swift内存管理

2022-12-13  本文已影响0人  iOS小洁

ARC

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种引用

weak、unowned的使用限制

weak、unowned只能用在类实例上面

Autoreleasepool

public func autoreleasepool<Result>(invoking body: () throws -> Result) rethrows -> Result

autoreleasepool { 
    let p = MJPerson(age: 20, name: "Jack") 
  p.run() 
}

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

在生命周期中可能会变为 nil 的使用 weak

初始化赋值后再也不会变为 nil 的使用 unowned

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Person对象无法释放（看不到Person的deinit被调用）

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

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

解决：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() }

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之后才能引用self）

class Person {
  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

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 Person {
  var age: Int = 0 
  lazy var getAge: Int = { self.age }() 
  deinit { print("deinit") }
}

@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

class Person {
  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()
    }
  }
}

逃逸闭包不可以捕获inout参数

typealias Fn = () -> () 
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
}

内存访问冲突（Conflicting Access to Memory）

内存访问冲突会在两个访问满足下列条件时发生：

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

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

// Ok 
func test() {
  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)
} 
test()

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

var age = 10 
func test1(_ ptr: UnsafeMutablePointer<Int>) { ptr.pointee += 10 } 
func test2(_ ptr: UnsafePointer<Int>) { print(ptr.pointee) } 
test1(&age) 
test2(&age) // 20 
print(age) // 20

var age = 10 
func test3(_ ptr: UnsafeMutableRawPointer) { ptr.storeBytes(of: 20, as: Int.self) } 
func test4(_ ptr: UnsafeRawPointer) { print(ptr.load(as: Int.self)) } 
test3(&age) 
test4(&age) // 20 
print(age) // 20

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

获得指向某个变量的指针

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

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

获得指向堆空间实例的指针

class Person {} 
var person = Person() 
var ptr = withUnsafePointer(to: &person) { UnsafeRawPointer($0) } 
var heapPtr = UnsafeRawPointer(bitPattern: ptr.load(as: UInt.self)) 
print(heapPtr!)

创建指针

var ptr = UnsafeRawPointer(bitPattern: 0x100001234)// 创建 


var ptr = malloc(16) 
// 存 
ptr?.storeBytes(of: 11, as: Int.self) 
ptr?.storeBytes(of: 22, toByteOffset: 8, as: Int.self) 
// 取 
print((ptr?.load(as: Int.self))!) // 11 
print((ptr?.load(fromByteOffset: 8, as: Int.self))!) // 22
// 销毁 
free(ptr)

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


var ptr = UnsafeMutablePointer<Int>.allocate(capacity: 3) 
ptr.initialize(to: 11) 
ptr.successor().initialize(to: 22) 
ptr.successor().successor().initialize(to: 33)
print(ptr.pointee) // 11 
print((ptr + 1).pointee) // 22 
print((ptr + 2).pointee) // 33
print(ptr[0]) // 11 
print(ptr[1]) // 22 
print(ptr[2]) // 33
ptr.deinitialize(count: 3) 
ptr.deallocate()



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

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

指针之间的转换

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)
ptr.assumingMemoryBound(to: Int.self).pointee = 11 
(ptr + 8).assumingMemoryBound(to: Double.self).pointee = 22.0

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

ptr.deallocate()

// unsafeBitCast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 Person {} 
var person = Person() 
var ptr = unsafeBitCast(person, to: UnsafeRawPointer.self) 
print(ptr)
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