How does one make an optional closure in swift?

Question

I\'m trying to declare an argument in Swift that takes an optional closure. The function I have declared looks like this:

class Promise {

 func then(onFulfi         


        

Answer 1

You should enclose the optional closure in parentheses. This will properly scope the ? operator.

func then(onFulfilled: ()->(), onReject: (()->())?){       
    if let callableRjector = onReject {
      // do stuff! 
    }
 }