Stop dispatch_after - swift

Question

I use an animation for specify a tip to help the interaction with delay using these:
let delay = 1.8 * Double(NSEC_PER_SEC)
let time = dispatch_time(DISPATCH_TIME_NOW, Int64(delay))
dispatch_after(time, dispatch_get_main_queue()) {
//call the method which have the steps after delay.
self.rain.alpha = 0
UIView.animateWithDuration(5, animations: {
self.rain.alpha = 1
})
self.tip.startAnimating()
}
But, I need to stop this delay process if, before animation start, user touch the screen.

Answer 1

iOS 8 and OS X Yosemite introduced dispatch_block_cancel that allow you to cancel them before they start executing
You declare one variable in class as follows:
var block: dispatch_block_t?
Init block variable and provide it in dispatch_after:
block = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS) {
print("I executed")
}
let time: dispatch_time_t = dispatch_time(DISPATCH_TIME_NOW, Int64(5 * NSEC_PER_SEC))
dispatch_after(time, dispatch_get_main_queue(), block!)
After that you can cancel it as follows:
dispatch_block_cancel(block!)

Answer 2

Swift 3.0 Example DispatchQueue cancel or stop
var dispatchQueue: DispatchQueue?
var dispatchWorkItem: DispatchWorkItem?
func someOnClickButtonStart() {
self.dispatchQueue = DispatchQueue.global(qos: .background) // create queue
self.dispatchWorkItem = DispatchWorkItem { // create work item
// async code goes here
}
if self.dispatchWorkItem != nil {
self.dispatchQueue?.asyncAfter(
deadline: .now() + .seconds(1),
execute: self.dispatchWorkItem!
) // schedule work item
}
}
func someOnClickButtonCancel() {
if let dispatchWorkItem = self.dispatchWorkItem {
dispatchWorkItem.cancel() // cancel work item
}
}

Answer 3

Here's a general solution I wrote to cancel a dispatch_after in Swift:
typealias cancellable_closure = (() -> ())?
func dispatch_after(#seconds:Double, queue: dispatch_queue_t = dispatch_get_main_queue(), closure:()->()) -> cancellable_closure {
var cancelled = false
let cancel_closure: cancellable_closure = {
cancelled = true
}
dispatch_after(
dispatch_time(DISPATCH_TIME_NOW, Int64(seconds * Double(NSEC_PER_SEC))), queue, {
if !cancelled {
closure()
}
}
)
return cancel_closure
}
func cancel_dispatch_after(cancel_closure: cancellable_closure) {
cancel_closure?()
}
Usage:
let doSomethingLater = dispatch_after(seconds: 3.0) {
something()
}
....
if shouldCancelForSomeReason {
cancel_dispatch_after(doSomethingLater)
}
By default it runs on the main queue, but you can pass in a parameter for it to run on another queue:
let doSomethingLater = dispatch_after(seconds: 3.0, queue: dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
something()
}

Answer 4

Just sharing, in Swift 4.x, I do this:
var block: DispatchWorkItem?
self.block = DispatchWorkItem { self.go(self) }
// execute task in 2 seconds
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3), execute: self.block!)
and then to cancel the block, self.block?.cancel()
Try this sample project:
import UIKit
class ViewController: UIViewController {
var block: DispatchWorkItem?
#IBAction func go(_ sender: Any) {
self.block?.cancel()
let vc2 = VC2()
self.navigationController?.pushViewController(vc2, animated: true)
}
override func viewDidLoad() {
super.viewDidLoad()
self.view.backgroundColor = .white
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
self.block = DispatchWorkItem { self.go(self) }
// execute task in 2 seconds
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3), execute: self.block!)
}
}
class VC2: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
self.view.backgroundColor = .green
}
}

Answer 5

You can create a boolean variable shouldCancelAnimation and test it inside the dispatch_after block to prevent the execution of your animation.
var shouldCancelAnimation = false // property of class
func runAnimation()
{
let delay = 1.8 * Double(NSEC_PER_SEC)
let time = dispatch_time(DISPATCH_TIME_NOW, Int64(delay))
dispatch_after(time, dispatch_get_main_queue()) {
if !self.shouldCancelAnimation
{
self.rain.alpha = 0
UIView.animateWithDuration(5, animations: {
self.rain.alpha = 1
})
self.tip.startAnimating()
}
self.shouldCancelAnimation = false
}
}
func viewWasTouched() // This could be touches began or taprecognizer event
{
shouldCancelAnimation = true
}