public abstract class BackpackGUI {

    /**
     * The inventory the player will be viewing as a GUI
     * */
    private Inventory gui;

    /**
     * The model the view will handle
     * */
    private BackpackModel model;

    /**
     * The GUI inventory the player sees
     * */
    private GUIView view;

    /**
     * The user viewing the inventory GUI
     * */
    protected BackpackPlayer viewer;

    /**
     * The current page of the GUI the user is on
     * */
    protected int page;

    protected BackpackGUI(BackpackPlayer viewer) {
        this.viewer = viewer;
        this.page = 0;
    }

    public int getPage() {
        return page;
    }

    public int getMaxPage() {
        return model.getBackpack().getTotalPagesStored();
    }

    public BackpackPlayer getViewer() {
        return viewer;
    }

    /**
     * Cycles to the next page in the specified direction
     * @return the next page in the cycle of the direction
     * */
    public int getNextPageInDirection(Direction direction) {
        page = page + (1 * direction.getDirection());
        return page;
    }

    /**
     * Opens the backpack to the first page
     * */
    public void open() {
        open(0);
    }

    /**
     * Opens the backpack to the specified page number
     * */
    public void open(int page) {
        this.model = buildModel(viewer, page);
        this.view = buildView(model);
        model.addObserver(view);

        gui = view.getView();
        Player player = viewer.getPlayer();
        player.openInventory(gui);
    }

    /**
     * Closes the GUI of the current viewer
     * */
    public void close() {
        InventoryPage viewedPage = model.getActivePage();
        Inventory pageInventory = viewedPage.getInventory();
        List<HumanEntity> viewers = pageInventory.getViewers();
        for(HumanEntity viewer : viewers) {
            viewer.closeInventory();
        }

        syncBackpackWithInventory(gui, model);
    }

    protected GUIView getGUIView() {
        return view;
    }

    private void syncBackpackWithInventory(Inventory latestInventory, BackpackModel model) {
        latestInventory = removeButtonsFromView(latestInventory);
        InventoryPage updatedPage = new InventoryPage(latestInventory);
        model.setActivePage(updatedPage);
    }

    /**
     * Allows the viewer to view another player's backpack
     * */
    public void viewBackpack(AbstractBackpack backpack) {
        Player owner = backpack.getOwner();
        BackpackPlayer backpackPlayer = UserManager.getOrLoadBackpackPlayer(owner);
        BackpackGUI gui = getGUIImpl(backpackPlayer, backpack);
        gui.open();
    }

    /**
     * Builds the {@link BackpackModel} the GUI uses for the view
     * */
    protected abstract BackpackModel<?> buildModel(BackpackPlayer player, int page);

    /**
     * Builds the GUI the viewer will see
     * */
    protected abstract GUIView buildView(BackpackModel model);

    /**
     * Updates the GUI inventory view to the new inventory view
     * */
    protected abstract void updateView(Inventory newInventoryView);

    /**
     * @return the Inventory for the specified page number
     * */
    protected abstract Inventory getPageForNumber(int pageNumber);

    /**
     * Removes the buttons from the GUI
     * @return the inventory with the buttons removed
     * */
    protected abstract Inventory removeButtonsFromView(Inventory gui);

    public static BackpackGUI getGUIImpl(BackpackPlayer player, AbstractBackpack backpack) {
        BackpackType type = backpack.getBackpackType();
        switch (type) {
            case PAGEABLE:
                return new PageableBackpackGUI(player);
            default:
                return new PageableBackpackGUI(player);
        }
    }

}