Wait itself within endless loop, but user can cancel every time - swift

Question

In Swift 3 I have a loop which can be canceled by user pressing a button. Within the loop some checks are made. After the check, the task can sleep for a minute. But when calling the task with
let delayQueue = DispatchQueue(label: "com.myApp.queue3", qos: .utility)
let additionalTime: DispatchTimeInterval = .seconds(3)
repeat {
delayQueue.asyncAfter(deadline: .now() + additionalTime) { self.update() }
} while !self.stop
the loop itself needs to run all the time waiting for the user
"stop", indicates, that user clicked on stop button.
Is that waste of CPU power? How could I avoid this loop to be done all the time?

Answer 1

You should use Timer instead.
var timer: Timer?
let timeInterval: TimeInterval = 3
func didPressCancelButton() {
timer?.invalidate()
}
func beginUpdates() {
timer = Timer.scheduledTimer(
timeInterval: timeInterval,
target: self,
selector: #selector(self.update),
userInfo: nil,
repeats: true
);
}
func update() {
print("Updated")
}

Answer 2

Instead of delaying execution in thread with an outer loop you can put your loop in thread instead and make it to sleep.
import Foundation
class YourUpdatingClass {
private let updateQueue: OperationQueue
init() {
updateQueue = OperationQueue()
updateQueue.name = "com.myApp.queue3"
updateQueue.qualityOfService = .utility
}
private var updateOperation: BlockOperation?
#IBAction func startUpdating() {
guard updateOperation == nil else {
// In case if updating already started
return
}
updateOperation = BlockOperation { [weak self] in
while true {
Thread.sleep(forTimeInterval: 3)
self?.update()
}
}
updateQueue.addOperation(updateOperation!) // we just created updateOperation, so we can use `!`, but use it with caution
}
#IBAction func stopUpdating() {
updateOperation?.cancel()
updateOperation = nil
}
private func update() {
print("update") // Whatever your update does
}
}
You updating is contained in eternal while loop which takes a nap every 3 seconds.
Stopping is managed by cancelling operation, instead of checking some var in the loop.