ios - Changing label text inside method

Question

I'm using delegation to pass back information from a view controller. This is the method

 func writeValueBack(value: String) {
  self.label.text = value
}

The function gets called and all is great apart from the label doesn't update.

The label has a value and is not returning nil, I checked with this line

println(self.label.text)

It prints the value of 'value'

So that means that the label's text is being set to 'value' but it's not updating.

I even tried using the main thread but no luck

func writeValueBack(value: String) {
    dispatch_async(dispatch_get_main_queue(), { () -> Void in
        self.label.text = value
    })
}

I just don't know what the problem is.

Any help would be great.

Protocol:

protocol writeValueBackDelegate {
func writeValueBack(value: String)
}

EDIT: Code for my view controller:

//
//  ViewController.swift
//  DelegateTesting
//
//  Created by Alex Catchpole on 30/11/2014.
//  Copyright (c) 2014 Alex Catchpole. All rights reserved.
//

import UIKit

class ViewController: UIViewController, writeValueBackDelegate {
@IBOutlet weak var label: UILabel!
@IBOutlet weak var textField: UITextField!


override func viewDidLoad() {
    super.viewDidLoad()
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "MainSegue" {
        var vc = segue.destinationViewController as SecondViewController
        vc.labelText = textField.text
        vc.delegate = self
    }
}

func writeValueBack(value: String) {
    dispatch_async(dispatch_get_main_queue(), { () -> Void in
        self.label.text = value
    })
}


@IBAction func button(sender: AnyObject) {
    self.label.text = textField.text
}

@IBAction func segue(sender: AnyObject) {
    performSegueWithIdentifier("MainSegue", sender: self)

}

}

Second ViewController source:

//
//  SecondViewController.swift
//  DelegateTesting
//
//  Created by Alex Catchpole on 30/11/2014.
//  Copyright (c) 2014 Alex Catchpole. All rights reserved.
//

import UIKit

class SecondViewController: UIViewController {
@IBOutlet weak var label: UILabel!
@IBOutlet weak var textField: UITextField!

var labelText: String!
var delegate: writeValueBackDelegate? = nil

override func viewDidLoad() {
    super.viewDidLoad()
    label.text = labelText

    // Do any additional setup after loading the view.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}
@IBAction func button(sender: AnyObject) {
    label.text = textField.text
}


@IBAction func segueBack(sender: AnyObject) {
    var editedText = label.text
    performSegueWithIdentifier("SecondSegue", sender: self)
    if (delegate != nil) {
        delegate?.writeValueBack(editedText)
        println("working")
    }
}
}

Answer 1

The issue is in your second view controller, which as rdelmar pointed out creates a new instance of your first view controller instead of navigating back to the original instance.

To fix this, you could use dismissViewControllerAnimated:completion instead of performing your second segue. But an unwind segue would lead to simpler code, and can be achieved by adding this to your first view controller:

@IBAction func unwindFromSecond(unwindSegue: UIStoryboardSegue) {
    if let secondViewController = unwindSegue.sourceViewController as? SecondViewController {
        label.text = secondViewController.label.text
        // Or whatever you need to retrieve data from the second controller
    }
}

Then in your storyboard create an unwind segue from the second view controller. For example if you have a Dismiss button, control-drag from this button to the Exit icon in your second view controller scene and choosing unwindFromSecond. For detailed steps see the answer to this other question: What are Unwind segues for and how do you use them?

You can now remove the writeValueBackDelegate declaration and associated variables, the writeValueBack method and the second view controller's segueBack method