1.内存管理

跟OC一样，Swift也是采取基于引用计数的ARC内存管理方案（针对堆空间）

• Swift的ARC中有3中引用

  • 强引用( strong reference ) : 默认情况下，引用都是强引用

• 弱引用( weak reference ) : 通过weak定义弱引用
  1.必须是可选类型的var，因为实例销毁后，ARC会自动将弱引用设置为nil
  2.ARC自动给弱引用设置nil时，不会触发属性观察器

• 无主引用( unowned reference ) : 通过unowned定义无主引用
  1.不会产生强引用，实例销毁后仍然存储着实例的内存地址（类似于OC中的unsafe_unretained）
  2.试图在实例销毁后访问无主引用，会产生运行时错误（野指针）

2.weak、unowned的使用限制

• weak、unowned只能用在类实例上面

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.循环引用(Reference Cycle)

• weak、unowned 都能解决循环引用的问题，unowned 要比weak 少一些性能消耗
  1.在生命周期中可能会变为 nil 的使用 weak
  2.初始化赋值后再也不会变为 nil 的使用 unowned

  
  
  image.png

5.闭包的循环引用

• 闭包表达式默认会对用到的外层对象产生额外的强引用（对外层对象进行了retain操作）
• 下面代码会产生循环引用，导致Person1对象无法释放（看不到Person1的deninit被调用）

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

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

/// 在闭包表达式的捕获列表声明weak或unowned引用，解决循环引用问题
/*
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.闭包的循环引用

• 如果想在定义闭包属性的同时引用self，这个闭包必须是lazy的（因为在实例初始化完毕后才能引用self）
• 如果lazy属性是闭包调用的结果，那么不用考虑循环引用的问题（因为闭包调用后，闭包的生命周期就结束了）

class Person2 {
    lazy var fn: (() -> ()) = {
        [weak self] in
        self?.run()
    }
    func run() { print("run") }
    deinit { print("deinit") }
}
/// 上边的闭包fn内部如果用到了实例成员（属性、方法）
/// 编译器会强制要求明确写出self
class Person3 {
    var age: Int = 0
    lazy var getAge: Int = {
        self.age
    }()
    deinit { print("deinit") }
}

7.@escaping

• 非逃逸闭包、逃逸闭包，一般都是当做参数传递给函数
• 非逃逸闭包:闭包调用发生在函数结束前，闭包调用在函数作用域内
• 逃逸闭包:闭包有可能在函数结束后调用，闭包调用逃离了函数的作用域，需要通过@escaping声明

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

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

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

class Person4 {
    var fn: Fn
    // fn是逃逸闭包
    init(fn: @escaping Fn) {
        self.fn = fn
    }
    func run() {
        // DispatchQueue.global().async也是一个逃逸闭包
        // 它用到了实例成员（属性、方法）,编译器会强制要求明确写出self
        DispatchQueue.global().async {
            self.fn()
        }
    }
}

8.逃逸闭包的注意点

• 逃逸闭包不可以捕获inout参数

func other1(_ fn: Fn) { fn() }
func other2(_ fn: @escaping Fn) { fn() }
/*
func test(value: inout Int) -> Fn {
    other1 { value += 1 }
//     error: 逃逸闭包不能捕获inout参数
    other2 { value += 1 }
    
    func plus() { value += 1 }
//     error: 逃逸闭包不能捕获inout参数
    return plus
}
*/

9.内存访问冲突

• 内存访问冲突会在两个访问满足下列条件时发生：
• 至少一个是写入操作
• 它们访问的是同一块内存
• 它们的访问时间重叠（比如在同一个函数内）

// 不存在内存访问冲突
func plus(_ num: inout Int) -> Int { num + 1 }
var number = 1
number = plus(&number)

//存在内存访问冲突
var step = 1
func increment(_ num: inout Int) { num += step }
increment(&step)

//解决内存访问冲突
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)

/// 如果下面的条件可以满足，就说明重叠访问结构体的属性是安全的 
/// 你只访问实例存储属性，不是计算属性或者类属性
/// 结构体是局部变量而非全局变量
/// 结构体要么没有被闭包捕获要么只被非逃逸闭包捕获

// 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.指针

• Swift中也有专门的指针类型，这些都被定性为"Unsafe"（不安全的），常见的有以下4种类型
• UnsafePoint<Pointee>类似于const Pointee*
• UnsafeMutablePoint<Pointee>类似于Pointee*
• UnsafeRawPoint类似于const void *
• UnsafeMutableRawPointer 类似于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指针的应用示例

var arr = NSArray(objects: 11, 22, 33, 44)
arr.enumerateObjects { (obj, idx, stop) in
    print(idx, obj)
    if idx == 2 {
        // 下标为2就停止遍历
        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获取指向某个变量的指针

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

//获得指向堆空间实例的指针
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创建指针

var testPtr = UnsafeRawPointer(bitPattern: 0x100001234)

//创建
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
//销毁
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指针之间的转换

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是忽略数据类型的强制转换，不会因为数据类型的变化而改变原来的内存数据
/// 类似于C++中的reinterpret_cast

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