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(invoking body: () throws -> Result) rethrows -> Result
public func autoreleasepool(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(©OfStep)
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
类似于const Pointee* - UnsafeMutablePoint
类似于Pointee* - UnsafeRawPoint类似于const void *
- UnsafeMutableRawPointer 类似于void*
var age = 10
func test2(_ ptr: UnsafeMutablePointer) {
ptr.pointee += 10
}
func test3(_ ptr: UnsafePointer) {
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.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.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.self).pointee) // 11
print(unsafeBitCast(ptr9 + 8, to: UnsafePointer.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)