Items

In InvUI, the UI Elements are just called Items.
Every item has an ItemProvider which returns ItemStacks based on player UUIDs. This behavior allows for easy localization of UI Elements, as you just need to inherit from ItemProvider and implement your localization code there.
Unlike other inventory libraries, InvUI does not redraw items every tick. To trigger an update, call item.notifyWindows();. This will notify all Windows that are currently displaying that Item to call Item#getItemProvider and ItemProvider#getFor and display the updated item to their viewer.

Default ItemProviders

There are also two default ItemProviders in InvUI: ItemWrapper and ItemBuilder.
Both implementations will ignore the provided UUID in ItemProvider#getFor, but you can always inherit from ItemProvider to change this behavior.

Default UI Items in InvUI

InvUI provides a few basic items, but you will probably have to create your own ones for more complicated behavior.

Item	Description
AsyncItem	Creates it's ItemProvider asynchronously and displays a placeholder ItemProvider until the task is done.
AutoCycleItem	Automatically cycles between an array of ItemProviders.
AutoUpdateItem	Automatically updates its ItemProvider on a timer.
CommandItem	Makes the player execute a command or write a message in chat when clicked.
CycleItem	Cycles through an array of ItemProviders when clicked.
SimpleItem	Just displays an ItemProvider.
SuppliedItem	Displays an ItemProvider from a Supplier.

If you don't need any behavior at all and just want to make your GUI look pretty, you can use the SimpleItem. It cannot update it's ItemProvider later on and does not have any on-click functionality.

Creating your own Item

To create your own Item, you'll need to inherit from either BaseItem (if you want to be able to change its appearance) or SimpleItem (if you don't need to change its appearance). In this example, I inherited from BaseItem. Every time a player clicks on the Item, a counter will be incremented and the number on the Item will change.

KotlinJava

class CountItem : BaseItem() {

    private var count = 0

    override fun getItemProvider(): ItemProvider {
        return ItemBuilder(Material.DIAMOND).setDisplayName("Count: $count")
    }

    override fun handleClick(clickType: ClickType, player: Player, event: InventoryClickEvent) {
        if (clickType.isLeftClick) {
            count++ // increment if left click
        } else {
            count-- // else decrement
        }
        notifyWindows() // this will update the ItemStack that is displayed to the player
    }

}

public class CountItem extends BaseItem {

    private int count;

    @Override
    public ItemProvider getItemProvider() {
        return new ItemBuilder(Material.DIAMOND).setDisplayName("Count: " + count);
    }

    @Override
    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
        if (clickType.isLeftClick()) {
            count++; // increment if left click
        } else {
            count--; // else decrement
        }

        notifyWindows(); // this will update the ItemStack that is displayed to the player
    }

}

After adding the Item to the GUI, it will look like this:

ControlItems

A ControlItem is a special sort of Item: When it is added to a GUI, the GUI will be saved inside the ControlItem.
You can also manually assign a GUI to a ControlItem by calling ControlItem#setGUI.
So, what are ControlItems used for? You guessed it, controlling GUIs. Page switching buttons are one example, as they need to call PagedGUI#goForward and PagedGUI#goBack which would not be possible without having the instance of the GUI to control saved somewhere.

This would be a simple implementation of ControlItem for a paged GUI:

KotlinJava

class ChangePageItem : ControlItem<PagedGUI>() {

    override fun handleClick(clickType: ClickType, player: Player, event: InventoryClickEvent) {
        if (clickType == ClickType.LEFT) {
            gui.goForward() // go one page forward on left-click
        } else if (clickType == ClickType.RIGHT) {
            gui.goBack() // go one page back on right-click
        }
    }

    override fun getItemProvider(gui: PagedGUI): ItemProvider {
        return ItemBuilder(Material.BLACK_STAINED_GLASS_PANE)
            .setDisplayName("§7Current page: " + (gui.currentPageIndex + 1)) // + 1 because we don't want to have "Current page: 0"
            .addLoreLines("§8Left-click to go forward", "§8Right-click to go back")
    }

}

public class ChangePageItem extends ControlItem<PagedGUI> {

    @Override
    public void handleClick(@NotNull ClickType clickType, @NotNull Player player, @NotNull InventoryClickEvent event) {
        if (clickType == ClickType.LEFT) {
            getGui().goForward(); // go one page forward on left-click
        } else if (clickType == ClickType.RIGHT) {
            getGui().goBack(); // go one page back on right-click
        }
    }

    @Override
    public ItemProvider getItemProvider(PagedGUI gui) {
        return new ItemBuilder(Material.BLACK_STAINED_GLASS_PANE)
            .setDisplayName("§7Current page: " + (gui.getCurrentPageIndex() + 1)) // + 1 because we don't want to have "Current page: 0"
            .addLoreLines("§8Left-click to go forward", "§8Right-click to go back");
    }

}

The getItemProvider method will get called when the page changes, as PagedGUI internally calls Controllable#updateControlItems, so the displayed page number in this example item will always be correct.