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

问题

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

回答1:

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.

回答2:

Simple Swift 4.0 example:

func method(arg: Bool, completion: (Bool) -> ()) {
    print("First line of code executed")
    // do stuff here to determine what you want to "send back".
    // we are just sending the Boolean value that was sent in "back"
    completion(arg)
}

How to use it:

method(arg: true, completion: { (success) -> Void in
    print("Second line of code executed")
    if success { // this will be equal to whatever value is set in this method call
          print("true")
    } else {
         print("false")
    }
})

回答3:

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: () -> 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!

回答4:

We can use Closures for this purpose. Try the following

func loadHealthCareList(completionClosure: (indexes: NSMutableArray)-> ()) {
      //some code here
      completionClosure(indexes: list)
}

At some point we can call this function as given below.

healthIndexManager.loadHealthCareList { (indexes) -> () 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

回答5:

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) -> 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?

回答6:

In addition to above : Trailing closure can be used .

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

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

来源：https://stackoverflow.com/questions/30401439/how-could-i-create-a-function-with-a-completion-handler-in-swift

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function

swift

completionhandler