dispatch_after - GCD in Swift?

Question

I've gone through the iBook from Apple, and couldn't find any definition of it:

Can someone explain the structure of dispatch_after?

dispatch_after(<#when: dispatch_time_t#>, <#queue: dispatch_queue_t?#>, <#block: dispatch_block_t?#>) 

Answer 1

I use dispatch_after so often that I wrote a top-level utility function to make the syntax simpler:

func delay(delay:Double, closure:()->()) {     dispatch_after(         dispatch_time(             DISPATCH_TIME_NOW,             Int64(delay * Double(NSEC_PER_SEC))         ),         dispatch_get_main_queue(), closure) } 

And now you can talk like this:

delay(0.4) {     // do stuff } 

Wow, a language where you can improve the language. What could be better?

---

Update for Swift 3, Xcode 8 Seed 6

Seems almost not worth bothering with, now that they've improved the calling syntax:

func delay(_ delay:Double, closure:@escaping ()->()) {     let when = DispatchTime.now() + delay     DispatchQueue.main.asyncAfter(deadline: when, execute: closure) } 

Answer 2

A clearer idea of the structure:

dispatch_after(when: dispatch_time_t, queue: dispatch_queue_t, block: dispatch_block_t?) 

dispatch_time_t is a UInt64. The dispatch_queue_t is actually type aliased to an NSObject, but you should just use your familiar GCD methods to get queues. The block is a Swift closure. Specifically, dispatch_block_t is defined as () -> Void, which is equivalent to () -> ().

Example usage:

let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC))) dispatch_after(delayTime, dispatch_get_main_queue()) {     print("test") } 

EDIT:

I recommend using @matt's really nice delay function.

EDIT 2:

In Swift 3, there will be new wrappers for GCD. See here: https://github.com/apple/swift-evolution/blob/master/proposals/0088-libdispatch-for-swift3.md

The original example would be written as follows in Swift 3:

let deadlineTime = DispatchTime.now() + .seconds(1) DispatchQueue.main.asyncAfter(deadline: deadlineTime) {     print("test") } 

Note that you can write the deadlineTime declaration as DispatchTime.now() + 1.0 and get the same result because the + operator is overridden as follows (similarly for -):

• func +(time: DispatchTime, seconds: Double) -> DispatchTime
• func +(time: DispatchWalltime, interval: DispatchTimeInterval) -> DispatchWalltime

This means that if you don't use the DispatchTimeInterval enum and just write a number, it is assumed that you are using seconds.