Nested closures does not like argument list

问题:

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.

回答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
})

回答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]
}