//****************解決循環(huán)引用 的兩種方式 Weak Reference , Unowned Reference
        // weak 引用可以隨時(shí)在實(shí)類的生命周期內(nèi) 置 nil
        // unowned 引用, 標(biāo)志著我們明確 這個(gè)實(shí)類一旦初始化后就不會(huì)置為 nil
        
        //----------Weak Reference---------
        // 使用關(guān)鍵字 weak 可以讓強(qiáng)引用 變?yōu)?弱引用. 阻止循環(huán)引用出現(xiàn)
        // 當(dāng)屬性為 weak 類型, 該屬性置 nil 時(shí) 觀察者模式不會(huì)觸發(fā)
        
        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// Apartment 的Person 屬性時(shí)弱引用 故可以置 nil
        unit4A = nil// 引用環(huán)已經(jīng)被打破, unit4A 也可以置 nil
        
        
        
        
        
        
        //-------------- Unowned Reference
        //和 weak 一樣 unowned 也是將強(qiáng)引用弱化為弱引用
        //和 weak 不同, unowned 的引用不能為 nil
        //unowned 修飾的屬性一定是 非可選值 不能置nil
        // unowned 必須引用一個(gè)有內(nèi)存的實(shí)類, 如果 unowned 引用指向一個(gè)已經(jīng)銷毀的實(shí)類, 會(huì)拋出異常
        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")
            }
        }
        
        var johna: Customer?
        
        johna = Customer(name: "John Appleseed")
        
        johna!.card = CreditCard(number:1234_5678_9012_3456, customer: johna!)
        // johna---(strong)-->CreditCard
        // CreditCard -----(unowned)--->johna
        johna = nil
        //John Appleseed is being deinitialized
        //Card #1234567890123456 is being deinitialized
        
        
        // 防止循環(huán)引用的幾條原則.
        // 1. 屬性都是可選值時(shí), 使用 weak
        // 2. 有一個(gè)屬性不是可選值時(shí), 使用 unowned
        // 3. 都不是可選值時(shí), 使用 強(qiáng)制解包的可選型 和 unowned 配合使用: 如下面的例子
        class Country{
            
            let name: String
            
            var capitalCity: City!// 默認(rèn)是nil
            
            init(name: String, capitalName: String) {
                
                self.name = name
                // 因?yàn)?capitalCity 是可選值, 默認(rèn)是nil, 在給 self.name 賦值之后, 可以認(rèn)為 self 已經(jīng)初始化成功, 所以下面的代碼可以使用 self
                self.capitalCity = City(name: capitalName, country: self)
                
            }
            
        }
        
        class City{
            
            let name: String
            
            unowned let country: Country
            
            init(name: String, country: Country) {
                
                self.name = name
                
                self.country = country
                
            }
            
        }
        
        var country = Country(name: "Canada", capitalName:"Ottawa")
        // capitalCity 是強(qiáng)制解包的可選值, 所以這里不需要解包.
        print("\(country.name)'s capital city is called \(country.capitalCity.name)")