Volume tracker

[defagos]

As a developer integrating Pillarbox I would like to be able to integrate custom volume controls.

Hints

MPVolumeView is the only API that makes it possible to change the system volume but offers limited customization. It can be almost be turned into a programmatic API with the following trick:

import SwiftUI
import Combine
import MediaPlayer

final class VolumeManager: ObservableObject {
    let volumeView = MPVolumeView()

    private weak var volumeSlider: UISlider? {
        volumeView.subviews.compactMap { $0 as? UISlider }.first
    }

    @Published var volume: Float = 0 {
        didSet {
            volumeSlider?.value = volume
        }
    }

    init() {
        AVAudioSession.sharedInstance().publisher(for: \.outputVolume)
            .assign(to: &$volume)
    }
}

struct ContentView: View {
    @StateObject private var volumeManager = VolumeManager()

    var body: some View {
        Slider(value: $volumeManager.volume)
    }
}

#Preview {
    ContentView()
}

This lets the associated UI be freely customized, but of course there is a private slider access that might be risky.

Note that the initial volume returned by MPVolumeView might not be up to date. For this reason it is better to read the volume via AVAudioSession and to write it with MPVolumeView (since the outputVolume property is read-only).

Acceptance criteria

• A way to create a custom volume control UI is provided.
• The demo provides a corresponding slider somewhere.

Tasks

• Provide component for volume tracking.
• Extract custom slider construction.
• Add volume slider with same look & feel as progress slider.