MPVolumeViewWrapper.swift

import MediaPlayer
import SwiftUI

struct ContentView: View {
    @State var sliderValue: Float = 0
    
    @StateObject var observer = VolumeObserver()
    
    var body: some View {
        VStack {
            MPVolumeViewWrapper(sliderValue: $sliderValue)
                .frame(height: 50)
            
            LabeledContent("MPVolumeView slider value") {
                Text(sliderValue, format: .number)
            }
            
            LabeledContent("AVAudioSession.outputVolume") {
                Text(observer.volume, format: .number)
            }
        }
        .padding()
    }
}

final class VolumeObserver: ObservableObject {
    @Published var volume: Float = 0
    
    let session = AVAudioSession.sharedInstance()
    
    var observation: NSKeyValueObservation?
    
    init() {
        try! session.setActive(true)
        observation = session.observe(\.outputVolume, options: [.initial, .new]) { [weak self] _, value in
            self?.volume = value.newValue ?? 0
        }
    }
}

private struct MPVolumeViewWrapper: UIViewRepresentable {
    @Binding var sliderValue: Float
    
    typealias UIViewType = MPVolumeView
    
    final class Coordinator {
        var parent: MPVolumeViewWrapper
        
        init(parent: MPVolumeViewWrapper) {
            self.parent = parent
        }
        
        @objc func updateValue(sender: UISlider) {
            parent.sliderValue = sender.value
        }
    }
    
    func makeCoordinator() -> Coordinator {
        .init(parent: self)
    }
    
    func makeUIView(context: Context) -> MPVolumeView {
        let uiView = MPVolumeView()
        uiView.slider?.addTarget(context.coordinator, action: #selector(Coordinator.updateValue(sender:)), for: .valueChanged)
        return uiView
    }
    
    func updateUIView(_ uiView: MPVolumeView, context: Context) {
        
    }
}

private extension MPVolumeView {
    var slider: UISlider? {
        subviews
            .compactMap { $0 as? UISlider }
            .first
    }
}

#Preview {
    ContentView()
}