UnitType.swift

enum UnitType: String {
  case milligrams = "mg"
  case grams = "g"
  case kilograms = "kg"
  case count = "count"
  case pounds = "lb"
  case ounces = "oz"
  case milliliters = "ml"
  case liters = "L"
  case cups = "cup"
  case teaspoons = "tsp"
  case tablespoons = "tbsp"

  static let dryUnits: [UnitType] = [.count, .milligrams, .grams, .kilograms, .pounds, .ounces]
  static let liquidUnits: [UnitType] = [.milliliters, .liters, .cups, .teaspoons, .tablespoons]

  var weights: [String] {
    var weights = ["Select Amount"]
    switch self {
    case .milligrams, .ounces, .teaspoons, .tablespoons, .count:
      for i in 1 ... 100 {
        weights.append(String(i))
      }
    case .kilograms, .liters:
      for i in stride(from: 0.05, to: 5.05, by: 0.05) {
        let val = String(format: "%.2f", i)
        weights.append(val)
      }
    case .cups, .pounds:
      for i in stride(from: 0.25, to: 5.25, by: 0.25) {
        let val = String(format: "%.2f", i)
        weights.append(val)
      }
    case .grams, .milliliters: // grams // RA: changed this from `default:`, not sure if you meant for milliliters to be using this case, but that's how the code was written, and shows why you should avoid using `default` ;)
      for i in 1...100 {
        weights.append(String(i))
      }
      for i in stride(from: 110, to: 1010, by: 5) {
        weights.append(String(i))
      }
    }
    return weights
  }
}

WeightPickerView.swift

import UIKit

class WeightPickerView: UIPickerView, UIPickerViewDelegate, UIPickerViewDataSource {
  
  typealias SelectedRowClosure = (Int) -> Void
  typealias TouchersBegan = () -> Void
  
  var selectedRowClosure: SelectedRowClosure = { _ in }
  var touchesBegan: TouchersBegan = { _ in }
  
  fileprivate var pickerWeights = [String]()
  var pickerUnitTypes: [UnitType] = [.grams, .milligrams, .kilograms, .count, .liters,
                                     .pounds, .ounces, .cups, .teaspoons, .tablespoons]
  
  func numberOfComponents(in pickerView: UIPickerView) -> Int {
    return 2
  }
  
  func pickerView(_ pickerView: UIPickerView,
                  numberOfRowsInComponent component: Int) -> Int {
    if component == 0 {
      return pickerWeights.count
    } else {
      return pickerUnitTypes.count
    }
  }
  
  func pickerView(_ pickerView: UIPickerView,
                  titleForRow row: Int,
                  forComponent component: Int) -> String? {
    if component == 0 {
      return pickerWeights[row]
    } else {
      return pickerUnitTypes[row].rawValue
    }
  }
  
  // MARK: pickerView(_:didSelectRow:inComponent:)
  func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
    touchesBegan() // I'd like to move this elsewhere, but not sure where :( 
    
    if component == 0 {
      selectedRowClosure(row)
    } else {
      pickerWeights = pickerUnitTypes[row].weights
      // Change what's shown in the 1st component based on selected unit in the 2nd component. 
      pickerView.reloadComponent(0)
    }
  }
}

extension WeightPickerView {
  // Selects row, triggering `didSelectRow` function that changes the 1st component
  private func pick(row: Int, inComponent component: Int) {
    selectRow(row, inComponent: component, animated: false)
    pickerView(self, didSelectRow: row, inComponent: component)
  }
  
  // Makes sure that units in 1st component are shown (only used in `viewDidLoad()`)
  func configureInitialRows() {
    pickerView(self, didSelectRow: 0, inComponent: 1)
  }
  
  // Selects rows in picker view based on parameters
  func pickRowsFor(unit: UnitType, weight: Double) {
    let unitRow = pickerUnitTypes.index(of: unit) ?? 0
    pick(row: unitRow, inComponent: 1)
    
    let weightInt = Int(weight) // Int(exactly: weight) didn't compile for me, so I just removed it... -RA
    if let row = pickerWeights.index(of: "\(weightInt)") {
      pick(row: row, inComponent: 0)
    } else if let row = pickerWeights.index(of: String(format: "%.2f", weight)) {
      pick(row: row, inComponent: 0)
    }
  }
  
  func selectedUnitType() -> UnitType {
    let row = selectedRow(inComponent: 1)
    
    return pickerUnitTypes[row]
  }
  
  func selectedWeight() -> Double? {
    let row = selectedRow(inComponent: 0)
    let weight = pickerWeights[row]
    
    return Double(weight)
  }
}