Completion blocks return nothing

Question

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {

        isSuccess(true, success: { (name) -> String in
            return "My name is \(name)"
        })

        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

    }


    func isSuccess(val:Bool, success: (name: String) -> String) {
        if val {
            success(name: "Jacky")
        }
    }

}

I expect it to return string "My name is Jacky",but it didn't .But if I change the isSuccess to :

    func isSuccess(val:Bool, success: (name: String) -> String) {
        if val {
            print(success(name: "Jacky"))
        }
    }

}

Then it worked properly, why is that? Thanks in advance!

how are you testing what it's returning? you aren't capturing it anywhere — Wain, Feb 24 '16 at 11:20
@Wain I'm not familiar with completion handlers, could you describe it in detail ? Thanks — ilovecomputer, Feb 24 '16 at 11:24

score 1 · Accepted Answer · answered Feb 24 '16 at 11:58

Your completion block returns a String. When you invoke it by calling

success(name: "Jacky")

the completion block returns the String My name is Jacky. But you do nothing with that string. You just returned it and never used it. In your second example, you actually used it - you took the string from the completion block, and printed it.

For example, instead the print, you could also write

let stringFromCompletionBlock = success(name: "Jacky")

That way you can see that it indeed returned a value.

And another thing is that the completion block should be call as the last thing in the function - this way you "notify" that the function has finished it's purpose, so it's not reasonable to use the value returned from a completion block inside the same function which called that completion block

I tried replacing print with let ,but it actually won't return anything still. — ilovecomputer, Feb 24 '16 at 13:37

score 0 · Answer 2 · answered Feb 24 '16 at 12:03

First of all the closure in the function isSuccess should be declared this way. The closure should not return a String, it should just accept a String as param.

func isSuccess(val:Bool, success: (name: String) -> ()) {
    if val {
        success(name: "Jacky")
    }
}

Next you could use that value to update the UI like follow

class ViewController: UIViewController {

    weak var label:UILabel!

    override func viewDidLoad() {

        isSuccess(true) { (name) -> () in
            dispatch_async(dispatch_get_main_queue()) {
                self.label.text = "My name is \(name)"
            }
        }            
        super.viewDidLoad()
    }

    func isSuccess(val:Bool, success: (name: String) -> ()) {
        if val {
            success(name: "Jacky")
        }
    }   
}

Completion blocks return nothing

2 Answers2