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  lt  when  dispatch time t  gt    lt  queue  dispatch queue t   gt    lt  block  dispatch block t   gt

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Apple has a dispatch after snippet for Objective-C   dispatch after dispatch time DISPATCH TIME NOW   int64 t   lt  delayInSeconds  gt    NSEC PER SEC    dispatch get main queue            lt  code to be executed after a specified delay  gt        Here is the same snippet ported to Swift 3   DispatchQueue main asyncAfter deadline  DispatchTime now      lt  delayInSeconds  gt        lt  code to be executed after a specified delay  gt

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Swift 3 0  version  Following closure function execute some task after delay on main thread    func performAfterDelay delay   Double  onCompletion   escaping   - gt  Void        DispatchQueue main asyncAfter deadline  DispatchTime now     delay  execute           onCompletion              Call this function like   performAfterDelay delay  4 0      print  test

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1  Add this method as a part of UIViewController Extension   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            Call this method on VC            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         - gt     in       Code Here        Compact Form  dispatch after dispatch time DISPATCH TIME NOW  2   dispatch get main queue            Code here

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I always prefer to use extension instead of free functions   Swift 4   public extension DispatchQueue      private class func delay delay  TimeInterval  closure   escaping    - gt  Void        let when   DispatchTime now     delay     DispatchQueue main asyncAfter deadline  when  execute  closure         class func performAction after seconds  TimeInterval  callBack   escaping     - gt  Void          DispatchQueue delay delay  seconds          callBack                  Use as follow   DispatchQueue performAction after  0 3         Code Here

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In Swift 3 0   Dispatch queues    DispatchQueue label   test   async             long running Background Task         for obj in 0   1000               print  async   obj                          UI update in main queue         DispatchQueue main async execute                 print  UI update on main queue                          DispatchQueue label   m   sync             long running Background Task         for obj in 0   1000               print  sync   obj                          UI update in main queue         DispatchQueue main sync execute                print  UI update on main queue                      Dispatch after 5 seconds      DispatchQueue main after when  DispatchTime now     5            print  Dispatch after 5 sec

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Here is synchronous version of asyncAfter in Swift   let deadline   DispatchTime now      seconds 3  let semaphore   DispatchSemaphore init value  0  DispatchQueue global   asyncAfter deadline  deadline        dispatchPrecondition condition   onQueue DispatchQueue global         semaphore signal      semaphore wait     Along with asynchronous one   let deadline   DispatchTime now      seconds 3  DispatchQueue main asyncAfter deadline  deadline        dispatchPrecondition condition   onQueue DispatchQueue global

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Another way is to extend Double like this   extension Double      var dispatchTime  dispatch time t          get              return dispatch time DISPATCH TIME NOW Int64 self   Double NSEC PER SEC                      Then you can use it like this   dispatch after Double 2 0  dispatchTime  dispatch get main queue         - gt  Void in             self dismissViewControllerAnimated true  completion  nil           I like matt s delay function but just out of preference I d rather limit passing closures around

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Now more than syntactic sugar for asynchronous dispatches in Grand Central Dispatch  GCD  in Swift   add Podfile  pod  AsyncSwift    Then you can use it like this   let seconds   3 0 Async main after  seconds    print  Is called after 3 seconds     background after  6 0    print  At least 3 0 seconds after previous block  and 6 0 after Async code is called

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Swift 3  amp  4   You can create a extension on DispatchQueue and add function delay which uses DispatchQueue asyncAfter function internally  extension DispatchQueue       static func delay   delay  DispatchTimeInterval  closure   escaping    - gt                let timeInterval   DispatchTime now     delay         DispatchQueue main asyncAfter deadline  timeInterval  execute  closure            use    DispatchQueue delay  seconds 1         print  This is after delay

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Swift 4 has a pretty short way of doing this   Timer scheduledTimer withTimeInterval  2  repeats  false     timer  in        Your stuff here     print  hello

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dispatch after dispatch time DISPATCH TIME NOW   int64 t  10   NSEC PER SEC    dispatch get main queue                        The dispatch after         function takes three parameters      a delay   a dispatch queue   a block or closure   The dispatch after         function invokes the block or closure on the dispatch queue that is passed to the function after a given delay  Note that the delay is created using the dispatch time       function  Remember this because we also use this function in Swift   I recommend to go through the tutorial Raywenderlich Dispatch tutorial

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matt s syntax is very nice and if you need to invalidate the block  you may want to use this    typealias dispatch cancelable closure    cancel   Bool  - gt  Void  func delay time NSTimeInterval  closure   - gt Void  - gt   dispatch cancelable closure         func dispatch later clsr   - gt Void            dispatch after              dispatch time                  DISPATCH TIME NOW                  Int64 time   Double NSEC PER SEC                              dispatch get main queue    clsr             var closure dispatch block t    closure     var cancelableClosure dispatch cancelable closure       let delayedClosure dispatch cancelable closure     cancel in         if closure    nil               if  cancel    false                    dispatch async dispatch get main queue    closure                                    closure   nil         cancelableClosure   nil            cancelableClosure   delayedClosure      dispatch later           if let delayedClosure   cancelableClosure               delayedClosure cancel  false                       return cancelableClosure     func cancel delay closure dispatch cancelable closure          if closure    nil           closure  cancel  true            Use as follow   let retVal   delay 2 0        println  Later     delay 1 0        cancel delay retVal      credits  Link above seems to be down  Original Objc code from Github

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For multiple functions use this  This is very helpful to use animations or Activity loader for static functions or any UI Update   DispatchQueue main asyncAfter deadline   now     0 9                   Call your function 1             DispatchQueue main asyncAfter deadline   now     0 5                       Call your function 2                           For example - Use a animation before a tableView reloads  Or any other UI update after the animation       Start your amination   self startAnimation   DispatchQueue main asyncAfter deadline   now     0 9                        The animation will execute depending on the delay time                  self stopAnimation                   DispatchQueue main asyncAfter deadline   now     0 5                            Now update your view                       self fetchData                        self updateUI

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This worked for me   Swift 3   let time1   8 23 let time2   3 42     Delay 2 seconds  DispatchQueue main asyncAfter deadline   now     2 0        print  Sum of times    time1   time2        Objective-C   CGFloat time1   3 49  CGFloat time2   8 13      Delay 2 seconds  dispatch after dispatch time DISPATCH TIME NOW   int64 t  2 0   NSEC PER SEC    dispatch get main queue           CGFloat newTime   time1   time2      NSLog   New time   f   newTime

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I use dispatch after so often that I wrote a top-level utility function to make the syntax simpler   func delay delay Double  closure   - gt           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   - gt           let when   DispatchTime now     delay     DispatchQueue main asyncAfter deadline  when  execute  closure

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Swift 3   This is super-easy and elegant in Swift 3    DispatchQueue main asyncAfter deadline   now     4 5                   Older Answer   To expand on Cezary s answer  which will execute after 1 nanosecond  I had to do the following to execute after 4 and a half seconds   let delay   4 5   Double NSEC PER SEC  let time   dispatch time DISPATCH TIME NOW  Int64 delay   dispatch after time  dispatch get main queue    block    Edit  I discovered that my original code was slightly wrong   Implicit typing causes a compile error if you don t cast NSEC PER SEC to a Double   If anyone can suggest a more optimal solution I d be keen to hear it

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In Swift 5  use in the below    DispatchQueue main asyncAfter deadline   now     0 2  execute  closure       time gap  specify unit is second DispatchQueue main asyncAfter deadline   now      seconds 2                 Singleton shared   printDate                default time gap is second  you can reduce it     DispatchQueue main asyncAfter deadline   now     0 2                 just do it

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Although not the original question by the OP  certain NSTimer related questions have been marked as duplicates of this question  so it is worth including an NSTimer answer here   NSTimer vs dispatch after   NSTimer is more high level while dispatch after is more low level  NSTimer is easier to cancel  Canceling dispatch after requires writing more code    Delaying a task with NSTimer  Create an NSTimer instance   var timer   NSTimer     Start the timer with the delay that you need      invalidate the timer if there is any chance that it could have been called before timer invalidate      delay of 2 seconds timer   NSTimer scheduledTimerWithTimeInterval 2 0  target  self  selector   selector delayedAction   userInfo  nil  repeats  false     Add a function to be called after the delay  using whatever name you used for the selector parameter above    func delayedAction         print  Delayed action has now started       Notes   If you need to cancel the action before it happens  simply call timer invalidate    For a repeated action use repeats  true   If you have a one time event with no need to cancel then there is no need to create the timer instance variable  The following will suffice   NSTimer scheduledTimerWithTimeInterval 2 0  target  self  selector   selector delayedAction   userInfo  nil  repeats  false    See my fuller answer here

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Another helper to delay your code that is 100  Swift in usage and optionally allows for choosing a different thread to run your delayed code from   public func delay bySeconds seconds  Double  dispatchLevel  DispatchLevel    main  closure   escaping    - gt  Void        let dispatchTime   DispatchTime now     seconds     dispatchLevel dispatchQueue asyncAfter deadline  dispatchTime  execute  closure     public enum DispatchLevel       case main  userInteractive  userInitiated  utility  background     var dispatchQueue  DispatchQueue           switch self           case  main                  return DispatchQueue main         case  userInteractive       return DispatchQueue global qos   userInteractive          case  userInitiated         return DispatchQueue global qos   userInitiated          case  utility               return DispatchQueue global qos   utility          case  background            return DispatchQueue global qos   background                      Now you simply delay your code on the Main thread like this   delay bySeconds  1 5            delayed code     If you want to delay your code to a different thread   delay bySeconds  1 5  dispatchLevel   background            delayed code that will run on background thread       If you prefer a Framework that also has some more handy features then checkout HandySwift  You can add it to your project via Carthage then use it exactly like in the examples above  e g    import HandySwift      delay bySeconds  1 5            delayed code

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Delaying GCD call using asyncAfter in swift  let delayQueue   DispatchQueue label   com theappmaker in   qos   userInitiated  let additionalTime  DispatchTimeInterval    seconds 2    We can delay as   microseconds milliseconds nanoseconds  delayQueue asyncAfter deadline   now     0 60        print Date       delayQueue asyncAfter deadline   now     additionalTime        print Date

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Simplest solution in Swift 3 0  amp  Swift 4 0  amp  Swift 5 0  func delayWithSeconds   seconds  Double  completion   escaping    - gt            DispatchQueue main asyncAfter deadline   now     seconds             completion             Usage  delayWithSeconds 1         Do something

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In Swift 4  Use this snippet       let delayInSec   1 0     DispatchQueue main asyncAfter deadline   now     delayInSec              code here        print  It works

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use this code to perform some UI related task after 2 0 seconds               let delay   2 0             let delayInNanoSeconds   dispatch time DISPATCH TIME NOW  Int64 delay   Double NSEC PER SEC                let mainQueue   dispatch get main queue                dispatch after delayInNanoSeconds  mainQueue                     print  Some UI related task after delay                    Swift 3 0  version  Following closure function execute some task after delay on main thread    func performAfterDelay delay   Double  onCompletion   escaping   - gt  Void        DispatchQueue main asyncAfter deadline  DispatchTime now     delay  execute           onCompletion              Call this function like   performAfterDelay delay  4 0      print  test

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To execute a funtion or code after a delay use the next method DispatchQueue main asyncAfter deadline   now      secondsOfDelay             your code here           Example - In this example the funtion getShowMovies will be executed after 1 second DispatchQueue main asyncAfter deadline   now     1            self getShowMovies

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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    - gt  Void  which is equivalent to    - gt       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  - gt  DispatchTime func   time  DispatchWalltime  interval  DispatchTimeInterval  - gt  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

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