Callback function syntax in Swift

Question

I am attempting pass a function to another function and then have the passed function executed passing to it a variable.

Here is my code:

func showStandardPrompt(prompt:String,view: UIViewController,numberInput: Bool, callback: (()->(String))?) {
    let alert = UIAlertController(title: "Input Data", message: prompt, preferredStyle: .Alert)
    alert.addTextFieldWithConfigurationHandler { (textField) in
        if numberInput {
            textField.keyboardType = .NumberPad
        }
    }

    let OKAction = UIAlertAction(title: "OK", style: .Default) { (action) in
        let field = alert.textFields![0] as UITextField
        callback?(field.text!)
    }

    alert.addAction(OKAction)
    let CancelAction = UIAlertAction(title: "Cancel", style: .Default, handler: nil)
    alert.addAction(CancelAction)
    view.presentViewController(alert,animated: true, completion: nil)
}

The error I get is in

callback?(field.text!)

The error is "Cannot convert value type of 'String' to expected argument type '()'. I know it's a matter of syntax - just don't know what it should be.

Please try to indent your code properly, because it's difficult to understand otherwise. I fixed it for you this time. — rob mayoff, Oct 22 '15 at 21:01

score 127 · Accepted Answer · edited Jun 11 '20 at 07:51

127

Rob's answer is correct, though I'd like to share an example of a simple working callback / completion handler, you can download an example project below and experiment with the getBoolValue's input.

Swift 5:

func getBoolValue(number : Int, completion: (Bool)->()) {
    if number > 5 {
        completion(true)
    } else {
        completion(false)
    }
}

getBoolValue(number: 2) { (result) -> () in
    // do stuff with the result
    print(result)
}

Important to understand:

(String)->() // takes a String returns void
()->(String) // takes void returns a String

edited Jun 11 '20 at 07:51

multitudes

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answered Oct 22 '15 at 21:08

Dan Beaulieu

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That is not a playground. – Price Ringo Oct 22 '15 at 21:37
1

@PriceRingo thanks, I thought I uploaded a playground. It was a while ago – Dan Beaulieu Oct 22 '15 at 21:38
1

`completion(result: number>5)` ? – GoZoner Oct 23 '15 at 04:13
Thanks for the example. That' exactly what I needed to make my project work! And, for me, this is so much better than the old way of UIAlertView where I had to set a tag and then check the button index and tag in the delegate method. – Floyd Resler Oct 23 '15 at 12:52
Not a problem. This exact example is what made callbacks clear to me. – Dan Beaulieu Oct 23 '15 at 12:54
Great, just what I need, in swift 4 the changes are a little bit different: in the parameters should be `func getBoolValue(number : Int, completion: @escaping (_ result: Bool)->()) {...}` and to set value should be `completion(true)` . I hope this clarification will be useful to you. – iGhost May 09 '19 at 04:29
2

this gonna be *Function types cannot have argument labels; use '_' before 'result' *error – Zaporozhchenko Oleksandr Aug 28 '19 at 11:28

score 24 · Answer 2 · answered Dec 30 '18 at 04:40

24

I use this way :

ViewController1

destination.callback = { (id) -> Void in
            print("callback")
            print(id)
        }

ViewController2

var callback: ((_ id: Int) -> Void)?
callback?(example_id)

answered Dec 30 '18 at 04:40

Álvaro Agüero

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score 16 · Answer 3 · answered Nov 11 '17 at 16:53

try below code updated for Swift 3

func getBoolValue(number : Int, completion: (Bool)->()) {
    if number > 5 {
        completion(true)
    } else {
        completion(false)
    }
}

getBoolValue(number : 8, completion:{ result in
    print(result)
})

score 7 · Answer 4 · answered Oct 22 '15 at 21:03

You've declared callback to take no arguments, and then you're trying to pass it an argument.

You specified type (()->(String))?, which is an optional function that takes no arguments and returns a String.

Perhaps you mean to specify ((String)->())?, which is an optional function that takes a String and returns nothing.

score 2 · Answer 5 · answered Oct 22 '15 at 21:06

In the parameters for showStandardPrompt, you declare callback to have the type ()->(String), which is a function which takes no parameters and returns a String.

You then call it with field.text! as a parameter, which obviously conflicts with the previously given type.

You need to correct the type given to callback to take a String argument and return nothing:

(String)->()

Callback function syntax in Swift

5 Answers5

Swift 5:

Linked