How to use background thread in swift

Question

How to use threading in swift   dispatchOnMainThread         NSLog   Block Executed On  s   dispatch queue get label dispatch get current queue

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Swift 4 x  Put this in some file    func background work   escaping    - gt            DispatchQueue global qos   userInitiated  async           work            func main work   escaping    - gt            DispatchQueue main async           work             and then call it where you need   background          background job      main            update UI  or what you need to do in main thread

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Good answers though  anyway I want to share my Object Oriented solution Up to date for swift 5  please check it out  AsyncTask Conceptually inspired by android s AsyncTask  I ve wrote my own class in Swift AsyncTask enables proper and easy use of the UI thread  This class allows to perform background operations and publish results on the UI thread  Here are few usage examples Example 1 - AsyncTask backgroundTask    p String - gt Void in  set BGParam to String and BGResult to Void         print p    print the value in background thread        execute  quot Hello async quot     execute with value  Hello async   Example 2 - let task2 AsyncTask beforeTask               print  quot pre execution quot     print  pre execution  before backgroundTask           backgroundTask   p Int - gt String in  set BGParam to Int  amp  BGResult to String             if p gt 0   check if execution value is bigger than zero                return  quot positive quot   pass String  quot poitive quot  to afterTask                           return  quot negative quot    otherwise pass String  quot negative quot             afterTask    p String  in             print p    print background task result             task2 execute 1    execute with value 1  It has 2 generic types   BGParam - the type of the parameter sent to the task upon execution   BGResult - the type of the result of the background computation  When you create an AsyncTask you can those types to whatever you need to pass in and out of the background task  but if you don t need those types  you can mark it as unused with just setting it to  Void or with shorter syntax       When an asynchronous task is executed  it goes through 3 steps   beforeTask   - gt Void invoked on the UI thread just before the task is executed  backgroundTask   param BGParam - gt BGResult invoked on the background thread immediately after afterTask  param BGResult - gt Void invoked on the UI thread with result from the background task

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Swift 5  To make it easy  create a file  DispatchQueue Extensions swift  with this content    import Foundation  typealias Dispatch   DispatchQueue  extension Dispatch        static func background   task   escaping    - gt                Dispatch global qos   background  async               task                        static func main   task   escaping    - gt                Dispatch main async               task                       Usage     Dispatch background          do stuff      Dispatch main               update UI

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In Swift 4 2 and Xcode 10 1  We have three types of Queues    1  Main Queue  Main queue is a serial queue which is created by the system and associated with the application main thread   2  Global Queue   Global queue is a concurrent queue which we can request with respect to the priority of the tasks   3  Custom queues   can be created by the user  Custom concurrent queues always mapped into one of the global queues by specifying a Quality of Service property  QoS     DispatchQueue main  Main thread DispatchQueue global qos   userInitiated    High Priority DispatchQueue global qos   userInteractive   High Priority  Little Higher than userInitiated  DispatchQueue global qos   background   Lowest Priority DispatchQueue global qos   default   Normal Priority  after High but before Low  DispatchQueue global qos   utility   Low Priority DispatchQueue global qos   unspecified   Absence of Quality   These all Queues can be executed in two ways  1  Synchronous execution  2  Asynchronous execution  DispatchQueue global qos   background  async          do your job here     DispatchQueue main async              update ui here            Perform some task and update UI immediately  DispatchQueue global qos   userInitiated  async            Perform task     DispatchQueue main async                Update UI         self tableView reloadData                To call or execute function after some time DispatchQueue main asyncAfter deadline   now     5 0          Here call your function      If you want to do changes in UI use this DispatchQueue main async execute          Update UI     self tableView reloadData        From AppCoda   https   www appcoda com grand-central-dispatch     This will print synchronously means  it will print 1-9  amp  100-109 func simpleQueues         let queue   DispatchQueue label   com appcoda myqueue        queue sync           for i in 0   lt 10               print     i                       for i in 100   lt 110           print       i             This will print asynchronously  func simpleQueues         let queue   DispatchQueue label   com appcoda myqueue        queue async           for i in 0   lt 10               print     i                       for i in 100   lt 110           print       i

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Dan Beaulieu s answer in swift5  also working since swift 3 0 1    Swift 5 0 1  extension DispatchQueue        static func background delay  Double   0 0  background     - gt Void     nil  completion      - gt  Void     nil            DispatchQueue global qos   background  async               background                if let completion   completion                   DispatchQueue main asyncAfter deadline   now     delay  execute                        completion                                                         Usage  DispatchQueue background delay  3 0  background           do something in background    completion           when background job finishes  wait 3 seconds and do something in main thread     DispatchQueue background background           do something in background    completion          when background job finished  do something in main thread     DispatchQueue background delay  3 0  completion          do something in main thread after 3 seconds

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Since the OP question has already been answered above I just want to add some speed considerations   I don t recommend running tasks with the  background thread priority especially on the iPhone X where the task seems to be allocated on the low power cores   Here is some real data from a computationally intensive function that reads from an XML file  with buffering  and performs data interpolation   Device name    background    utility    default    userInitiated    userInteractive   iPhone X  18 7s   6 3s   1 8s   1 8s   1 8s iPhone 7  4 6s   3 1s   3 0s   2 8s   2 6s iPhone 5s  7 3s   6 1s   4 0s   4 0s   3 8s   Note that the data set is not the same for all devices  It s the biggest on the iPhone X and the smallest on the iPhone 5s

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The best practice is to define a reusable function that can be accessed multiple times   REUSABLE FUNCTION   e g  somewhere like AppDelegate swift as a Global Function   func backgroundThread   delay  Double   0 0  background      - gt  Void     nil  completion      - gt  Void     nil        dispatch async dispatch get global queue Int QOS CLASS USER INITIATED value   0             background             let popTime   dispatch time DISPATCH TIME NOW  Int64 delay   Double NSEC PER SEC            dispatch after popTime  dispatch get main queue                  completion                        Note  in Swift 2 0  replace QOS CLASS USER INITIATED value above with QOS CLASS USER INITIATED rawValue instead  USAGE   A  To run a process in the background with a delay of 3 seconds       backgroundThread 3 0  background                   Your background function here          B  To run a process in the background then run a completion in the foreground       backgroundThread background                   Your function here to run in the background            completion                   A function to run in the foreground when the background thread is complete          C  To delay by 3 seconds - note use of completion parameter without background parameter       backgroundThread 3 0  completion                   Your delayed function here to be run in the foreground

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Swift 3 0   A lot has been modernized in Swift 3 0  Running something on the background thread looks like this   DispatchQueue global qos   background  async       print  This is run on the background queue        DispatchQueue main async           print  This is run on the main queue  after the previous code in outer block             Swift 1 2 through 2 3  let qualityOfServiceClass   QOS CLASS BACKGROUND let backgroundQueue   dispatch get global queue qualityOfServiceClass  0  dispatch async backgroundQueue        print  This is run on the background queue        dispatch async dispatch get main queue         - gt  Void in         print  This is run on the main queue  after the previous code in outer block               Pre Swift 1 2      Known issue  As of Swift 1 1 Apple didn t support the above syntax without some modifications  Passing QOS CLASS BACKGROUND didn t actually work  instead use Int QOS CLASS BACKGROUND value    For more information see Apples documentation

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dispatch async dispatch get global queue QOS CLASS BACKGROUND  0            Conversion into base64 string     self uploadImageString    uploadPhotoDataJPEG base64EncodedStringWithOptions NSDataBase64EncodingOptions EncodingEndLineWithCarriageReturn

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You have to separate out the changes that you want to run in the background from the updates you want to run on the UI   dispatch async dispatch get global queue DISPATCH QUEUE PRIORITY DEFAULT  0            do your task      dispatch async dispatch get main queue                 update some UI

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in Swift 4 2 this works   import Foundation  class myThread  Thread       override func main             while true                print  Running in the Thread                Thread sleep forTimeInterval  4                      let t   myThread    t start     while true        print  Main Loop        sleep 5

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Swift 3 version  Swift 3 utilizes new DispatchQueue class to manage queues and threads  To run something on the background thread you would use   let backgroundQueue   DispatchQueue label   com app queue   qos   background  backgroundQueue async       print  Run on background thread       Or if you want something in two lines of code   DispatchQueue global qos   background  async       print  Run on background thread        DispatchQueue main async           print  We finished that               only back on the main thread  may you access UI          label text    Done             You can also get some in-depth info about GDC in Swift 3 in this tutorial

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Multi purpose function for thread  public enum QueueType           case Main         case Background         case LowPriority         case HighPriority          var queue  DispatchQueue               switch self               case  Main                  return DispatchQueue main             case  Background                  return DispatchQueue label   com app queue                                        qos   background                                       target  nil              case  LowPriority                  return DispatchQueue global qos   userInitiated              case  HighPriority                  return DispatchQueue global qos   userInitiated                                     func performOn   queueType  QueueType  closure   escaping    - gt  Void            queueType queue async execute  closure          Use it like     performOn  Background          Code

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From Jameson Quave s tutorial  Swift 2  dispatch async dispatch get global queue DISPATCH QUEUE PRIORITY DEFAULT  0           All stuff here

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Grand Central Dispatch is used to handle multitasking in our iOS apps   You can use this code      Using time interval  DispatchQueue main asyncAfter deadline  DispatchTime now   1        print  Hello World         Background thread queue sync        for i in 0   lt 10             print  Hello   i               Main thread for i in 20   lt 30        print  Hello   i      More information use this link   https   www programminghub us 2018 07 integrate-dispatcher-in-swift html

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I really like Dan Beaulieu s answer  but it doesn t work with Swift 2 2 and I think we can avoid those nasty forced unwraps   func backgroundThread delay  Double   0 0  background      - gt  Void     nil  completion      - gt  Void     nil         dispatch async dispatch get global queue QOS CLASS USER INITIATED  0              background             if let completion   completion              let popTime   dispatch time DISPATCH TIME NOW  Int64 delay   Double NSEC PER SEC                dispatch after popTime  dispatch get main queue                      completion

[ios] How to use background thread in swift?

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