Where to implement UserDefaults to save data when viewcontroller is closed

I am working on a tip tracker app that lets the user input as many tip values that they want, while showing them their input and also calculating a total and average. I am trying to use UserDefaults to save two doubles (totalValueOfAllTips, numberOfTipsTracked) and one string (sampleLog.text)

I want it to show the same data in `sampleLog`, `totalLabel`, and `avgLabel` every time the user re-opens the app.

Here is my TipTrackerViewController class, along with protocols that calculate the total, number of tips, and average value:

import UIKit
protocol TipTrackerProtocol {
    var totalValueOfAllTips: Double { get }
    var numberOfTipsTracked: Int { get }
}
extension TipTrackerProtocol {
    var averageTip: Double {
        return round((totalValueOfAllTips / Double(numberOfTipsTracked)) * 100) / 100 
    }
}
class TipTrackerViewController: UIViewController, TipTrackerProtocol {
  
    var numberOfTipsTracked = 0       // store as user default  
    var valDollar = 0                
    var valCent = 0                  
    var totalValueOfAllTips = 0.0     // store as user default
  
   
    @IBOutlet weak var dollarLabel: UILabel!       
    @IBOutlet weak var centLabel: UILabel!
    @IBOutlet weak var sampleLog: UITextView!     // store all values in sampleLog as user defaults            
    @IBOutlet weak var totalLabel: UILabel!       
    @IBOutlet weak var avgLabel: UILabel!
       
    @IBOutlet weak var dollarsStepper: UIStepper!
    @IBOutlet weak var centsStepper: UIStepper!
  
     // Dollar Stepper
    @IBAction func stepper(_ sender: UIStepper) {
        dollarLabel.text = "$\(Int(sender.value))"
        valDollar = Int(sender.value)      
    }
  
     // Cent Stepper
    @IBAction func stepperCent(_ sender: UIStepper) {
        centLabel.text = ".\(Int(sender.value))"
        if sender.value == 0 {
            centLabel.text = ".00"         
        }
        valCent = Int(Double(sender.value)) 
    }
  
    @IBAction func addTipButton(_ sender: UIButton) {
        let numberFormatter = NumberFormatter()
      
       
        numberFormatter.minimumFractionDigits = 2
        numberFormatter.maximumFractionDigits = 2
        numberFormatter.minimumIntegerDigits = 1  /
      
        let tipDollarCent = (dollarLabel.text ?? "$0") + (centLabel.text ?? ".00")
        let tipValue = getTipValue()
        totalValueOfAllTips += tipValue
        numberOfTipsTracked += 1                         
        sampleLog.text! += "\(tipDollarCent)\n"
      
       
        let totalLabelStringValue = numberFormatter.string(from: totalValueOfAllTips as NSNumber)
        let averageLabelStringValue = numberFormatter.string(from: averageTip as NSNumber)
        totalLabel.text! = "$" + totalLabelStringValue!    
        avgLabel.text! = "$" + averageLabelStringValue!   
        dollarLabel.text = "$0"            
        centLabel.text = ".00"             
      
       
        valDollar = 0
        valCent = 0
        dollarsStepper.value = 0
        centsStepper.value = 0
      
    }
  
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
  
    /
    @IBAction func backButtonPressed(_ sender: Any) {
        navigationController?.popViewController(animated: true)
    }
  
   
    private func getTipValue() -> Double {
       
        let valueOfCents = Double(valCent) / 100
        return Double(valDollar) + valueOfCents
    }
  
}

I tried the following implementation inside of my viewDidLoad function, at least for the total tip value:

UserDefaults.standard.set(totalValueOfAllTips, forKey: _totalValueOfAllTips)     // write to user defaults 
UserDefaults.standard.double(forKey: _totalValueOfAllTips)                       // read from user default when view loads

Based on my research and the errors Xcode gives me, I am not putting the UserDefault commands within the proper scope, because I'm sure my syntax is correct.