README.md

Spinlock

The Dispatch framework does not provide a spinlock, likely for good reason. Here a spinlock protocol and class implementation are defined with simple code. The Playground can be used to try it out.

There really is no good use for a spinlock given that Dispatch already allows for sync logic which handles locking and priority inversion concerns. While a spinlock is waiting with the sleep calls no other work can be done on the main queue. It is essentially the same as using sync to put work on another queue while locking the current queue as a consequence.

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DispatchSpinlock.swift

import Foundation

public class DispatchSpinlock: Spinlock {
    private let sleepInterval: TimeInterval
    private var isDone: Bool = false
    private var timeout: DispatchTime = .distantFuture

    private var isTimedOut: Bool {
        let timedOut = DispatchTime.now() >= timeout
        return timedOut
    }

    public init(sleepInterval: TimeInterval = 0.1) {
        self.sleepInterval = sleepInterval
    }

    public func done() {
        isDone = true
    }

    public func wait() {
        wait(timeout: .distantFuture)
    }

    @discardableResult
    public func wait(timeout: DispatchTime) -> DispatchTimeoutResult {
        let result: DispatchTimeoutResult

        self.timeout = timeout

        while !isDone && !isTimedOut {
            print("Spinning")
            Thread.sleep(forTimeInterval: sleepInterval)
        }

        if isTimedOut {
            result = .timedOut
        } else {
            result = .success
        }
        return result
    }
}

Playground.swift

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

print("Creating spinlock")
let spinlock = DispatchSpinlock(sleepInterval: 1.0)

DispatchQueue.global().asyncAfter(deadline: .now() + 2.0) {
    spinlock.done()
}

print("Waiting...")
let result = spinlock.wait(timeout: .now() + 5.0)

print("Finished waiting...")

switch result {
case .success:
    print("success")
case .timedOut:
    print("timed out")
}

print("Done")
PlaygroundPage.current.finishExecution()

Spinlock.swift

import Foundation

public protocol Spinlock {
    func done()
    func wait()
    func wait(timeout: DispatchTime) -> DispatchTimeoutResult
}