Bdtjtk

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.

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.

download example project

func getBoolValue(number : Int, completion: (result: Bool)->()) {

    if number > 5 {

        completion(result: true)

    } else {

        completion(result: false)

    }

}



getBoolValue(8) { (result) -> () in

    // do stuff with the result

    print(result)

}

Important to understand:

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

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

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.

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)

})

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)->()

I use this way :

ViewController1

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

            print("callback")

            print(id)

        }

ViewController2

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

callback?(example_id)