How do I atomically increment a variable in Swift?

Question

I want to be able to increment a counter atomically and I can\'t find any reference on how to do it.

Adding more information based on comments:

• Are yo

Answer 1

Details

• Xcode 10.1 (10B61)
• Swift 4.2

Solution

import Foundation

struct AtomicInteger: 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(_ source: T) where T : BinaryInteger {
        value = Type(source)
    }

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

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

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

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

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

    init(_ 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) -> (quotient: AtomicInteger, remainder: AtomicInteger) {
        _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 {
        _wait(); defer { _signal() }
        return AtomicInteger(value.signum())
    }
}


extension AtomicInteger {

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

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

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

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

// Comparing Operator Functions

extension AtomicInteger {

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

    static func > (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 >= (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)
    }
}

Usage

// Usage Samples
let numA = AtomicInteger(0)
let numB = AtomicInteger(0)
let numC = AtomicInteger(0)
let numD = AtomicInteger(0)

var num1 = AtomicInteger(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
}

Full Sample

import Foundation

var x = AtomicInteger(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