Another small wrapper for multiple DispatchWorkItems

Scheduler2.swift

PlaygroundPage.current.needsIndefiniteExecution = true

typealias DispatcherIdentifier = String

extension DispatchQueue {
    static var associatedValueKey = 0

    func schedule(after timeInterval: TimeInterval,
                  with identifier: DispatcherIdentifier,
                  action: @escaping () -> Void) {
        var items =
            objc_getAssociatedObject(self, &DispatchQueue.associatedValueKey) as? [DispatcherIdentifier: DispatchWorkItem]
                ?? [:]

        cancelAction(with: identifier)

        print("Scheduled \(identifier)")
        let item = DispatchWorkItem(block: action)
        items[identifier] = item
        objc_setAssociatedObject(self, &DispatchQueue.associatedValueKey, items, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)

        self.asyncAfter(deadline: .now() + timeInterval, execute: item)
    }

    @discardableResult
    func cancelAction(with identifier: DispatcherIdentifier) -> Bool {
        guard
            var items = objc_getAssociatedObject(self, &DispatchQueue.associatedValueKey) as? [DispatcherIdentifier: DispatchWorkItem],
            let item = items[identifier]
            else {
                return false
        }

        defer {
            objc_setAssociatedObject(self, &DispatchQueue.associatedValueKey, items, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }

        guard !item.isCancelled else {
            items[identifier] = nil
            return false
        }

        item.cancel()
        print("Cancelled \(identifier)")
        return true
    }
}

let dispatcher = DispatchQueue.main

dispatcher.schedule(after: 2, with: "first") {
    print("---> Hey, I'm first")
}

dispatcher.schedule(after: 3, with: "first") {
    print("---> No dude, I'm first")
}

dispatcher.schedule(after: 4, with: "second") {
    print("---> I'm second, but I'll be cancelled")
}

dispatcher.schedule(after: 5, with: "third") {
    print("---> And I'm third")
}

DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
    dispatcher.cancelAction(with: "second")
}