Swift 中的强引用和弱引用

Swift 中的强引用和弱引用是处理对象之间关系的重要概念。强引用是指通过变量或常量来引用一个对象，这个引用会增加对象的引用计数。只有当所有强引用都被释放时，对象的内存才会被回收。而弱引用则是一种不会增加对象引用计数的引用方式，当对象的所有强引用都被释放时，弱引用会自动变为 nil，对象的内存也会被回收。

swift
class Person {
    let name: String
    weak var car: Car?
    
    init(name: String) {
        self.name = name
    }
    
    deinit {
        print("\(name) is being deinitialized")
    }
}
class Car {
    let model: String
    var owner: Person?
    
    init(model: String) {
        self.model = model
    }
    
    deinit {
        print("Car \(model) is being deinitialized")
    }
}
var john: Person?
var bmw: Car?
john = Person(name: "John")
bmw = Car(model: "BMW")
john?.car = bmw
bmw?.owner = john
john = nil
bmw = nil

swift
class Country {
    let name: String
    var capital: City!
    
    init(name: String, capitalName: String) {
        self.name = name
        self.capital = City(name: capitalName, country: self)
    }
    
    deinit {
        print("Country \(name) is being deinitialized")
    }
}
class City {
    let name: String
    unowned var country: Country
    
    init(name: String, country: Country) {
        self.name = name
        self.country = country
    }
    
    deinit {
        print("City \(name) is being deinitialized")
    }
}
var china: Country?
china = Country(name: "China", capitalName: "Beijing")
china = nil