Why does a function in my iOS app get called twice?

Question

I made my very first iOS app. But there are two annoying bugs which I cannot get rid of. I hope somebody can help me!

The app is supposed to train to read musical notation. The user specifies his instrument and level (on the previous viewcontroller) and based on that, it places random notes in musical notation on the screen. The user should match those notes in textfields and the app keeps track of the score and advances a level after ten right answers.

However, somehow I'm having problems with the function which generates the random notes. The function for some reason gets called twice, the first time it generates the notes, saves them in a global variable and creates the labels with the notes. The second time, it changes the global variable but not the labels. It returns the following error message this time: 2018-09-29 23:08:37.279170+0200 MyProject[57733:4748212] Warning: Attempt to present <MyProject.ThirdViewController: 0x7fc709125890> on <MyProject.SecondViewController: 0x7fc70900fcd0> whose view is not in the window hierarchy! Because of this, the user answers the question on the screen, but the app thinks it's the wrong answer, because it has the second answer stored.

The second time the user answers a question, the function is only called once, but the read-out from the text fields doesn't update to the new values, but keeps the same as with the first question.

Here is the code which gives the problems:

import UIKit

class ThirdViewController: UIViewController
{

// snip

func setupLabels() {
    // snip
    // here the random notes are created, this is function is called multiple times for some reason
    let antwoord = Noten()
    let antwoordReturn = antwoord.generateNoten(instrument: instrument, ijkpunt: ijkpunt, aantalNoten: aantalNoten-1)
    let sleutel = antwoordReturn.0
    let heleOpgave = antwoordReturn.1
    print(heleOpgave)
    print(PassOpgave.shared.category)
    let heleOpgaveNummers = antwoordReturn.2
    // snip
    var a = 0
    while a < aantalNoten {
        // the labels are created, no problems there
        let myTekstveld = UITextField(frame: CGRect(x: labelX, y: labelY + 150, width: labelWidth, height: labelHeight / 2))
        myTekstveld.backgroundColor = UIColor.white
        myTekstveld.textAlignment = .center
        myTekstveld.placeholder = "?"
        myTekstveld.keyboardType = UIKeyboardType.default
        myTekstveld.borderStyle = UITextField.BorderStyle.line
        myTekstveld.autocorrectionType = .no
        myTekstveld.returnKeyType = UIReturnKeyType.done
        myTekstveld.textColor = UIColor.init(displayP3Red: CGFloat(96.0/255.0), green: CGFloat(35.0/255.0), blue: CGFloat(123.0/255.0), alpha: 1)
        myTekstveld.delegate = self as? UITextFieldDelegate
        myTekstveld.tag = a + 1
        view.addSubview(myTekstveld)

        a += 1
        labelX += labelWidth
    }

    // the button is created
}

override func viewDidLoad()
{
    super.viewDidLoad()
    // snip
    setupLabels()
}

@objc func buttonAction(sender: UIButton!) {
    // snip
    // here the text from the text fields is read, but this only works the first time the buttonAction is called, the next times, it simply returns the first user input.
    while a <= aantalNoten {
        if let theLabel = view.viewWithTag(a) as? UITextField {
            let tekstInput = theLabel.text!
            userInput.append(tekstInput)
        }
        a += 1
    }

    // snip 

    setupLabels()
    return
}

// snip

Answer 1

You have two instances of ThirdViewController when you don't mean to.

This error is very telling:

2018-09-29 23:08:37.279170+0200 MyProject[57733:4748212] Warning: Attempt to present <MyProject.ThirdViewController: 0x7fc709125890> on <MyProject.SecondViewController: 0x7fc70900fcd0> whose view is not in the window hierarchy!

This is telling you that SecondViewController is trying to create ThirdViewController when SecondViewController is not even on the screen. This suggests that the mistake is in SecondViewController (perhaps observing notifications or other behaviors when not on screen). It's possible of course that you also have two instances of SecondViewController.

I suspect you're trying to build all of this by hand rather than letting Storyboards do the work for you. That's fine, but these kinds of mistakes are a bit more common in that case. The best way to debug this further is to set some breakpoints and carefully check the address of the objects (0x7fc709125890 for example). You'll need to hunt down where you're creating an extra one.

Answer 2

Your genreteNoten method is being called multiple times because it is called from setupLabels which is In turn called from viewDidLoad.

viewDidLoad may be called multiple times and your code should account for that. As it says in this answer to a similar question:

If you have code that only needs to run once for your controller use -awakeFromNib.

Answer 3

I managed to partially solve my second problem myself (that the read-out from the text fields was not updating to the second answer) by not creating them again. I added some code to setupLabels function to only create the text fields if there was no input already:

let myTekstveld = UITextField()
if (view.viewWithTag(a+1) as? UITextField) != nil {  
}
else {
myTekstveld.frame = CGRect(x: labelX, y: labelY + 100, width: labelWidth, height: labelHeight / 2)
// snip
myTekstveld.tag = a + 1
view.addSubview(myTekstveld)
}

The app works as expected now, the only problem is that the text fields are not cleared after each question.