1.內(nèi)存管理

跟OC一樣，Swift也是采取基于引用計數(shù)的ARC內(nèi)存管理方案（針對堆空間）

• Swift的ARC中有3中引用

  • 強引用( strong reference ) : 默認情況下愿汰，引用都是強引用

• 弱引用( weak reference ) : 通過weak定義弱引用
  1.必須是可選類型的var困后，因為實例銷毀后，ARC會自動將弱引用設(shè)置為nil
  2.ARC自動給弱引用設(shè)置nil時衬廷，不會觸發(fā)屬性觀察器

• 無主引用( unowned reference ) : 通過unowned定義無主引用
  1.不會產(chǎn)生強引用摇予，實例銷毀后仍然存儲著實例的內(nèi)存地址（類似于OC中的unsafe_unretained）
  2.試圖在實例銷毀后訪問無主引用，會產(chǎn)生運行時錯誤（野指針）

2.weak吗跋、unowned的使用限制

• weak侧戴、unowned只能用在類實例上面

protocol Liveable : AnyObject {}
class Person {}

weak var p0: Person?
weak var p1: AnyObject?
weak var p2: Liveable?

unowned var p10: Person?
unowned var p11: AnyObject?
unowned var p12: Liveable?

3.Autoreleasepool

 // public func autoreleasepool<Result>(invoking body: () throws -> Result) rethrows -> Result
public func autoreleasepool<Result>(invoking body: () throws -> Result) rethrows -> Result
autoreleasepool {
    let p = MJPerson(age: 20, name: "Jack")
     p.run()
}

4.循環(huán)引用(Reference Cycle)

• weak、unowned 都能解決循環(huán)引用的問題跌宛，unowned 要比weak 少一些性能消耗
  1.在生命周期中可能會變?yōu)?nil 的使用 weak
  2.初始化賦值后再也不會變?yōu)?nil 的使用 unowned

  
  
  image.png

5.閉包的循環(huán)引用

• 閉包表達式默認會對用到的外層對象產(chǎn)生額外的強引用（對外層對象進行了retain操作）
• 下面代碼會產(chǎn)生循環(huán)引用酗宋，導(dǎo)致Person1對象無法釋放（看不到Person1的deninit被調(diào)用）

class Person1 {
    var fn: (() -> ())?
    func run(){ print("run") }
    deinit { print("deinit") }
}

func test() {
    let p = Person1()
    p.fn = { p.run() }
}
test()

/// 在閉包表達式的捕獲列表聲明weak或unowned引用，解決循環(huán)引用問題
/*
p.fn = {
    [weak p] in
    p?.run()
}

p.fn = {
    [unowned p] in
    p.run()
}

p.fn = {
    [weak wp = p, unowned up = p, a = 10 + 20] in
    wp?.run()
}
 */

6.閉包的循環(huán)引用

• 如果想在定義閉包屬性的同時引用self疆拘，這個閉包必須是lazy的（因為在實例初始化完畢后才能引用self）
• 如果lazy屬性是閉包調(diào)用的結(jié)果蜕猫，那么不用考慮循環(huán)引用的問題（因為閉包調(diào)用后，閉包的生命周期就結(jié)束了）

class Person2 {
    lazy var fn: (() -> ()) = {
        [weak self] in
        self?.run()
    }
    func run() { print("run") }
    deinit { print("deinit") }
}
/// 上邊的閉包fn內(nèi)部如果用到了實例成員（屬性哎迄、方法）
/// 編譯器會強制要求明確寫出self
class Person3 {
    var age: Int = 0
    lazy var getAge: Int = {
        self.age
    }()
    deinit { print("deinit") }
}

7.@escaping

• 非逃逸閉包回右、逃逸閉包，一般都是當做參數(shù)傳遞給函數(shù)
• 非逃逸閉包:閉包調(diào)用發(fā)生在函數(shù)結(jié)束前漱挚，閉包調(diào)用在函數(shù)作用域內(nèi)
• 逃逸閉包:閉包有可能在函數(shù)結(jié)束后調(diào)用翔烁，閉包調(diào)用逃離了函數(shù)的作用域布蔗，需要通過@escaping聲明

typealias Fn = () -> ()
// fn 是非逃逸閉包
func test1(_ fn: Fn) { fn() }

// fn是逃逸閉包
var gFn: Fn?
func test2(_ fn: @escaping Fn) { gFn = fn }

// fn是逃逸閉包
func test3(_ fn: @escaping Fn) {
    DispatchQueue.global().async {
        fn()
    }
}

class Person4 {
    var fn: Fn
    // fn是逃逸閉包
    init(fn: @escaping Fn) {
        self.fn = fn
    }
    func run() {
        // DispatchQueue.global().async也是一個逃逸閉包
        // 它用到了實例成員（屬性泄伪、方法）,編譯器會強制要求明確寫出self
        DispatchQueue.global().async {
            self.fn()
        }
    }
}

8.逃逸閉包的注意點

• 逃逸閉包不可以捕獲inout參數(shù)

func other1(_ fn: Fn) { fn() }
func other2(_ fn: @escaping Fn) { fn() }
/*
func test(value: inout Int) -> Fn {
    other1 { value += 1 }
//     error: 逃逸閉包不能捕獲inout參數(shù)
    other2 { value += 1 }
    
    func plus() { value += 1 }
//     error: 逃逸閉包不能捕獲inout參數(shù)
    return plus
}
*/

9.內(nèi)存訪問沖突

• 內(nèi)存訪問沖突會在兩個訪問滿足下列條件時發(fā)生：
• 至少一個是寫入操作
• 它們訪問的是同一塊內(nèi)存
• 它們的訪問時間重疊（比如在同一個函數(shù)內(nèi)）

// 不存在內(nèi)存訪問沖突
func plus(_ num: inout Int) -> Int { num + 1 }
var number = 1
number = plus(&number)

//存在內(nèi)存訪問沖突
var step = 1
func increment(_ num: inout Int) { num += step }
increment(&step)

//解決內(nèi)存訪問沖突
var copyOfStep = step
increment(&copyOfStep)
step = copyOfStep

func balance(_ x: inout Int, _ y: inout Int) {
    let sum = x + y
    x = sum / 2
    y = sum - x
}

var num1 = 42
var num2 = 30
balance(&num1, &num2) // OK
//balance(&num1, &num1) // Error

struct Player {
    var name: String
    var health: Int
    var energy: Int
    mutating func shareHealth(with teammate: inout Player) {
        balance(&teammate.health, &health)
    }
}

var oscar = Player(name: "Oscar", health: 10, energy: 10)
var maria = Player(name: "Maria", health: 5, energy: 10)
oscar.shareHealth(with: &maria) // OK
//oscar.shareHealth(with: &oscar) // Error

var tuple = (health: 10, energy: 20)
// Error
//balance(&tuple.health, &tuple.energy)

var holly = Player(name: "Holly", health: 10, energy: 10)
//Error
//balance(&holly.health, &holly.energy)

/// 如果下面的條件可以滿足，就說明重疊訪問結(jié)構(gòu)體的屬性是安全的 
/// 你只訪問實例存儲屬性卸留，不是計算屬性或者類屬性
/// 結(jié)構(gòu)體是局部變量而非全局變量
/// 結(jié)構(gòu)體要么沒有被閉包捕獲要么只被非逃逸閉包捕獲

// Ok
func test1() {
    var tulpe = (health: 10, energy: 20)
    balance(&tulpe.health, &tulpe.energy)
    
    var holly = Player(name: "Holly", health: 10, energy: 10)
    balance(&holly.health, &holly.energy)
}
test1()

10.指針

• Swift中也有專門的指針類型白华，這些都被定性為"Unsafe"（不安全的）哩治，常見的有以下4種類型
• UnsafePoint<Pointee>類似于const Pointee*
• UnsafeMutablePoint<Pointee>類似于Pointee*
• UnsafeRawPoint類似于const void *
• UnsafeMutableRawPointer 類似于void*

var age = 10
func test2(_ ptr: UnsafeMutablePointer<Int>) {
    ptr.pointee += 10
}
func test3(_ ptr: UnsafePointer<Int>) {
    print(ptr.pointee)
}
test2(&age)
test3(&age) // 20
print(age)  // 20

var age1 = 10
func test4(_ ptr: UnsafeMutableRawPointer) {
    ptr.storeBytes(of: 20, as: Int.self)
}
func test5(_ ptr: UnsafeRawPointer) {
    print(ptr.load(as: Int.self))
}
test4(&age1)
test5(&age1) // 20
print(age1) // 20

10.1指針的應(yīng)用示例

var arr = NSArray(objects: 11, 22, 33, 44)
arr.enumerateObjects { (obj, idx, stop) in
    print(idx, obj)
    if idx == 2 {
        // 下標為2就停止遍歷
        stop.pointee = true
    }
}

var arr1 = NSArray(objects: 11, 22, 33, 44)
for (idx, obj) in arr1.enumerated() {
    print(idx, obj)
    if idx == 2 {
        break
    }
}

10.2獲取指向某個變量的指針

var age2 = 11
var ptr1 = withUnsafeMutablePointer(to: &age2) { $0 }
var ptr2 = withUnsafePointer(to: &age2) { $0 }
ptr1.pointee = 22
print(ptr2.pointee) // 22
print(age2) // 22

var ptr3 = withUnsafeMutablePointer(to: &age2) { UnsafeMutableRawPointer($0) }
var ptr4 = withUnsafePointer(to: &age2) { UnsafeRawPointer($0) }
ptr3.storeBytes(of: 33, as: Int.self)
print(ptr4.load(as: Int.self))  // 33
print(age2) // 33

//獲得指向堆空間實例的指針
class Person5 {}
var person5 = Person5()
var ptr5 = withUnsafePointer(to: &person5) { UnsafeRawPointer($0) }
var heapPtr = UnsafeRawPointer(bitPattern: ptr5.load(as: UInt.self))
print(heapPtr!)

10.3創(chuàng)建指針

var testPtr = UnsafeRawPointer(bitPattern: 0x100001234)

//創(chuàng)建
var testPtr1 = malloc(16)
//存
testPtr1?.storeBytes(of: 11, as: Int.self)
testPtr1?.storeBytes(of: 22, toByteOffset: 8, as: Int.self)
//取
print((testPtr1?.load(as: Int.self))!) // 11
print((testPtr1?.load(fromByteOffset: 8 , as: Int.self))!)  // 22
//銷毀
free(testPtr1)

var ptr6 = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)
ptr6.storeBytes(of: 11, as: Int.self)
ptr6.advanced(by: 8).storeBytes(of: 22, as: Int.self)
print(ptr6.load(as: Int.self))   // 11
print(ptr6.advanced(by: 8).load(as: Int.self))   //22
ptr6.deallocate()

var ptr7 = UnsafeMutablePointer<Int>.allocate(capacity: 3)
ptr7.initialize(to: 11)
ptr7.successor().initialize(to: 22)
ptr7.successor().successor().initialize(to: 33)

print(ptr7.pointee) // 11
print((ptr7 + 1).pointee) // 22
print((ptr7 + 2).pointee) // 33

print(ptr7[0])   //11
print(ptr7[1])   //22
print(ptr7[2])   //33

ptr7.deinitialize(count: 3)
ptr7.deallocate()

class Person6 {
    var age: Int
    var name: String
    init(age: Int, name: String) {
        self.age = age
        self.name = name
    }
    deinit { print(name, "deinit") }
}

var ptr8 = UnsafeMutablePointer<Person6>.allocate(capacity: 3)
ptr8.initialize(to: Person6(age: 10, name: "Jack"))
(ptr8 + 1).initialize(to: Person6(age: 11, name: "Rose"))
(ptr8 + 2).initialize(to: Person6(age: 12, name: "Kate"))
// Jack deinit
// Rose deinit
// Kate deinit
ptr8.deinitialize(count: 3)
ptr8.deallocate()

10.4指針之間的轉(zhuǎn)換

var ptr9 = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)

ptr9.assumingMemoryBound(to: Int.self).pointee = 11
(ptr9 + 8).assumingMemoryBound(to: Double.self).pointee = 22.0

print(unsafeBitCast(ptr9, to: UnsafePointer<Int>.self).pointee) // 11
print(unsafeBitCast(ptr9 + 8, to: UnsafePointer<Int>.self).pointee) // 22.0

ptr9.deallocate()

/// unsafeBitCase是忽略數(shù)據(jù)類型的強制轉(zhuǎn)換，不會因為數(shù)據(jù)類型的變化而改變原來的內(nèi)存數(shù)據(jù)
/// 類似于C++中的reinterpret_cast

class Person7 {}
var person7 = Person7()
var ptrr7 = unsafeBitCast(person7, to: UnsafeRawPointer.self)
print(ptrr7)