Unexpected non-void return value in void function Swift3

Question

I have a function that returns either a class object or nil. The function's purpose is to check if a Chat exists. The chat ID's are stored in MySQL. If the ID exists, I perform a Firebase reference to get a snapshot and then get the object. If the ID does not exist, I return nil:

func findChat(string: String) -> Chat? {

    var returnValue: (Chat?)
    let url = getChatsURL
    let Parameters = [ "title" : string ]

    Alamofire.request("\(url)", method: .post, parameters: Parameters).validate().responseString { response in
            if let anyResponse = response.result.value {
                self.responseFromServer = anyResponse
            }

            if self.responseFromServer == "" {
              returnValue = nil
            } else {
                let ref = DatabaseReference.chats.reference()
                let query = ref.queryOrdered(byChild: "uid").queryEqual(toValue: (self.responseFromServer))
                query.observe(.childAdded, with: { snapshot in
                returnValue = Chat(dictionary: snapshot.value as! [String : Any])
            })
        }
        return returnValue
    }

}

However, at return returnValue I am getting

Unexpected non-void return value in void function.

Any thoughts of what I could be missing?

Similar to [this issue](https://stackoverflow.com/a/32121273/6541007). — OOPer, Aug 28 '17 at 19:05

score 11 · Accepted Answer · answered Aug 28 '17 at 19:10

The problem is that you are trying to return a non-void value from inside a closure, which only returns from the closure, but since that closure expects a void return value, you receive the error.

You cannot return from an asynchronous function using the standard return ... syntax, you have to declare your function to accept a completion handler and return the value from the async network call inside the completion handler.

func findChat(string: String, completion: @escaping (Chat?)->()) {
    var returnValue: (Chat?)
    let url = getChatsURL
    let Parameters = [ "title" : string ]

    Alamofire.request("\(url)", method: .post, parameters: Parameters).validate().responseString { response in
        if let anyResponse = response.result.value {
            self.responseFromServer = anyResponse
        }
        if self.responseFromServer == "" {
            completion(nil)
        } else {
            let ref = DatabaseReference.chats.reference()
            let query = ref.queryOrdered(byChild: "uid").queryEqual(toValue: (self.responseFromServer))
                query.observe(.childAdded, with: { snapshot in
                completion(Chat(dictionary: snapshot.value as! [String : Any]))
            })
        }
    }
}

Then you can call this function and use the return value like this:

findChat(string: "inputString", completion: { chat in
    if let chat = chat {
        //use the return value
    } else {
        //handle nil response
    }
})

Wow! Thank you very much. The solution works perfectly. I will need to study up some more to understand Why though! — Sente, Aug 28 '17 at 19:44
Glad I could help. If you want to know more about the topic, look into asynchronous functions and completion handlers (if you are really keen, even closures in general, completion handlers are actually closures). — Dávid Pásztor, Aug 28 '17 at 20:05

score 3 · Answer 2 · answered Aug 28 '17 at 19:04

Your block is executed asynchronously, but you're trying to return a value from the enclosing function. It doesn't work that way. Your findChat function needs to take a completion block itself instead of returning a value, and then you can call that completion block from the point where you're trying to say return returnValue.

Unexpected non-void return value in void function Swift3

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