Swift GCD Based Throttle.swift

import Foundation

class Throttle {
    private let queue: DispatchQueue
    private let delay: Double
    private var delayedBlock: (() -> Void)?
    private var cancelBlock: (() -> Void)?
    private var timer: DispatchSourceTimer?
    private var isReady = true
    private var hasDelayedBlock: Bool { delayedBlock != nil }

    init(queue: DispatchQueue, delay: Double) {
        self.queue = queue
        self.delay = delay
    }

    func callAsFunction(action: @escaping () -> Void, onCancel: @escaping () -> Void) {
        queue.async { [weak self] in
            guard let self = self else {
                onCancel()
                return
            }
            if self.isReady {
                self.isReady = false
                action()
                self.scheduleTimer()
            } else {
                self.cancelBlock?()
                self.cancelBlock = onCancel
                self.delayedBlock = action
            }
        }
    }

    private func scheduleTimer() {
        let timer = DispatchSource.makeTimerSource(queue: queue)
        timer.setEventHandler { [weak self] in
            guard let self = self else { return }
            if self.hasDelayedBlock {
                self.cancelBlock = nil
                self.delayedBlock?()
                self.delayedBlock = nil
                self.scheduleTimer()
            } else {
                self.isReady = true
            }
        }
        timer.schedule(deadline: .now() + delay)
        timer.resume()
        self.timer = timer
    }
}

// MARK: - Tests -
func print(_ msg: String) {
    let formatter = DateFormatter()
    formatter.dateFormat = "ss.SSSS"
    Swift.print("\(formatter.string(from: Date())) \(msg)")
}

let queue = DispatchQueue(label: "ThrottleQueue")
let throttle: Throttle = Throttle(queue: queue, delay: 0.5)

throttle {
    print("action 1 done")
} onCancel: {
    print("action 1 cancelled")
}

DispatchQueue.global().async {
    throttle {
        print("action 2 done")
    } onCancel: {
        print("action 2 cancelled")
    }
}

queue.asyncAfter(deadline: DispatchTime.now() + 0.2) {
    throttle {
        print("action 3 done")
    } onCancel: {
        print("action 3 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 0.3) {
    throttle {
        print("action 4 done")
    } onCancel: {
        print("action 4 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 0.4) {
    throttle {
        print("action 5 done")
    } onCancel: {
        print("action 5 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 0.5) {
    throttle {
        print("action 6 done")
    } onCancel: {
        print("action 6 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 0.7) {
    throttle {
        print("action 7 done")
    } onCancel: {
        print("action 7 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 0.75) {
    throttle {
        print("action 8 done")
    } onCancel: {
        print("action 8 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 0.8) {
    throttle {
        print("action 9 done")
    } onCancel: {
        print("action 9 cancelled")
    }
}

DispatchQueue.global().asyncAfter(deadline: DispatchTime.now() + 1.3) {
    throttle {
        print("action 10 done")
    } onCancel: {
        print("action 10 cancelled")
    }
}

queue.asyncAfter(deadline: DispatchTime.now() + 1.5) {
    throttle {
        print("action 11 done")
    } onCancel: {
        print("action 11 cancelled")
    }
}

queue.asyncAfter(deadline: DispatchTime.now() + 2.5) {
    throttle {
        print("action 12 done")
    } onCancel: {
        print("action 12 cancelled")
    }
}