19-内存管理

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种引用

  1. 强引用（strong reference）：默认情况下，引用都是强引用
  2. 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
  3. 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

闭包的循环引用

@escaping

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

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
  1. UnsafePointer<Pointee> 类似于const Pointee *
  2. UnsafeMutablePointer<Pointee> 类似于Pointee *
  3. UnsafeRawPointer 类似于const void *
  4. UnsafeMutableRawPointer 类似于void *

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

// 仿照oc的遍历写法
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
    }
}

// Swift自带的遍历写法
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

// 返回相应的初始化器UnsafeMutableRawPointer和UnsafeRawPointer
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!)

创建指针一

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 = UnsafeRawPointer(bitPattern: 0x100001234)

创建指针二

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()

创建指针三

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)