How to send back data using Closures in Swift iOS?-CodePudding

I'm following this tutorial to send data back using Closures. https://betterprogramming.pub/5-ways-to-pass-data-between-view-controllers-18acb467f5ec

in this tutorial point no 4 that is "Closures". I have two VC's one for selecting pet (FormsVC) and one for displaying selected pet (ProfileVC).

below is a code for ProfileVC:

// ProfileVC

// MARK: - Set Fav Pet Name
    
    func setPetName(pet: String) {
        lblFavouritePet.text = pet
    }

// MARK: - Button Select Your Fav Pet Event

    @IBAction func btnSelectYourFavPet_Event(_ sender: UIButton) {
        
        let vc = FormsVC()
        
        self.present(vc, animated: true)
    }

below is a code for FormsVC:

// FormsVC

// MARK: - Variable Declaration
    
    var favoritePet = String()

// MARK: - viewDidLoad Method

    override func viewDidLoad() {
        super.viewDidLoad()

        setUpFormsVC()
        
    }

// MARK: - Set Up FormsVC
    
    func setUpFormsVC() {
       
        btnDog.titleLabel?.text = "Dog"
        btnCat.titleLabel?.text = "Cat"
        btnRabbit.titleLabel?.text = "Rabbit"
        btnBird.titleLabel?.text = "Bird"
        
    }

// MARK: - Button Selected Pet Event
    
    @IBAction func selectedPetEvent(_ sender: UIButton) {
        
        favoritePet = sender.titleLabel?.text ?? "Dog"
        
    }

// MARK: - Selected Pet Name
    
    func getFavoritePet() -> String {
        return favoritePet
    }

// MARK: - Button OK Event

    @IBAction func btnOk_Event(_ sender: UIButton) {
        
        let vc = ProfileVC()
        
        self.dismiss(animated: true, completion: {
            vc.setPetName(pet: self.getFavoritePet())
        })

// problem occurs when I dismiss FormsVC after selecting pet, the label displaying selected pet name (lblFavouritePet) throwing error of "Unexpectedly found nil while implicitly unwrapping an Optional value"
        
    }
}

Problem occurs when I dismiss FormsVC after selecting pet, the label displaying selected pet name (lblFavouritePet) throwing error of "Unexpectedly found nil while implicitly unwrapping an Optional value". I have no idea why it is found nil because I have assigned favoritePet's value of selected pet. Sorry for this dumb question, Could anyone help me ?

CodePudding user response：

First of all, as @matt mentioned, you should take the presenting view controller, not create the new instance. It's stated in the tutorial you use:

if let vc = presentingViewController as? Profile...

Your app crashes, because you use storyboards, and lblFavoritePet is an @IBOutlet implicitly unwrapped optional, hence, you should initialize it from the storyboard. But you initialize it without using the storyboard, and the property remains nil.

So, don't make a new instance, use the code that is stated in the tutorial.

And follow the naming conventions.

CodePudding user response：

First of all, you have to declarer the closer where you want to pass data.

// FormsVC
// MARK: - Variable Declaration
let completion: ((String)->Void)? = nil

// MARK: - Button OK Event
@IBAction func btnOk_Event(_ sender: UIButton) {
    
    completion?(self.getFavoritePet())
    self.dismiss(animated: true)
}

The second part is you have to write the code to receive the data

// ProfileVC
// MARK: - Button Select Your Fav Pet Event

@IBAction func btnSelectYourFavPet_Event(_ sender: UIButton) {
    
    let vc = FormsVC()
    vc.completion = { petName in
        self.setPetName(pet: petName)
    }

    self.present(vc, animated: true)
}