How could I create a function with a completion handler in Swift

Question

I was just curious as to how I would approach this  If I had a function  and I wanted something to happen when it was fully executed  how would I add this into the function  Thanks

User · Accepted Answer

Say you have a download function to download a file from network, and want to be notified when download task has finished.

typealias CompletionHandler = (success:Bool) -> Void

func downloadFileFromURL(url: NSURL,completionHandler: CompletionHandler) {

    // download code.

    let flag = true // true if download succeed,false otherwise

    completionHandler(success: flag)
}

// How to use it.

downloadFileFromURL(NSURL(string: "url_str")!, { (success) -> Void in

    // When download completes,control flow goes here.
    if success {
        // download success
    } else {
        // download fail
    }
})

Hope it helps.

User · Answer

In addition to above   Trailing closure can be used    downloadFileFromURL NSURL string   url str         success  - gt  Void in       When download completes control flow goes here    if success            download success     else          download fail

User · Answer

I m a little confused about custom made completion handlers  In your example   Say you have a download function to download a file from network and want to be notified when download task has finished   typealias CompletionHandler    success Bool  - gt  Void  func downloadFileFromURL url  NSURL completionHandler  CompletionHandler            download code       let flag   true    true if download succeed false otherwise      completionHandler success  flag      Your    download code will still be ran asynchronously  Why wouldn t the code go straight to your let flag   true and completion Handler success  flag  without waiting for your download code to be finished

User · Answer

Simple Example  func method arg  Bool  completion   Bool  - gt            print  quot First line of code executed quot          do stuff here to determine what you want to  quot send back quot          we are just sending the Boolean value that was sent in  quot back quot      completion arg     How to use it  method arg  true  completion     success  - gt  Void in     print  quot Second line of code executed quot       if success      this will be equal to whatever value is set in this method call           print  quot true quot         else            print  quot false quot

User · Answer

I had trouble understanding the answers so I m assuming any other beginner like myself might have the same problem as me    My solution does the same as the top answer but hopefully a little more clear and easy to understand for beginners or people just having trouble understanding in general   To create a function with a completion handler  func yourFunctionName finished     - gt  Void          print  Doing something          finished        to use the function       override func viewDidLoad                yourFunctionName                do something here after running your function            print  Tada                            Your output will be     Doing something      Tada      Hope this helps

User · Answer

Swift 5 0     Simple and Short example  Style 1     func methodName completionBlock     - gt  Void                print  quot block Completion quot             completionBlock          Style 2     func methodName completionBlock     - gt                  print  quot block Completion quot           completionBlock          Use      override func viewDidLoad             super viewDidLoad                    methodName                print  quot Doing something after Block Completion   quot                    Output  block Completion Doing something after Block Completion

User · Answer

We can use Closures for this purpose  Try the following   func loadHealthCareList completionClosure   indexes  NSMutableArray - gt                some code here       completionClosure indexes  list      At some point we can call this function as given below   healthIndexManager loadHealthCareList    indexes  - gt     in             print indexes      Please refer the following link for more information regarding Closures   https   developer apple com library ios documentation Swift Conceptual Swift Programming Language Closures html

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