Swift Public protocols with Internal functions and properties
<p>I am wondering what the best practice is when I want some functions to be public and some to me internal when working with protocols.<br><br>I am writing an <a href="https://github.com/Sajjon/SwiftAudioManager" rel="noreferrer">AudioManager</a> in <strong>Swift 3</strong> wrapping <code>AVPlayer</code> as a framework.<br><br> I want some methods to be public, so that e.g. a ViewController making use of the AudioManager can access some methods, but some methods would not be exposed outside the framework<br> -> i.e. having the access modifier <code>internal</code> instead of <code>public</code>.<br><br> I am writing the framework with protocol driven design, almost every part should have a protocol.<br>So protocols are talking to protocols within the framework.<br> E.g. the main class - <code>AudioManager</code> - has an <code>AudioPlayer</code>, and should be able to call some <code>internal</code> functions on it,<br> e.g. <code>pause(reason:)</code> but that method should be <code>internal</code> and not exposed outside the framework.<br><br>Here is an example.</p>

<pre><code>internal enum PauseReason {
    case byUser
    case routeChange
}

// Compilation error: `Public protocol cannot refine an internal protocol`
public protocol AudioPlayerProtocol: InternalAudioPlayerProtocol { 
   func pause() // I want 
}

internal protocol InternalAudioPlayerProtocol {
    func pause(reason: PauseReason) // Should only be accessible within the framework
}

public class AudioPlayer: AudioPlayerProtocol {
    public func pause() {
        pause(reason: .byUser)
    }

    // This would probably not compile because it is inside a public class...
    internal func pause(reason: PauseReason) { //I want this to be internal
        // save reason and to stuff with it later on
    }
}

public protocol AudioManagerProtocol {
    var audioPlayer: AudioPlayerProtocol { get }
}

public class AudioManager: AudioManagerProtocol {
    public let audioPlayer: AudioPlayerProtocol

    init() {
        audioPlayer = AudioPlayer()
        NotificationCenter.default.addObserver(self, selector: #selector(handleRouteChange(_:)), name: NSNotification.Name.AVAudioSessionRouteChange, object: nil)
    }

    func handleRouteChange(_ notification: Notification) {
        guard
        let userInfo = notification.userInfo,
        let reasonRaw = userInfo[AVAudioSessionRouteChangeReasonKey] as? NSNumber,
        let reason = AVAudioSessionRouteChangeReason(rawValue: reasonRaw.uintValue)
        else { print("what could not get route change") }
        switch reason {
        case .oldDeviceUnavailable:
            pauseBecauseOfRouteChange()
        default:
            break
        }
    }
}

private extension AudioManager {
    func pauseBecauseOfRouteChange() {
        audioPlayer.pause(reason: .routeChange)
    }
}

// Outside of Audio framework
class PlayerViewController: UIViewController {
    fileprivate let audioManager: AudioManagerProtocol 
    @IBAction didPressPauseButton(_ sender: UIButton) {
        // I want the `user of the Audio framwwork` (in this case a ViewController)
        // to only be able to `see` `pause()` and not `pause(reason:)` 
        audioManager.audioPlayer.pause()
    }
}
</code></pre>

<p><strong>I know I can get it to work by changing the method <code>pauseBecauseOfRouteChange</code> to look like this:</strong></p>

<pre><code>func pauseBecauseOfRouteChange() {
    guard let internalPlayer = audioPlayer as? InternalAudioPlayerProtocol else { return }
    internalPlayer.pause(reason: .routeChange)
}
</code></pre>

<p><strong>But I am wondering if there is a more elegant solution?<br></strong> Something like marking that the <code>AudioPlayerProtocol</code> refines the <code>InternalAudioPlayerProtocol</code>...</p>

<p>Or how do you fellow programmers do it?<br>The framework is more beautiful if it does not expose methods and variables that are intended for internal use!</p>

<p>Thanks!</p>