Nested closures does not like argument list

Question

A UIView needs to change a warning label depending on the completion handler of a custom control:

    voucherInputView.completionHandler = {[weak self] (success: Bool) -> Void in

        self?.proceedButton.enabled = success
        self?.warningLabel.alpha = 1.0

        if success
        {
            self?.warningLabel.text = "Code you entered is correct"
            self?.warningLabel.backgroundColor = UIColor.greenColor()
        }
        else
        {
            self?.warningLabel.text = "Code you entered is incorrect"
            self?.warningLabel.backgroundColor = UIColor.orangeColor()
        }


        UIView.animateWithDuration(NSTimeInterval(1.0), animations:{ ()-> Void in
            self?.warningLabel.alpha = 1.0
        })

The final animation block shows an error in the form.

Cannot invoke 'animateWithDuration' with an argument list of type '(NSTimeInterval), animations: ()-> Void)'

If i call this somewhere outside of the completion closure it works.

Ain

Answer 1

The problem is that the closure is implicitly returning the result of this expression:

self?.warningLabel.alpha = 1.0

but the closure itself is declared as returning Void.

Adding an explicit return should resolve the problem:

UIView.animateWithDuration(NSTimeInterval(1.0), animations: { ()-> Void in
    self?.warningLabel.alpha = 1.0
    return
})

Ger

This fixed it for me, but would someone mind to explain why this behavior is so weird and unexpected for many people? BTW, in your example you can replace ()->Void with _ and append return using ; return to the same line. ALSO, you can write ; () instead of a single-line return. :) – Guv

@badcat When you write a closure without a return keyword, the closure automatically returns the last statement. It does this so array.sort {$0.index < $1.index} works. If your last statement is an assignment of optional type, it will return Void?. You can see this if you let Xcode infer the type: () -> ()?. The empty return statement implicitly returns Void (the non-optional type). Fun fact: Void is a typealias for (), the empty tuple, which all functions return if you don't type a return statement. (1/2) – Accommodation

(2/2) Functions can handle optional Void as results from the last line as they don't need to guess any returns. All returns must be explicit. – Accommodation

@badcat What is the sweetness in writing ; return or ; () over just return? Above all, ; return has more characters to type than return? Or if Antonio can answer, please do. Thanks in advance. – Crispin

@Unheilig: You can write the closure in a single line - in that case the 2 statements must be separated: self?.warningLabel.alpha = 1.0; return – Ger

@Ger Now I see it. Already upvoted before your reply. Thanks. – Crispin

Answer 2

Antonio's solution also applies with nested closures, like doing an AFNetworking request within UITableViewRowAction handler.

override func tableView(tableView: UITableView, editActionsForRowAtIndexPath indexPath: NSIndexPath) -> [AnyObject]? {

    let cleanRowAction = UITableViewRowAction(style: UITableViewRowActionStyle.Default, title: "Do Stuff", handler: {[weak self](action: UITableViewRowAction!, indexPath: NSIndexPath!) in

        AFHTTPSessionManager(baseURL: NSURL(string: "http://baseurl")).PUT("/api/", parameters: nil, success: { (task: NSURLSessionDataTask!, response: AnyObject!) -> Void in

                // Handle success

                self?.endEditing()
                return
            }, failure: { (task: NSURLSessionDataTask!, error: NSError!) -> Void in

                // Handle error

                self?.endEditing()
                return
        })
        return

    })

    cleanRowAction.backgroundColor = UIColor.greenColor()
    return [cleanRowAction]
}

Mastin