[swift-users] Mutex/queue access in Swift 3

Adrian Zubarev adrian.zubarev at devandartist.com
Fri Sep 23 11:09:32 CDT 2016

Whops sry wrong receiver mail. :/ My bad.

Adrian Zubarev
Sent with Airmail

Am 23. September 2016 um 17:51:44, Adrian Zubarev (adrian.zubarev at devandartist.com) schrieb:

Do you care about the order in which your queue is filled?
Do you have multiple queues - I’m asking because your serial queue is not part of Queue<T>?!
You could add your DispatchQueue right into Queue<T>. It depends on your situation and your goal but sometime you could optimise your code with something like self._queue.async(flags: .barrier) { … }.

This required a concurrent dispatch queue, it will not block (like sync does) but it won’t run asynchronously with any other task in your dispatch queue (flags: .barrier).

Adrian Zubarev
Sent with Airmail

Am 23. September 2016 um 17:34:56, Ron Olson via swift-users (swift-users at swift.org) schrieb:

Hey all-

I used Erica's queue example (http://ericasadun.com/2016/03/08/swift-queue-fun/) to implement a queue in a multi-threaded Swift app (quick aside, is it 'correct' to say multithreaded when using DispatchQueue?). I spawn a number of objects on a .concurrent DispatchQueue and they all throw strings into the queue, where the main thread pops from the queue and prints it out.

In C/C++ I'd create a mutex and use that for pushing and popping. In Swift 3 I did wrapped the methods in a serial queue:

let serialQueue = DispatchQueue(label: "log.queue")

// http://ericasadun.com/2016/03/08/swift-queue-fun/
public struct Queue<T>: ExpressibleByArrayLiteral {
/// backing array store
public private(set) var elements: Array<T> = []

/// introduce a new element to the queue in O(1) time
public mutating func push(_ value: T) {
    serialQueue.sync {

/// remove the front of the queue in O(`count` time
public mutating func pop() -> T? {
    var retValue: T? = nil

    serialQueue.sync {
        if isEmpty == false {
            retValue = elements.removeFirst()

    return retValue

/// test whether the queue is empty
public var isEmpty: Bool { return elements.isEmpty }

/// queue size, computed property
public var count: Int {
    var count: Int = 0

    serialQueue.sync {
        count = elements.count
    return count

/// offer `ArrayLiteralConvertible` support
public init(arrayLiteral elements: T...) {
    serialQueue.sync {
        self.elements = elements

This is working; I have tested it with 50, uh, threads, and have had zero problems. So I'm content to go on my merry way and use it, but wanted to get some thoughts about whether this is the 'right' way and if there is something more Swift-y/libDispatch-y that I should use instead.

Thanks for any info,


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