Question

我希望能够原子地增加一个计数器，我找不到任何关于如何做的参考。

根据评论添加更多信息：

你在使用GCD吗？不，我没有使用GDC。必须使用队列系统来增加数字似乎有点过分了。
您了解基本的线程安全性吗？是的，否则我不会询问原子增量。
这个变量是本地的吗？没有。
是实例级别吗？是的，它应该是单个实例的一部分。

我想做这样的事情：

 class Counter {
      private var mux Mutex
      private (set) value Int
      func increment (){
          mux.lock()
          value += 1
          mux.unlock()
      }
 }

Answer 1

来自Low-Level Concurrency APIs：

有一长串OSAtomicIncrement和OSAtomicDecrement 允许您递增和递减整数值的函数以原子方式 - 线程安全，无需锁定（或使用队列）。如果您需要增加全局计数器，这些可能很有用从多个线程进行统计。如果你所做的只是增加一个全局计数器，无障碍OSAtomicIncrement版本很好，当没有争用时，他们打电话便宜。

这些函数适用于固定大小的整数，您可以选择 32位或64位变体，具体取决于您的需求：

class Counter {
    private (set) var value : Int32 = 0
    func increment () {
        OSAtomicIncrement32(&value)
    }
}

（注意：正如Erik Aigner正确注意到的那样，OSAtomicIncrement32和朋友们从macOS 10.12 / iOS 10.10开始被弃用。 Xcode 8建议使用<stdatomic.h>中的函数。然而，这似乎很难，比较Swift 3: atomic_compare_exchange_strong和https://openradar.appspot.com/27161329。因此，以下基于GCD的方法似乎是最好的解决方案现在。）

或者，可以使用GCD队列进行同步。来自“并发编程指南”中的Dispatch Queues：

...使用调度队列，您可以将两个任务都添加到序列中调度队列以确保只有一个任务修改了资源任何给定的时间。这种基于队列的同步更多比锁更有效，因为锁总是需要昂贵的内核陷入有争议和无争议的案件，而派遣 queue主要在应用程序的进程空间中工作在绝对必要时调用内核。

在你的情况下

// Swift 2:
class Counter {
    private var queue = dispatch_queue_create("your.queue.identifier", DISPATCH_QUEUE_SERIAL)
    private (set) var value: Int = 0

    func increment() {
        dispatch_sync(queue) {
            value += 1
        }
    }
}

// Swift 3:
class Counter {
    private var queue = DispatchQueue(label: "your.queue.identifier") 
    private (set) var value: Int = 0

    func increment() {
        queue.sync {
            value += 1
        }
    }
}

有关更复杂的示例，请参阅Adding items to Swift array across multiple threads causing issues (because arrays aren't thread safe) - how do I get around that?或GCD with static functions of a struct。这个帖子 What advantage(s) does dispatch_sync have over @synchronized?也非常有趣。

Answer 2

在这种情况下，队列是一种矫枉过正。您可以为此目的使用 Swift 3 中引入的DispatchSemaphore，如下所示：

import Foundation

public class AtomicInteger {

    private let lock = DispatchSemaphore(value: 1)
    private var value = 0

    // You need to lock on the value when reading it too since
    // there are no volatile variables in Swift as of today.
    public func get() -> Int {

        lock.wait()
        defer { lock.signal() }
        return value
    }

    public func set(_ newValue: Int) {

        lock.wait()
        defer { lock.signal() }
        value = newValue
    }

    public func incrementAndGet() -> Int {

        lock.wait()
        defer { lock.signal() }
        value += 1
        return value
    }
}

该课程的最新版本可用over here。

Answer 3

我知道这个问题已经有点老了，但我最近偶然发现了同样的问题。经过一番研究和阅读http://www.cocoawithlove.com/blog/2016/06/02/threads-and-mutexes.html这样的帖子后，我想出了一个原子计数器的解决方案。也许它也会帮助别人。

import Foundation

class AtomicCounter {

  private var mutex = pthread_mutex_t()
  private var counter: UInt = 0

  init() {
    pthread_mutex_init(&mutex, nil)
  }

  deinit {
    pthread_mutex_destroy(&mutex)
  }

  func incrementAndGet() -> UInt {
    pthread_mutex_lock(&mutex)
    defer {
      pthread_mutex_unlock(&mutex)
    }
    counter += 1
    return counter
  }
}

Answer 4

详细

Xcode 10.1（10B61）
Swift 4.2

解决方案

import Foundation

struct AtomicInteger<Type>: BinaryInteger where Type: BinaryInteger {

    typealias Magnitude = Type.Magnitude
    typealias IntegerLiteralType = Type.IntegerLiteralType
    typealias Words = Type.Words
    fileprivate var value: Type

    private var semaphore = DispatchSemaphore(value: 1)
    fileprivate func _wait() { semaphore.wait() }
    fileprivate func _signal() { semaphore.signal() }

    init() { value = Type() }

    init(integerLiteral value: AtomicInteger.IntegerLiteralType) {
        self.value = Type(integerLiteral: value)
    }

    init<T>(_ source: T) where T : BinaryInteger {
        value = Type(source)
    }

    init(_ source: Int) {
        value = Type(source)
    }

    init<T>(clamping source: T) where T : BinaryInteger {
        value = Type(clamping: source)
    }

    init?<T>(exactly source: T) where T : BinaryInteger {
        guard let value = Type(exactly: source) else { return nil }
        self.value = value
    }

    init<T>(truncatingIfNeeded source: T) where T : BinaryInteger {
        value = Type(truncatingIfNeeded: source)
    }

    init?<T>(exactly source: T) where T : BinaryFloatingPoint {
        guard let value = Type(exactly: source) else { return nil }
        self.value = value
    }

    init<T>(_ source: T) where T : BinaryFloatingPoint {
        value = Type(source)
    }
}

// Instance Properties

extension AtomicInteger {
    var words: Type.Words {
        _wait(); defer { _signal() }
        return value.words
    }
    var bitWidth: Int {
        _wait(); defer { _signal() }
        return value.bitWidth
    }
    var trailingZeroBitCount: Int {
        _wait(); defer { _signal() }
        return value.trailingZeroBitCount
    }
    var magnitude: Type.Magnitude {
        _wait(); defer { _signal() }
        return value.magnitude
    }
}

// Type Properties

extension AtomicInteger {
    static var isSigned: Bool { return Type.isSigned }
}

// Instance Methods

extension AtomicInteger {

    func quotientAndRemainder(dividingBy rhs: AtomicInteger<Type>) -> (quotient: AtomicInteger<Type>, remainder: AtomicInteger<Type>) {
        _wait(); defer { _signal() }
        rhs._wait(); defer { rhs._signal() }
        let result = value.quotientAndRemainder(dividingBy: rhs.value)
        return (AtomicInteger(result.quotient), AtomicInteger(result.remainder))
    }

    func signum() -> AtomicInteger<Type> {
        _wait(); defer { _signal() }
        return AtomicInteger(value.signum())
    }
}


extension AtomicInteger {

    fileprivate static func atomicAction<Result, Other>(lhs: AtomicInteger<Type>,
                                                        rhs: Other, closure: (Type, Type) -> (Result)) -> Result where Other : BinaryInteger {
        lhs._wait(); defer { lhs._signal() }
        var rhsValue = Type(rhs)
        if let rhs = rhs as? AtomicInteger {
            rhs._wait(); defer { rhs._signal() }
            rhsValue = rhs.value
        }
        let result = closure(lhs.value, rhsValue)
        return result
    }

    fileprivate static func atomicActionAndResultSaving<Other>(lhs: inout AtomicInteger<Type>,
                                                               rhs: Other, closure: (Type, Type) -> (Type)) where Other : BinaryInteger {
        lhs._wait(); defer { lhs._signal() }
        var rhsValue = Type(rhs)
        if let rhs = rhs as? AtomicInteger {
            rhs._wait(); defer { rhs._signal() }
            rhsValue = rhs.value
        }
        let result = closure(lhs.value, rhsValue)
        lhs.value = result
    }
}

// Math Operator Functions

extension AtomicInteger {

    static func != <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 != $1 }
    }

    static func != (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 != $1 }
    }

    static func % (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 % $1 }
        return self.init(value)
    }

    static func %= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 % $1 }
    }

    static func & (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 & $1 }
        return self.init(value)
    }

    static func &= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 & $1 }
    }

    static func * (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 * $1 }
        return self.init(value)
    }

    static func *= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 * $1 }
    }

    static func + (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 + $1 }
        return self.init(value)
    }
    static func += (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 + $1 }
    }

    static func - (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 - $1 }
        return self.init(value)
    }

    static func -= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 - $1 }
    }

    static func / (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 / $1 }
        return self.init(value)
    }

    static func /= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 / $1 }
    }
}


// Shifting Operator Functions

extension AtomicInteger {
    static func << <RHS>(lhs:  AtomicInteger<Type>, rhs: RHS) -> AtomicInteger where RHS : BinaryInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 << $1 }
        return self.init(value)
    }

    static func <<= <RHS>(lhs: inout AtomicInteger, rhs: RHS) where RHS : BinaryInteger {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 << $1 }
    }

    static func >> <RHS>(lhs: AtomicInteger, rhs: RHS) -> AtomicInteger where RHS : BinaryInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 >> $1 }
        return self.init(value)
    }

    static func >>= <RHS>(lhs: inout AtomicInteger, rhs: RHS) where RHS : BinaryInteger {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 >> $1 }
    }
}

// Comparing Operator Functions

extension AtomicInteger {

    static func < <Other>(lhs: AtomicInteger<Type>, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 < $1 }
    }

    static func <= (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 <= $1 }
    }

    static func == <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 == $1 }
    }

    static func > <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 > $1 }
    }

    static func > (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 > $1 }
    }

    static func >= (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 >= $1 }
    }

    static func >= <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 >= $1 }
    }
}

// Binary Math Operator Functions

extension AtomicInteger {

    static func ^ (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 ^ $1 }
        return self.init(value)
    }

    static func ^= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 ^ $1 }
    }

    static func | (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 | $1 }
        return self.init(value)
    }

    static func |= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 | $1 }
    }

    static prefix func ~ (x: AtomicInteger) -> AtomicInteger {
        x._wait(); defer { x._signal() }
        return self.init(x.value)
    }
}

// Hashable

extension AtomicInteger {

    var hashValue: Int {
        _wait(); defer { _signal() }
        return value.hashValue
    }

    func hash(into hasher: inout Hasher) {
        _wait(); defer { _signal() }
        value.hash(into: &hasher)
    }
}

// Get/Set

extension AtomicInteger {

    // Single  actions

    func get() -> Type {
        _wait(); defer { _signal() }
        return value
    }

    mutating func set(value: Type) {
        _wait(); defer { _signal() }
        self.value = value
    }

    // Multi-actions

    func get(closure: (Type)->()) {
        _wait(); defer { _signal() }
        closure(value)
    }

    mutating func set(closure: (Type)->(Type)) {
        _wait(); defer { _signal() }
        self.value = closure(value)
    }
}

用法

// Usage Samples
let numA = AtomicInteger<Int8>(0)
let numB = AtomicInteger<Int16>(0)
let numC = AtomicInteger<Int32>(0)
let numD = AtomicInteger<Int64>(0)

var num1 = AtomicInteger<Int>(0)
num1 += 1
num1 -= 1
num1 = 10
num1 = num1/2

var num2 = 0
num2 = num1.get()
num1.set(value: num2*5)

// lock num1 to do several actions
num1.get { value in
    //...
}

num1.set { value in
    //...
    return value
}

完整样本

import Foundation

var x = AtomicInteger<Int>(0)
let dispatchGroup = DispatchGroup()
private func async(dispatch: DispatchQueue, closure: @escaping (DispatchQueue)->()) {
    for _ in 0 ..< 100 {
        dispatchGroup.enter()
        dispatch.async {
            print("Queue: \(dispatch.qos.qosClass)")
            closure(dispatch)
            dispatchGroup.leave()
        }
    }
}

func sample() {
    let closure1: (DispatchQueue)->() = { _ in x += 1 }
    let closure2: (DispatchQueue)->() = { _ in x -= 1 }
    async(dispatch: .global(qos: .userInitiated), closure: closure1) // result: x += 100
    async(dispatch: .global(qos: .utility), closure: closure1) // result: x += 100
    async(dispatch: .global(qos: .background), closure: closure2) // result: x -= 100
    async(dispatch: .global(qos: .default), closure: closure2) // result: x -= 100
}

sample()
dispatchGroup.wait()
print(x) // expected result x = 0

Answer 5

我通过使用一些重载运算符改进了@florian的答案：

import Foundation

class AtomicInt {

    private var mutex = pthread_mutex_t()
    private var integer: Int = 0
    var value : Int {
        return integer
    }


    //MARK: - lifecycle


    init(_ value: Int = 0) {
        pthread_mutex_init(&mutex, nil)
        integer = value
    }

    deinit {
        pthread_mutex_destroy(&mutex)
    }


    //MARK: - Public API


    func increment() {
        pthread_mutex_lock(&mutex)
        defer {
            pthread_mutex_unlock(&mutex)
        }
        integer += 1
    }

    func incrementAndGet() -> Int {
        pthread_mutex_lock(&mutex)
        defer {
            pthread_mutex_unlock(&mutex)
        }
        integer += 1
        return integer
    }

    func decrement() {
        pthread_mutex_lock(&mutex)
        defer {
            pthread_mutex_unlock(&mutex)
        }
        integer -= 1
    }

    func decrementAndGet() -> Int {
        pthread_mutex_lock(&mutex)
        defer {
            pthread_mutex_unlock(&mutex)
        }
        integer -= 1
        return integer
    }


    //MARK: - overloaded operators

   static func > (lhs: AtomicInt, rhs: Int) -> Bool {
        return lhs.integer > rhs
    }

    static func < (lhs: AtomicInt, rhs: Int) -> Bool {
        return lhs.integer < rhs
    }

    static func == (lhs: AtomicInt, rhs: Int) -> Bool {
        return lhs.integer == rhs
    }

    static func > (lhs: Int, rhs: AtomicInt) -> Bool {
        return lhs > rhs.integer
    }

    static func < (lhs: Int, rhs: AtomicInt) -> Bool {
        return lhs < rhs.integer
    }

    static func == (lhs: Int, rhs: AtomicInt) -> Bool {
        return lhs == rhs.integer
    }

    func test() {
        let atomicInt = AtomicInt(0)
        atomicInt.increment()
        atomicInt.decrement()
        if atomicInt > 10  { print("bigger than 10") }
        if atomicInt < 10  { print("smaller than 10") }
        if atomicInt == 10 { print("its 10") }
        if 10 > atomicInt  { print("10 is bigger") }
        if 10 < atomicInt  { print("10 is smaller") }
        if 10 == atomicInt { print("its 10") }
    }

}

Answer 6

您可以查看由 Apple 托管的支持通用类型的 Swift Atomics 库

[Concurrency theory]

Answer 7

在撰写本文时，这被证明是最简单的解决方案，直到我们获得适当的Swift原子类型为止。您可以使用该通用类来设置任何种类的原子value。

为此滥用DispatchQueue是没有道理的。首先，您只能使用基于块的同步，创建可能不必要的闭包/堆栈框架，仅用于内部DispatchSemaphore队列，而不是直接创建队列。

final class Atomic<T> {

    private let sema = DispatchSemaphore(value: 1)
    private var _value: T

    init (_ value: T) {
        _value = value
    }

    var value: T {
        get {
            sema.wait()
            defer {
                sema.signal()
            }
            return _value
        }
        set {
            sema.wait()
            _value = newValue
            sema.signal()
        }
    }

    func swap(_ value: T) -> T {
        sema.wait()
        let v = _value
        _value = value
        sema.signal()
        return v
    }
}

用法：

let atomicBool = Atomic<Bool>(false)
atomicBool.value = true

let atomicInt = Atomic<Int>(0)
let oldInt = atomicInt.swap(1)

如果您想增加Int，请使用以下扩展名

extension Atomic where T == Int {

    func increment(n: Int) -> Int {
        sema.wait()
        let v = _value + n
        _value = v
        sema.signal()
        return v
    }
}

Answer 8

我们可以使用多种方法以原子方式快速递增变量，here中对此进行了讨论。

还有一个快速的建议SE-0283，可以在本地快速添加原子变量。

如何在Swift中以原子方式递增变量？

8 个答案:

详细

解决方案

用法

完整样本