Swift Memory Layout 和 Runtime

[ShannonChenCHN]

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[ShannonChenCHN]

Swift 源代码

• https://github.com/apple/swift
• How to Read the Swift Standard Library Source - Ole Begemann
• Exploring the Swift standard library source code

Runtime

• Swift5.0 的 Runtime 机制浅析
• Swift Runtime分析：还像OC Runtime一样吗？

热修复

• JSPatch 基础用法：Swift
• iOS 5分钟集成热修复(JSPatch)
• iOS 热更新解读（三）—— JSPatch 之于 Swift

内存布局

• Size, Stride, Alignment - Swift Unboxed⭐️
• Swift进阶之内存模型和方法调度
• 🎬GOTO 2016 • Exploring Swift Memory Layout • Mike Ash（文字版：Mike Ash: Exploring Swift Memory Layout - Realm Academy）
• Understanding Swift Value Semantics
• Introduction To Swift Memory Layout
• Swift 对象内存模型探究（一） - Bugly
• 【基本功】深入剖析Swift性能优化 - 美团技术团队
• 🎬Advanced Swift L1 - Memory Layout & Pointers
• ABI Stability Manifesto - swift
• 指针
  • Unsafe Swift: Using Pointers and Interacting With C - raywenderlich
  • Manual Memory Management | Apple Developer Documentation
  • Swift 中的指针使用
• 为什么Swift比OC快？

[ShannonChenCHN]

元类型（ AnyObject, AnyClass, .Type, .Self, .self, type(of:)）

class Cat {
    func eat() {
        let classType_: AnyClass = type(of: self)
        print("Class type is \(classType_)")
    }
    
    static func eat() {
        let classType_: AnyClass = self
        print("Class type is \(classType_)")
    }
}

class Garfield: Cat { }

let aCat = Cat()
aCat.eat() // Class type is Cat
Cat.eat()  // Class type is Cat

let aGarfield = Garfield()
aGarfield.eat()  // Class type is Garfield
Garfield.eat()   // Class type is Garfield

let typeOfCat/*: Cat.Type */= Cat.self
let typeOfCatObj/*: Cat.Type */= type(of: aCat)
let classType: AnyClass = typeOfCat
let classType2: AnyClass = typeOfCatObj
let object: AnyObject = aCat

print(typeOfCat == typeOfCatObj) // true

print(typeOfCat)    // Cat
print(typeOfCatObj) // Cat
print(classType)    // Cat
print(classType2)   // Cat

let str = NSString("Jack Ma")
let typeOfNSString = NSString.self
let typeOfStr = type(of: str)

print(typeOfNSString)
print(typeOfStr)
if typeOfStr is NSString.Type {
    print("str is a NSSting Object")
}



extension BinaryInteger {
    func squared() -> Self {  // 这里的 Self 指的就是实体类型本身
        return self * self
    }
}

let a: Int = 3
print(a.squared()) // 9

参考

• What's .self, .Type and .Protocol? Understanding Swift Metatypes
• 理解 Swift 中的元类型：.Type 与 .self
• type(of:) - Swift Standard Library | Apple Developer Documentation
• ANYCLASS，元类型和 .SELF - Swifter
• Self vs self - what's the difference?
• 类型元数据
  • ABI Stability Manifesto - swift

[ShannonChenCHN]

Size, Stride, Alignment

struct Year {
  let year: Int  // 8
}

struct YearWithMonth {
  let year: Int
  let month: Int
}

let sizeOfYear = MemoryLayout<Year>.size
let instanceOfYear = Year(year: 1984)
let sizeOfYearInstance = MemoryLayout.size(ofValue: instanceOfYear)
print(sizeOfYear)               // 8
print(sizeOfYearInstance)       // 8

let sizeOfYearWithMonth = MemoryLayout<YearWithMonth>.size
let instanceOfYearWithMonth = YearWithMonth(year: 1984, month: 10)
let sizeOfYearWithMonthInstance = MemoryLayout.size(ofValue: instanceOfYearWithMonth)
print(sizeOfYearWithMonth)          // 16
print(sizeOfYearWithMonthInstance)  // 16


//__________________________________________________________________________________

struct Puppy {
    let age: Int // 8
    let name: String // 16
    let isTrained: Bool  // 1
}

let sizeOfProperties = MemoryLayout<Int>.size + MemoryLayout<String>.size + MemoryLayout<Bool>.size
print(sizeOfProperties) // returns 25, from 8 + 16 + 1

let sizeOfPuppy = MemoryLayout<Puppy>.size
print(sizeOfPuppy) // returns 25


let strideOfPuppy = MemoryLayout<Puppy>.stride
print(strideOfPuppy)  // returns 32
// We’ve preserved the alignment requirements of all the values inside the struct: the second integer is at byte 16, which is a multiple of 8.
// That’s why the struct’s stride can be greater than its size: to add enough padding to fulfill alignment requirements.


let alignmentOfPuppy = MemoryLayout<Puppy>.alignment
print(alignmentOfPuppy) // returns 8
// The stride then becomes the size rounded up to the next multiple of the alignment. In our case:
// the size is 25
// 25 is not a multiple of 8
// the next multiple of 8 after 25 is 32
// therefore, the stride is 32


// Size is the number of bytes to read from the pointer to reach all the data.
// Stride is the number of bytes to advance to reach the next item in the buffer.
// Alignment is the the “evenly divisible by” number that each instance needs to be at. If you’re allocating memory to copy data into, you need to specify the correct alignment (e.g. allocate(byteCount: 100, alignment: 4)).
// https://swiftunboxed.com/images/size-stride-alignment-summary.png

参考

• Size, Stride, Alignment - Swift Unboxed