AVAudioPlayer fade in and fade out operation

FadeOperation.swift

//
//  FadeOperation.swift
//  SomeProject
//
//  Created by Josip Bernat on 03/11/2017.
//  Copyright © 2017 Josip's Home. All rights reserved.
//

import Foundation
import AVFoundation

class FadeOperation: Operation {
    
    let player: AVAudioPlayer
    let fade: Fade
    fileprivate let fadeStep = 0.01 //0.01 step gives us fine fade in / out.
    
    enum Fade: Int {
        case anIn = 0
        case out
    }
    
    init(player: AVAudioPlayer, fade: Fade) {
        self.player = player
        self.fade = fade
        super.init()
    }
    
    override var isAsynchronous: Bool { return true }
    override var isExecuting: Bool { return state == .executing }
    override var isFinished: Bool { return state == .finished }
    
    var state = State.ready {
        willSet {
            willChangeValue(forKey: state.keyPath)
            willChangeValue(forKey: newValue.keyPath)
        }
        didSet {
            didChangeValue(forKey: state.keyPath)
            didChangeValue(forKey: oldValue.keyPath)
        }
    }
    
    enum State: String {
        case ready = "Ready"
        case executing = "Executing"
        case finished = "Finished"
        fileprivate var keyPath: String { return "is" + self.rawValue }
    }
    
    override func start() {
        if self.isCancelled {
            state = .finished
        } else {
            state = .ready
            main()
        }
    }
    
    override func main() {
        if self.isCancelled {
            state = .finished
        }
        else {
            state = .executing
            if fade == .anIn {
                fadeIn()
            }
            else {
                fadeOut()
            }
        }
    }
    
    func fadeIn() {
        if isCancelled {
            state = .finished
            return
        }
        
        if player.volume == 1.0 {
            player.volume = 0.0
        }
        player.prepareToPlay()
        player.play()
        
        commitFadeIn()
    }
    
    deinit {
        print("FadeOperation deinit")
    }
    
    func commitFadeIn() {
        if isCancelled {
            state = .finished
            return
        }
        
        if player.volume < 1.0 {
            // Fade
            player.volume += Float(fadeStep)
            if player.volume > 1.0 {
                player.volume = 1.0
            }
            DispatchQueue.global().asyncAfter(deadline: .now() + fadeStep, execute: { [weak self] in
                
                if self?.isCancelled == false {
                    self?.commitFadeIn()
                }
                else {
                    self?.state = .finished
                }
            })
        }
        else {
            state = .finished
        }
    }
    
    func fadeOut() {
        if isCancelled {
            state = .finished
            return
        }
        
        commitFadeOut()
    }
    
    @objc fileprivate func commitFadeOut() {
        
        if isCancelled {
            state = .finished
            return
        }
        
        if player.volume > 0.0 {
            // Fade
            player.volume -= Float(fadeStep)
            if player.volume < 0.0 {
                player.volume = 0.0
            }
            DispatchQueue.global().asyncAfter(deadline: .now() + fadeStep, execute: { [weak self] in
                
                if self?.isCancelled == false {
                    self?.commitFadeOut()
                }
                else {
                    self?.state = .finished
                }
            })
        }
        else {
            // Stop and get the sound ready for playing again
            player.pause()
            state = .finished
        }
    }
}

ViewController.swift

// This example is using one OperationQueue per player so 
// cancel all operations if user clicks play / pause fast.

func play() {
  queue.cancelAllOperations()
  queue.addOperation(FadeOperation.init(player: player, fade: .anIn))
}

func pause() {
  queue.cancelAllOperations()
  queue.addOperation(FadeOperation.init(player: player, fade: .out))
}