Swift - Optional Void

Question

I was busy using NSURLProtocolClient's URLProtocol function:

welf?.client?.URLProtocol(welf!, didReceiveResponse: operation.response, cacheStoragePolicy: NSURLCacheStoragePolicy.NotAllowed)

I was expecting it to return Void. But to my surprise it returns Void?

Why is it necessary to make a distinction between Void and Void?

I have read that Void is a type alias for the empty tuple type. So, does this have something to do with a distinction between the empty tuple type vs nil?

As drewag says. Looking at the documentation confirms the return type (there is no mention of an optional return value). — lindon fox
– lindon fox, Commented Nov 10, 2014 at 6:41
I was also surprised. In my case I was passing a closure like { some?.call() } and Swift complained since the receiving parameter was expecting a function of type () -> () but that closure had an implicit type of () -> ()?. I finally casted the closure to () -> () but I don't know if this is correct. — Ferran Maylinch
– Ferran Maylinch, Commented Aug 21, 2017 at 14:33

drewag · Accepted Answer · 2014-11-10 06:39:17Z

7

This is simply because you are using Optional Chaining. The method returns Void, but it is possible for the whole chain to return nil before the method is ever called.

Essentially, a return value of Void will mean the call was actually made (self and client both have values) while a nil result will mean that one of those were nil.

answered Nov 10, 2014 at 6:39

drewag

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1 Comment

BastiBen Over a year ago

This is actually a very nice tip! Thanks. Where did you learn about this?

rintaro · Accepted Answer · 2014-11-10 06:53:10Z

6

Note that, () and nil is different:

let a:Void? = ()
let b:Void? = nil

a == nil // -> false
b == nil // -> true

Using this, you can judge the method has really been invoked or not.

let result = welf?.client?.URLProtocol(welf!, didReceiveResponse: operation.response, cacheStoragePolicy: NSURLCacheStoragePolicy.NotAllowed)
if result != nil {
    // success
}
else {
    // `welf?.client` was `nil`
}

answered Nov 10, 2014 at 6:53

rintaro

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