Stop a DispatchQueue that is running on the main thread

Question

I have this block of code:

    DispatchQueue.main.asyncAfter(deadline: .now() + (delay * Double(isDelayAccounted.hashValue)) + extraDelay) {
        self.isShootingOnHold = false
        self.shoot()
        self.shootingEngine = Timer.scheduledTimer(timeInterval: (Double(60)/Double(self.ratePerMinute)), target: self, selector: #selector(ShootingEnemy.shoot), userInfo: nil, repeats: true)   
    }

Now, I want to be able to stop this thread from executing. How can I stop it from being executed? For instance, after 3 seconds, I decide I don't want that to execute anymore so I want to stop it.

You can invalidate the block https://stackoverflow.com/a/25120393/2000162 — TomCobo, Jun 19 '17 at 15:14

score 32 · Accepted Answer · answered Jun 19 '17 at 15:15

You can use DispatchWorkItems. They can be scheduled on DispatchQueues and cancelled before their execution.

let work = DispatchWorkItem(block: {
    self.isShootingOnHold = false
    self.shoot()
    self.shootingEngine = Timer.scheduledTimer(timeInterval: (Double(60)/Double(self.ratePerMinute)), target: self, selector: #selector(ShootingEnemy.shoot), userInfo: nil, repeats: true)
})
DispatchQueue.main.asyncAfter(deadline: .now() + (delay * Double(isDelayAccounted.hashValue)) + extraDelay, execute: work)
work.cancel()

vadian · Answer 2 · 2017-06-19T15:47:11.880

You could use an one-shot DispatchSourceTimer rather than asyncAfter

var oneShot : DispatchSourceTimer!

 oneShot = DispatchSource.makeTimerSource(queue: DispatchQueue.main)
 oneShot.scheduleOneshot(deadline: .now() + (delay * Double(isDelayAccounted.hashValue)) + extraDelay))
 oneShot.setEventHandler {
     self.isShootingOnHold = false
     self.shoot()
     self.shootingEngine = Timer.scheduledTimer(timeInterval: (Double(60)/Double(self.ratePerMinute)), target: self, selector: #selector(ShootingEnemy.shoot), userInfo: nil, repeats: true)   
 }
 oneShot.setCancelHandler {
     // do something after cancellation
 }

 oneShot.resume()

and cancel the execution with

oneShot?.cancel()
oneShot = nil

Stop a DispatchQueue that is running on the main thread

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