Execute a method when a variable value changes in Swift

Question

I need to execute a function when a variable value changes.

I have a singleton class containing a shared variable called labelChange. Values of this variable are taken from another class called Model. I have two VC classes, one of them has a button and a label and the second only a button.

When the button in the first VC class is pressed I am updating the label with this func:

func updateLabel(){
    self.label.text = SharingManager.sharedInstance.labelChange
}

But I want to call the same method whenever the value of the labelChange is changed. So in button click I will only update the labelChange value and when this thing happen I want to update the label with the new value of the labelChange. Also in the second VC I am able to update the labelChange value but I am not able to update the label when this value is changed.

Maybe properties are the solution but can anyone show me how to do so.

Edited second time:

Singleton Class:

class SharingManager {
    func updateLabel() {
        println(labelChange)
        ViewController().label.text = SharingManager.sharedInstance.labelChange     
    }
    var labelChange: String = Model().callElements() {
        willSet {
            updateLabel()
        }
    }
    static let sharedInstance = SharingManager()
}

First VC:

class ViewController: UIViewController {
    @IBOutlet weak var label: UILabel!
    @IBAction func Button(sender: UIButton) {    
       SViewController().updateMessageAndDismiss()
    }
}

Second VC:

func updateMessageAndDismiss() {
        SharingManager.sharedInstance.labelChange = modelFromS.callElements()
        self.dismissViewControllerAnimated(true, completion: nil)
    }
@IBAction func b2(sender: UIButton) { 
        updateMessageAndDismiss()
}

I made some improvements but I need to reference a label from the first VC class in singleton. Therefore I will update that label of VC in singleton.

When I print the value of labelChange the value is being updated and everything is fine. But when I try to update that value on label from singleton I receive an error:

unexpectedly found nil while unwrapping an Optional value

and the error is pointing in 4th line of singleton class.

Answer 1

You can simply use a property observer for the variable, labelChange, and call the function that you want to call inside didSet (or willSet if you want to call it before it has been set):

class SharingManager {
    var labelChange: String = Model().callElements() {
        didSet {
            updateLabel()
        }
    }
    static let sharedInstance = SharingManager()
}

This is explained in Property Observers.

I'm not sure why this didn't work when you tried it, but if you are having trouble because the function you are trying to call (updateLabel) is in a different class, you could add a variable in the SharingManager class to store the function to call when didSet has been called, which you would set to updateLabel in this case.

Edited:

So if you want to edit a label from the ViewController, you would want to have that updateLabel() function in the ViewController class to update the label, but store that function in the singleton class so it can know which function to call:

class SharingManager {
    static let sharedInstance = SharingManager()
    var updateLabel: (() -> Void)?
    var labelChange: String = Model().callElements() {
        didSet {
            updateLabel?()
        }
    }
}

and then set it in whichever class that you have the function that you want to be called, like (assuming updateLabel is the function that you want to call):

SharingManager.sharedInstance.updateLabel = updateLabel

Of course, you will want to make sure that the view controller that is responsible for that function still exists, so the singleton class can call the function.

If you need to call different functions depending on which view controller is visible, you might want to consider Key-Value Observing to get notifications whenever the value for certain variables change.

Also, you never want to initialize a view controller like that and then immediately set the IBOutlets of the view controller, since IBOutlets don't get initialized until the its view actually get loaded. You need to use an existing view controller object in some way.

Hope this helps.