CalculatorBrain.swift

import UIKit

struct CalculatorBrain {
    
    var bmi: BMI?
    
    func getBMIValue() -> String {
        let bmiTo1DecimalPlace = String(format: "%.1f", bmi?.value ?? "0.0")
        return bmiTo1DecimalPlace
    }
    
    //1. Create the getAdvice() method to fetch bmi.advice, returns "No Advice" if bmi is nil.
    func getAdvice() -> String {
        return bmi?.advice ?? "No Advice"
    }
    
    //2. Create the getColor() method to fetch bmi.color, returns a white color if bmi is nil.
    func getColor() -> UIColor {
        return bmi?.color ?? #colorLiteral(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
    }
    
    mutating func calculateBMI(height: Float, weight: Float) {
        let bmiValue = weight / (height * height)
        
        if bmiValue < 18.5 {
            bmi = BMI(value: bmiValue, advice: "Eat more pies!", color: #colorLiteral(red: 0.4745098054, green: 0.8392156959, blue: 0.9764705896, alpha: 1))
        } else if bmiValue < 24.9 {
            bmi = BMI(value: bmiValue, advice: "Fit as fiddle!", color: #colorLiteral(red: 0.721568644, green: 0.8862745166, blue: 0.5921568871, alpha: 1))
        } else {
            bmi = BMI(value: bmiValue, advice: "Go easy on the snacks.", color: #colorLiteral(red: 0.9098039269, green: 0.4784313738, blue: 0.6431372762, alpha: 1))
        }
    }
    
}

ResultViewController.swift

import UIKit

class ResultViewController: UIViewController {
    
    var bmiValue: String?
    
    //3. Create the advice and color properties as Optionals
    var advice: String?
    var color: UIColor?

    @IBOutlet weak var bmiLabel: UILabel!
    @IBOutlet weak var adviceLabel: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()

        bmiLabel.text = bmiValue
        
        //4. set the advice label text as the advice String.
        
        adviceLabel.text = advice
        //5. Set the background color of the view as the color property value.
        view.backgroundColor = color
    }

    @IBAction func recalculatePressed(_ sender: UIButton) {
        dismiss(animated: true, completion: nil)
    }
}