/// <summary>
        /// The constructor is private: loading screens should
        /// be activated via the static Load method instead.
        /// </summary>
        private LoadingScreen(ScreenManager.ScreenManager screenManager, bool loadingIsSlow, GameScreen[] screensToLoad)
        {
            this.loadingIsSlow = loadingIsSlow;
            this.screensToLoad = screensToLoad;

            TransitionOnTime = TimeSpan.FromSeconds(0.5);

            // If this is going to be a slow load operation, create a background
            // thread that will update the network session and draw the load screen
            // animation while the load is taking place.
            if (loadingIsSlow)
            {
                backgroundThread = new Thread(BackgroundWorkerThread);
                backgroundThreadExit = new ManualResetEvent(false);

                graphicsDevice = screenManager.GraphicsDevice;
            }
        }
        /// <summary>
        /// Removes a screen from the screen manager. You should normally
        /// use GameScreen.ExitScreen instead of calling this directly, so
        /// the screen can gradually transition off rather than just being
        /// instantly removed.
        /// </summary>
        public void RemoveScreen(GameScreen screen)
        {
            // If we have a graphics device, tell the screen to unload content.
            if (isInitialized)
                screen.UnloadContent();

            screens.Remove(screen);
            screensToUpdate.Remove(screen);
        }
        /// <summary>
        /// Adds a new screen to the screen manager.
        /// </summary>
        public void AddScreen(GameScreen screen)
        {
            screen.ScreenManager = this;
            screen.IsExiting = false;

            // If we have a graphics device, tell the screen to load content.
            if (isInitialized)
                screen.LoadContent();

            screens.Add(screen);
        }