customer(name:"John Appleseed")

john!.card = CreditCard(number:1234_5678_9012_3456, customer:john!)

john =nil
//John Appleseed is being deinitialized
//Card #1234567890123456 is being deinitialized 因为都没抢引用了

例子3：无主引用以及隐式解析可选属性”

class Country {

   let name:String
    var capitalCity:City!
    init(name:String, capitalName:String) {
        self.name = name
        self.capitalCity = City(name: capitalName, country:self)
    }
}

class City {
    let name:String
    unownedlet country:Country
    init(name:String, country:Country) {
        self.name = name
        self.country = country
    }
}


var country =Country(name:"Canada", capitalName:"Ottawa")
print("\(country.name)'s capital city is called \(country.capitalCity.name)的国家是\(country.capitalCity.country.name)")
// 打印 Canada's capital city is called Ottawa的国家是 Canada

例子4：闭包采用补货列表来避免

class HTMLElement {

    let name: String

    let text: String?

    lazy var asHTML: Void -> String = {

        [unowned self] in

        if let text = self.text {

            return "<\(self.name)>\(text)</\(self.name)>"

        } else {

            return "<\(self.name) />"

        }

    }

    init(name: String, text: String? = nil) {

        self.name = name

        self.text = text

    }

    deinit {

        print("\(name) is being deinitialized")

    }

}