我翻遍了苹果的iBook,找不到任何关于它的定义:
有人能解释一下dispatch_after的结构吗?
dispatch_after(<#when: dispatch_time_t#>, <#queue: dispatch_queue_t?#>, <#block: dispatch_block_t?#>)
我翻遍了苹果的iBook,找不到任何关于它的定义:
有人能解释一下dispatch_after的结构吗?
dispatch_after(<#when: dispatch_time_t#>, <#queue: dispatch_queue_t?#>, <#block: dispatch_block_t?#>)
当前回答
保留当前队列!
除了很好地回答这个问题之外,您还可以考虑保留当前队列以防止不必要的主队列操作(例如,当您试图延迟一些异步操作时)。
func after(_ delay: TimeInterval,
perform block: @escaping ()->(),
on queue: DispatchQueue = OperationQueue.current?.underlyingQueue ?? .main) { // So this `queue` preserves the current queue and defaulted to the `main`. Also the caller can pass in the desired queue explicitly
queue.asyncAfter(deadline: .now() + delay, execute: block)
}
用法:
after(3) {
// will be executed on the caller's queue
print(Date())
}
其他回答
Apple为Objective-C提供了dispatch_after代码段:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(<#delayInSeconds#> * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
<#code to be executed after a specified delay#>
});
以下是移植到Swift 3的相同代码片段:
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + <#delayInSeconds#>) {
<#code to be executed after a specified delay#>
}
我经常使用dispatch_after,所以我写了一个顶级实用函数来简化语法:
func delay(delay:Double, closure:()->()) {
dispatch_after(
dispatch_time(
DISPATCH_TIME_NOW,
Int64(delay * Double(NSEC_PER_SEC))
),
dispatch_get_main_queue(), closure)
}
现在你可以这样说:
delay(0.4) {
// do stuff
}
哇,一种你可以改进的语言。还有什么比这更好的呢?
更新Swift 3, Xcode 8种子6
看起来几乎不值得费心,现在他们已经改进了调用语法:
func delay(_ delay:Double, closure:@escaping ()->()) {
let when = DispatchTime.now() + delay
DispatchQueue.main.asyncAfter(deadline: when, execute: closure)
}
要在延迟后执行函数或代码,请使用下一个方法
DispatchQueue.main.asyncAfter(deadline: .now() + 'secondsOfDelay') {
your code here...
}
在本例中,函数getShowMovies将在1秒后执行
DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
self.getShowMovies()
}
1)添加这个方法作为UIViewController Extension的一部分。
extension UIViewController{
func runAfterDelay(delay: NSTimeInterval, block: dispatch_block_t) {
let time = dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC)))
dispatch_after(time, dispatch_get_main_queue(), block)
}
}
在VC上调用这个方法:
self.runAfterDelay(5.0, block: {
//Add code to this block
print("run After Delay Success")
})
2)
performSelector("yourMethod Name", withObject: nil, afterDelay: 1)
3)
override func viewWillAppear(animated: Bool) {
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2), dispatch_get_main_queue(), { () -> () in
//Code Here
})
/ /紧凑的形式
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2), dispatch_get_main_queue()) {
//Code here
}
}
在Swift 4中
使用以下片段:
let delayInSec = 1.0
DispatchQueue.main.asyncAfter(deadline: .now() + delayInSec) {
// code here
print("It works")
}