我使用动画来指定提示以帮助延迟交互：
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()
}
但是，如果在动画开始之前，用户触摸了屏幕，我需要停止这个延迟过程。
【问题讨论】：
iOS 8 和 OS X Yosemite 引入了dispatch_block_cancel，允许您在它们开始执行之前取消它们
你在类中声明一个变量如下：
var block: dispatch_block_t?
初始化block变量并在dispatch_after中提供：
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!)
之后您可以按如下方式取消它：
dispatch_block_cancel(block!)
【讨论】：
var block = DispatchWorkItem { //do stuff }，然后是 DispatchQueue.global().async(execute: block)，最后是 block.cancel()
Swift 3.0 示例 DispatchQueue 取消或停止
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
}
}
【讨论】：
DispatchWorkItem的原则是Swift中的正确解决方案。
这是我为在 Swift 中取消 dispatch_after 编写的通用解决方案：
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?()
}
用法：
let doSomethingLater = dispatch_after(seconds: 3.0) {
something()
}
....
if shouldCancelForSomeReason {
cancel_dispatch_after(doSomethingLater)
}
默认情况下它在主队列上运行，但你可以传入一个参数让它在另一个队列上运行：
let doSomethingLater = dispatch_after(seconds: 3.0, queue: dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
something()
}
【讨论】：
只是分享，在 Swift 4.x 中，我这样做：
var block: DispatchWorkItem?
self.block = DispatchWorkItem { self.go(self) }
// execute task in 2 seconds
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3), execute: self.block!)
然后取消阻塞，self.block?.cancel()
试试这个示例项目：
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
}
}
【讨论】：
您可以创建一个布尔变量shouldCancelAnimation 并在dispatch_after 块内对其进行测试，以防止您的动画执行。
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
}
【讨论】：