(Swift) NSTimer Stop when Scrolling

Question

Help me. I tried to make NSTimer with UIScrollView. but NSTimer stop during Scrolling on UIScroll View..

How can I keep work NSTimer during scrolling?

Answer 1

I created a simple project with a scrollView and a label that is updated with an NSTimer. When creating the timer with scheduledTimerWithInterval, the timer does not run when scrolling.

The solution is to create the timer with NSTimer:timeInterval:target:selector:userInfo:repeats and then call addTimer on NSRunLoop.mainRunLoop() with mode NSRunLoopCommonModes. This allows the timer to update while scrolling.

Here it is running:

Here is my demo code:

class ViewController: UIViewController {

    @IBOutlet weak var timerLabel: UILabel!
    var count = 0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timerLabel.text = "0"

        // This doesn't work when scrolling
        // let timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "update", userInfo: nil, repeats: true)

        // Do these two lines instead:
        let timer = NSTimer(timeInterval: 1, target: self, selector: "update", userInfo: nil, repeats: true)

        NSRunLoop.mainRunLoop().addTimer(timer, forMode: NSRunLoopCommonModes)
    }

    func update() {
        count += 1
        timerLabel.text = "\(count)"
    }
}

Swift 3:

let timer = Timer(timeInterval: 1, target: self, selector: #selector(update), userInfo: nil, repeats: true)
RunLoop.main.add(timer, forMode: RunLoopMode.commonModes)

Swift 4, 5:

let timer = Timer(timeInterval: 1, target: self, selector: #selector(update), userInfo: nil, repeats: true)
RunLoop.main.add(timer, forMode: RunLoop.Mode.common)