内存管理
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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)
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Swift的ARC中有3种引用
□ 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 ✔️ 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
✔️ 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 ✔️ 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) ✔️ 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) • Fatal error: Attempted to read an unowned reference but object 0x0 was already deallocated
weak、unowned的使用限制
- weak、unowned只能用在类实例上面
protocol Livable : AnyObject {}
class Person {}
weak var p0: Person?
weak var p1: AnyObject?
weak var p2: Livable?
unowned var p10: Person?
unowned var p11: AnyObject?
unowned var p12: Livable?
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
image.png
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)
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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()
- 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()
}
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之后才能引用self)
class Person {
lazy var fn: (() -> ()) = {
[weak self] in
self?.run()
}
func run() { print("run") }
deinit { print("deinit") }
}
□ 左边的闭包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
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非逃逸闭包、逃逸闭包，一般都是当做参数传递给函数
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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
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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
import Dispatch
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 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()
}
}
}
逃逸闭包的注意点
- 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:
□ 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
// Simultaneous accesses to 0x0, but modification requires exclusive access var step = 1
func increment(_ num: inout Int) { num += step }
increment(&step)
// 解决内存访问冲突
var copyOfStep = step increment(©OfStep) step = copyOfStep
内存访问冲突
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 tulpe = (health: 10, energy: 20)
// Error
balance(&tulpe.health, &tulpe.energy)
var holly = Player(name: "Holly", health: 10, energy: 10)
// Error
balance(&holly.health, &holly.energy)
内存访问冲突
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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
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你只访问实例存储属性，不是计算属性或者类属性
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结构体是局部变量而非全局变量
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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 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
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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
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UnsafePointer<Pointee> 类似于 const Pointee *
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UnsafeMutablePointer<Pointee> 类似于 Pointee *
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UnsafeRawPointer 类似于 const void *
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UnsafeMutableRawPointer 类似于 void *
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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()
创建指针
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()
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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
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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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