In Swift how to call method with parameters on GCD main thread

Question

In my app I have a function that makes an NSRURLSession and sends out an NSURLRequest using  sesh dataTaskWithRequest req  completionHandler    data  response  error    In the completion block for this task  I need to do some computation that adds a UIImage to the calling viewcontroller  I have a func called  func displayQRCode receiveAddr  withAmountInBTC amountBTC    that does the UIImage-adding computation  If I try to run the view-adding code inside of the completion block  Xcode throws an error saying that I can t use the layout engine while in a background process  So I found some code on SO that tries to queue a method on the main thread   let time   dispatch time DISPATCH TIME NOW  Int64 0 0   Double NSEC PER MSEC     dispatch after time  dispatch get main queue          let returned   UIApplication sharedApplication   sendAction  displayQRCode    to  self delegate  from  self  forEvent  nil       However  I don t know how to add the parameters  receiveAddr  and  amountBTC  to this function call  How would I do this  or can someone suggest an optimal way for adding a method call to the application s main queue

User · Answer

Reload collectionView on Main Thread  DispatchQueue main async       self collectionView reloadData

User · Answer

Modern versions of Swift use DispatchQueue main async to dispatch to the main thread   DispatchQueue main async         your code here     To dispatch after on the main queue  use   DispatchQueue main asyncAfter deadline   now     0 1         your code here     Older versions of Swift used   dispatch async dispatch get main queue        let delegateObj   UIApplication sharedApplication   delegate as YourAppDelegateClass   delegateObj addUIImage  yourstring

User · Answer

Perform some task and update UI immediately  DispatchQueue global qos   userInitiated  async            Call your function here     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

User · Answer

The proper way to do this is to use dispatch async in the main queue  as I did in the following code  dispatch async dispatch get main queue           self delegate as TBGQRCodeViewController  displayQRCode receiveAddr  withAmountInBTC amountBTC

User · Answer

Don t forget to weakify self if you are using self inside of the closure   dispatch async dispatch get main queue      weak self     - gt     in     if let strongSelf   self           self  doSomething

User · Answer

Swift 2  Using Trailing Closures this becomes    dispatch async dispatch get main queue          self tableView reloadData       Trailing Closures is Swift syntactic sugar that enables defining the closure outside of the function parameter scope  For more information see Trailing Closures in Swift 2 2 Programming Language Guide   In dispatch async case the API is func dispatch async queue  dispatch queue t    block  dispatch block t  since dispatch block t is type alias for    - gt  Void - A closure that receives 0 parameters and does not have a return value  and block being the last parameter of the function we can define the closure in the outer scope of dispatch async

User · Answer

Here s the nicer  IMO  Swifty Cocoa style syntax to achieve the same result as the other answers   NSOperationQueue mainQueue   addOperationWithBlock          Your code here      Or you could grab the popular Async Swift library for even less code and more functionality   Async main          Your code here

User · Answer

Swift 3   amp  Swift 4 version   DispatchQueue main async       print  Hello       Swift 3 and Xcode 9 2   dispatch async on main queue       print  Hello

User · Answer

Here s a nice little global function you can add for a nicer syntax   func dispatch on main block  dispatch block t        dispatch async dispatch get main queue    block      And usage  dispatch on main          Do some UI stuff

[parameters] In Swift how to call method with parameters on GCD main thread?

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