typealias OnShuffleChangedSwignal = Swignal1Arg<Bool>

class AudioPlayer {
    static let sharedInstance = AudioPlayer()

    let onShuffleChanged = OnShuffleChangedSwignal()
    var shuffle: Bool = false {
        didSet {
            onShuffleChanged.fire(shuffle)
        }
    }
}

class ControlsViewController: UIViewController {
    init() {
        AudioPlayer.sharedInstance.onShuffleChanged.addObserver(self) { (observer, arg1) in
        // note: you can rename the variables in the callback such as
        // callback: { (weakSelf, shuffle) in
            if let favoriteTracksDataSource = observer.tracksDataSource as? FavoriteTracksDataSource {
                favoriteTracksDataSource.shuffle = arg1
                favoriteTracksDataSource.refresh()
            }
        }
    }

    func updateViewBasedOnShuffle(shuffle: Bool) {
        // do important things
    }
}