Lazy Singletons.swift

class MusicPlayer {
    
    init() {
        print("Music Player has started")
    }
}

class Singer {
    
    /*
      The static part means this property is shared by the class rather than instances of the class.
        Meaning that you use Singer.musicPlayer rather than object.musicPlayer.
     */
    static let musicPlayer = MusicPlayer()
    
    init() {
        print("Singer has begun singing")
    }
}

let singer = Singer()

// Only after calling Singer.musicPlayer, does the MusicPlayer class get initialised
Singer.musicPlayer

/*
Yes: all Swift static let singletons are automatically lazy – they only get created when they are
needed. It’s so easy to do, and yet perfectly efficient too
*/