Optional closures in Swift 4

Question

I need a closure method executed on demand.

In Swift 3 I used to do something like this:

// Declare closure
var checksPerformed: ((Void) -> Void)? // Declaration

// Call when needed
checksPerformed?() 

//Only executes when checksPerformed is called
checksPerformed = { _ in 
    // do stuff here
}

In Swift 4 this is no longer the case.

And after fixing this warning nothing works as before. What is the new way of doing this?

If I'm updating the declaration to: var checksPerformed: (() -> ())? I'm getting

Just update it as: `var checksPerformed: (() -> Void)?` or `var checksPerformed: (() -> ())?` either way is fine. I personally avoid using `Void` instead I just use `()` where appropriate. — oyalhi, Jul 22 '17 at 10:57
@oyalhi The consensus is to use `Void` for return values only. — Sulthan, Jul 22 '17 at 11:07
Please check [this thread](https://stackoverflow.com/q/45183961/6541007). — OOPer, Jul 22 '17 at 11:11

score 6 · Accepted Answer · answered Jul 22 '17 at 11:12

Distinguish between single-tuple and multiple-argument function types

// Declare closure
var checksPerformed: (() -> ())? // Declaration

// Call when needed
checksPerformed?()

//Only executes when checksPerformed is called
checksPerformed = {
    // do stuff here
}

HixField · Answer 2 · 2017-07-22T11:24:08.543

0

Use

var checksPerformed: (()->(Void))?

If you do that everything should work as expected.

Dont use the _ in when assigning the closure:

//Only executes when checksPerformed is called
checksPerformed = { 
    // do stuff here
}

//Call when needed
checksPerformed?()

edited Jul 22 '17 at 11:24

answered Jul 22 '17 at 10:54

HixField

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In the last paragraph I said I did. It didn't work. I'll update my original message to include the error. And I cannot do this: `` var checksPerformed: ()->(Void)?`` . The ``Class has no initializers`` is thrown – kernelpanic Jul 22 '17 at 11:00
You have to do (()->(Void))? this declares it as an optional closure; sorry in my org answer the ? Was part of the question sentence. – HixField Jul 22 '17 at 11:04
Updated the answer to made it more clear – HixField Jul 22 '17 at 11:07
Your answer is the same as in the comments below my questions. And it still doesn't work (see my updated answer with a screenshot) – kernelpanic Jul 22 '17 at 11:09
@kernelpanic Remove the `_ in`. There are no parameters. Do you know that `_` is a placeholder for a parameter? In your original code you actually had a closure with one parameter of type `Void`, not a closure without parameters. – Sulthan Jul 22 '17 at 11:09
I agree the _ should be removed in assigning the closure – HixField Jul 22 '17 at 11:12
Make sure you first assign the closure before calling it. In the code the order is wrong – HixField Jul 22 '17 at 11:14

Optional closures in Swift 4

2 Answers2