Pass a closure as an optional parameter of the function

Question

I have a function which takes two parameters, the last parameter is a callback closure:

func myAsycTask(name: String, callback: @escaping ()->Void) {
   myQueue.async{
      self.doTask(name)
      callback()
   }
} 

func doTask(name: String) {...}

I would like to make the 2nd callback closure parameter optional. I tried to re-define the function above to:

func myAsycTask(name: String, callback: @escaping ()->Void? = nil) {
       myQueue.async{
          self.doTask(name)
          callback()
       }
}

I get a compiler error:

Nil default argument value of cannot be converted to type '() ->'

How can I achieve what I need then?

Well [answered](https://stackoverflow.com/questions/39618803/swift-3-optional-escaping-closure-parameter) — Jack, Nov 07 '17 at 09:02

score 5 · Answer 1 · answered Nov 07 '17 at 09:00

5

Your current code means that Void is an optional return in the closure (which does not make much sense, since Void is already nothing). You should enclose the parameter in brackets and then make it optional.

func myAsycTask(name: String, callback: (() -> Void)? = nil)

answered Nov 07 '17 at 09:00

Tamás Sengel

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But then the closure can not be `@escaping` since it's not a closure anymore but an optional closure. – Au Ris Nov 07 '17 at 09:04
Oh well spotted! Thanks! I wasn't aware of that myself, so good to know. – Au Ris Nov 07 '17 at 09:08

Oleh Zayats · Answer 2 · 2018-11-09T07:05:36.350

Try making your callback closure Optional and remove @escaping. @escaping annotation is pointless because your parameter is basically an enum (Optional is an enum with 2 cases: some(Value) and none) If your closure is owned by another type it is implicitly escaping.

import UIKit

// Also you can use a typealias to keep code more readable
typealias Callback = (() -> Void)

class Test {

    let myQueue = DispatchQueue(label: "com.playground")

    func doTask(name: String) {
        // something...
    }

    func myAsycTask(name: String, callback: Callback? = nil) {
        myQueue.async { [weak self] in
            self?.doTask(name: name)
            callback?()
        }
    }
}

Pass a closure as an optional parameter of the function

2 Answers2