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Chapter 4: "This Binding<Double> syntax ... works ... but this might be a bug."

“This Binding syntax for declaring a binding works better in this not-strictly-SwiftUI setting, but this might be a bug.”

I’m confused, can you help me understand what this means?

hi Bill! the usual way to declare a binding is @Binding, but that didn’t work, at the time I wrote this chapter. I can’t see any reason why it shouldn’t, so I thought it might be something that Apple will fix eventually.

SwiftUI is still evolving :wink:

Audry, thanks for the reply. After digging into this further, I decided to apply another pattern that seems to be common when Coordinator is being used for target/action.

import SwiftUI
import UIKit

struct ColorUISlider: UIViewRepresentable {

  class Coordinator: NSObject {
    var slider: ColorUISlider
    init(_ slider: ColorUISlider) {
      self.slider = slider
    @objc func valueChanged(_ sender: UISlider) {
      slider.value = Double(sender.value)

  var color: UIColor
  @Binding var value: Double
  func makeCoordinator() -> ColorUISlider.Coordinator {

  func makeUIView(context: Context) -> UISlider {
    let slider = UISlider(frame: .zero)
    slider.thumbTintColor = color
    slider.value = Float(value)

    action: #selector(Coordinator.valueChanged(_:)), for: .valueChanged)

    return slider

  func updateUIView(_ uiView: UISlider, context: Context) {
    uiView.value = Float(self.value)

struct ColorUISliderPreviews: PreviewProvider {
    static var previews: some View {
      ColorUISlider(color: .red, value: .constant(0.5))

Thanks again.

thanks Bill! that looks really straightforward — I think this is one of the things that started out a little complicated before they got around to making it simpler.