值类型

struct A {
    var a = 10
}

struct B {
    var a = A() {
        didSet {
            print(1, a.a)
        }
    }
    
    var aa = A() {
        didSet {
            print(11, aa.a)
        }
    }
    
    mutating func set(a: Int) {
        self.a.a = a
        set(aa: a)
    }
    
    mutating func set(aa: Int) {
        self.aa.a = aa
    }
}

struct C {
    var b = B() {
        didSet {
            print("didSet")
        }
    }
    
    mutating func set(a: Int) {
        b.set(a: a)
    }
}

var c = C()
c.set(a: 11)

// 打印顺序
// 1 11
// 11 11
// didSet

看这段代码，理解值类型的实现。

struct Thing {
    var property: Int = 0
}

var optionalThing: Thing? {
    didSet { print("didSet") }
}

optionalThing?.property = 1
// 打印
// didSet

struct A { 
    var foo: Int { 
        didSet { print("fooDidSet") } // not called
    }
}

var a: A? {
    didSet { print("didSet") } // called
}

a?.foo = 5

看上面两段代码，理解可选链断开的时机，也可能是 Swift 的 Bug。

值类型 malloc: Double free of object

class SS {}
struct Store {
    var a = 0
    var k = SS()
}

func test() {
    var store = Store()
    DispatchQueue.concurrentPerform(iterations: 1_000_000) { i in
        store = Store()
    }
}

上面这段代码，会报 malloc: Double free of object 0x1007d22a0，当把 SS 改成 struct，就不会挂掉。值类型在存在堆内存维护的情况下，会出现对引用类型的重复释放问题。wwdc

String 关于触发引用类型

func test() {
    var store = ""
    DispatchQueue.concurrentPerform(iterations: 1_000_000) { i in
        store = "\(1111):\(1111):\(1111):\(1111)"
    }
}

当 store = "(1111):(1111):(1111)" 的时候不会挂掉，但当是上面的代码就会挂掉，触发发了引用类型。

func test() {
    var store = ""
    DispatchQueue.concurrentPerform(iterations: 1_000_000) { i in
        store = "alsdfjlakjsdfaklsjdflakjsdfolakjsdflkajsdfklajsdlkfjasldkfja"
    }
}

上面这段代码没挂掉，因为字符串常量存放在固定的常量区。

综上，Swift String Key 的使用，尽量使用无引用类型 struct 来代替，因为不会触发引用类型。

Property observers being called on a nil struct instance upon attempt to mutate a member property
Bug or intended? “thing?.property = something” will trigger didSet observer even if thing is nil, but ONLY IF thing is a struct type

https://forums.swift.org/t/understanding-swifts-value-type-thread-safety/41406