If you want to default to nil: func foo(completionBlock: ((String) -> ())? = nil) { } If your default completion block is very simple, you can put it right in the function’s definition: // A default completion block that is simple enough to fit on one line func foo(completionBlock: (String) -> () = { result … Read more
The Swift designers decided to prohibit argument labels for function types. The reasoning is explained here: https://github.com/apple/swift-evolution/blob/master/proposals/0111-remove-arg-label-type-significance.md This is a frustrating and questionable choice, as prohibiting argument labels makes it much easier to incorrectly invoke closures, which seems more important than simplifying the language’s type system. Usability > ideology.
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: … Read more