LDLib2 UI — Agent Guide

This document is for AI agents. It provides a structured workflow for building UIs with LDLib2. Follow the decision tree, use the minimal templates, then navigate to detailed pages as needed.

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Step 1: Determine Language / Format

Ask the user which format they want. This determines syntax for all subsequent code.

Format	When to use	Language-specific doc
Kotlin	Mod development with Kotlin DSL (most concise)	kotlin_support.md
Java	Mod development with Java	getting_start.md
KubeJS	Modpack scripting, no compilation needed	kjs_support.md
XML	Declarative UI structure, visual editing	xml.md + XSD schema

XML note: XML defines only the UI tree and styles. You still need Java/Kotlin/KubeJS code to load the XML, do data bindings, and wrap it in a ModularUI. Guide the agent to read the XSD file for available tags and attributes.

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Step 2: Determine UI Type

Key rule: If the UI needs Player data or server-side state, it must be a Menu-based UI with ModularUI.of(ui, player).

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Step 3: Minimal Templates

Each template is a complete, runnable starting point. Pick the one matching Step 2.

Client-only Screen

ModularUI createUI() {
    var root = new UIElement().addClass("panel_bg").addChildren(
        new Label().setText("Hello")
    );
    return ModularUI.of(UI.of(root, StylesheetManager.INSTANCE.getStylesheetSafe(StylesheetManager.GDP)));
}
// Open: Minecraft.getInstance().setScreen(new ModularUIScreen(createUI(), Component.empty()));

fun createUI(): ModularUI {
    val root = element({ cls = { +"panel_bg" } }) {
        label({ text("Hello") })
    }
    return ModularUI(UI.of(root, StylesheetManager.GDP))
}
// Open: Minecraft.getInstance().setScreen(ModularUIScreen(createUI(), Component.empty()))

// Client-only screen must be opened from client_scripts
let root = new UIElement().addClass("panel_bg").addChildren(
    new Label().setText("Hello")
);
let mui = ModularUI.of(UI.of(root));
// Open via client-side trigger

Menu-based UI (Server-synced)

For Menu UIs, use the built-in factories. See factory.md for full details.

public class MyBlock extends Block implements BlockUIMenuType.BlockUI {
    @Override
    public ModularUI createUI(BlockUIMenuType.BlockUIHolder holder) {
        var root = new UIElement().addClass("panel_bg").addChildren(
            new Label().setText("Block UI"),
            new InventorySlots()
        );
        return ModularUI.of(
            UI.of(root, StylesheetManager.INSTANCE.getStylesheetSafe(StylesheetManager.GDP)),
            holder.player
        );
    }
    // Open: BlockUIMenuType.openUI((ServerPlayer) player, pos);
}

class MyBlock : Block(...), BlockUIMenuType.BlockUI {
    override fun createUI(holder: BlockUIMenuType.BlockUIHolder): ModularUI {
        val root = element({ cls = { +"panel_bg" } }) {
            label({ text("Block UI") })
            inventorySlots()
        }
        return ModularUI(UI.of(root, StylesheetManager.GDP), holder.player)
    }
    // Open: BlockUIMenuType.openUI(player as ServerPlayer, pos)
}

// startup_scripts/main.js — runs on both sides
LDLib2UI.block("mymod:my_block_ui", event => {
    event.modularUI = ModularUI.of(UI.of(
        new UIElement().addClass("panel_bg").addChildren(
            new Label().setText("Block UI"),
            new InventorySlots()
        )
    ), event.player);
});
// server_scripts: LDLib2UIFactory.openBlockUI(event.player, event.block.pos, "mymod:my_block_ui");

Other triggers: Replace BlockUIMenuType with HeldItemUIMenuType (item) or PlayerUIMenuType (arbitrary). See factory.md.

HUD Overlay

// Client-side only. Register in RegisterGuiLayersEvent.
var muiCache = Suppliers.memoize(() -> ModularUI.of(UI.of(
    new UIElement().layout(l -> l.widthPercent(100).heightPercent(100).paddingAll(10))
)));
event.registerAboveAll(MyMod.id("my_hud"), (ModularHudLayer) muiCache::get);

See hud.md for details.

XML-based UI

<?xml version="1.0" encoding="UTF-8" ?>
<ldlib2-ui xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:noNamespaceSchemaLocation="https://raw.githubusercontent.com/Low-Drag-MC/LDLib2/refs/heads/1.21/ldlib2-ui.xsd">
    <stylesheet location="ldlib2:lss/mc.lss"/>
    <root class="panel_bg">
        <label text="Hello from XML"/>
        <button text="Click me"/>
    </root>
</ldlib2-ui>

// Load and use
var xml = XmlUtils.loadXml(ResourceLocation.parse("mymod:my_ui.xml"));
var ui = UI.of(xml);
// Query elements: ui.select("#my_id"), ui.select(".my_class > button")
return ModularUI.of(ui, player); // or ModularUI.of(ui) for client-only

For XML auto-completion and validation, read the XSD schema to know all available tags, attributes, and structure.

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Step 4: Data Binding Patterns

This is the most critical knowledge for producing correct UI code. There are 3 patterns:

Pattern A: Client-only (no server sync)

Use bindDataSource / bindObserver or Kotlin's dataSource / observer. No network involved.

// Consumer: display dynamic data
new Label().bindDataSource(SupplierDataSource.of(() -> Component.literal("Value: " + myVar)));
// Producer: react to user input
new TextField().bindObserver(value -> myVar = value);

Pattern B: Bidirectional sync (Menu UI)

Use DataBindingBuilder + .bind(). Server getter/setter only — client side is automatic.

new Switch().bind(DataBindingBuilder.bool(() -> serverBool, v -> serverBool = v).build());
new TextField().bind(DataBindingBuilder.string(() -> serverStr, v -> serverStr = v).build());
new ItemSlot().bind(itemHandler, 0); // inventory shorthand
new FluidSlot().bind(fluidTank, 0);  // fluid shorthand

Kotlin shorthand: switch { bind(::serverBool) }, textField { bind(::serverStr) }

Pattern C: Server-to-client read-only

Use S2C variants. Client cannot modify the value.

new Label().bind(DataBindingBuilder.componentS2C(() -> Component.literal(serverData)).build());

Kotlin: label { bindS2C({ Component.literal(serverData) }) }

For complex bindings (custom types, RPCEvent, List sync, remote getter/setter): read data_bindings.md.

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Step 5: Navigation Map

Use this table to find detailed documentation by need.

Core Concepts

Need	Read	Path
How ModularUI works	ModularUI	docs/ldlib2/ui/preliminary/modularui.md
Screen vs Menu	Screen & Menu	docs/ldlib2/ui/preliminary/screen_and_menu.md
UI Factories (Block/Item/Player)	Factory	docs/ldlib2/ui/factory.md
Layout (flexbox, grid, size, padding)	Layout	docs/ldlib2/ui/preliminary/layout.md
Stylesheet / LSS	Stylesheet	docs/ldlib2/ui/preliminary/stylesheet.md
Events (mouse, keyboard, lifecycle)	Events	docs/ldlib2/ui/preliminary/event.md
Data Bindings & RPC	Data Bindings	docs/ldlib2/ui/preliminary/data_bindings.md
Style Animation	Style Animation	docs/ldlib2/ui/preliminary/style_animation.md
HUD Overlays	HUD	docs/ldlib2/ui/hud.md
XEI (JEI/REI/EMI)	XEI	docs/ldlib2/ui/xei_support.md
XML UI	XML	docs/ldlib2/ui/xml.md
Kotlin DSL	Kotlin	docs/ldlib2/ui/kotlin_support.md
KubeJS bindings	KubeJS	docs/ldlib2/ui/kjs_support.md

Components Quick Reference

Component	Class	Key method	Doc
Base element	UIElement	.layout(), .style(), .addClass()	element.md
Label	Label	.setText(), .bind()	label.md
Button	Button	.setText(), .setOnClick(), .setOnServerClick()	button.md
Text field	TextField	.setText(), .bind(), .setNumbersOnlyInt()	text-field.md
Text area	TextArea	.setText(), .bind()	text-area.md
Toggle	Toggle	.setText(), .bind()	toggle.md
Switch	Switch	.bind()	switch.md
Selector	Selector	.setCandidates(), .bind()	selector.md
Progress bar	ProgressBar	.setProgress(), .bind(), .label()	progress-bar.md
Scroller	Scroller / Scroller.Horizontal	.bind()	scroller.md
Item slot	ItemSlot	.bind(handler, slot), .setItem()	item-slot.md
Fluid slot	FluidSlot	.bind(tank, slot), .setFluid()	fluid-slot.md
Inventory slots	InventorySlots	(auto player inventory)	inventory-slots.md
Tab / TabView	Tab, TabView	.addTab()	tab.md, tab-view.md
Toggle group	ToggleGroup	.addToggle()	toggle-group.md
Scroller view	ScrollerView	(scrollable container)	scroller-view.md
Split view	SplitView	(resizable split)	split-view.md
Color selector	ColorSelector	.bind()	color-selector.md
Tag field	TagField	.bind()	tag-field.md
Search	SearchComponent	.bind()	search-component.md
Tree list	TreeList	—	tree-list.md
Scene (3D)	Scene	—	scene.md
Graph view	GraphView	—	graph-view.md
Code editor	CodeEditor	—	code-editor.md
Inspector	Inspector	—	inspector.md
Template	Template	(load from XML)	template.md
Bindable value	BindableValue<T>	.bind() (hidden sync helper)	bindable-value.md
Rich text	Text	.setText()	text.md

Textures Quick Reference

Texture	Class	Doc
Sprite (image)	SpriteTexture	sprite.md
Color rectangle	ColorRectTexture	color-rect.md
Color border	ColorBorderTexture	color-border.md
SDF rectangle	SDFRectTexture	sdf-rect.md
Resource rect	RectTexture	rect.md
Item stack	ItemStackTexture	item-stack.md
Fluid stack	FluidStackTexture	fluid-stack.md
Text	TextTexture	text.md
Animation	AnimationTexture	animation.md
Group	GroupTexture	group.md
Shader	ShaderTexture	shader.md
UI Resource	UIResourceTexture	ui-resource.md
LSS texture syntax	—	lss.md

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Step 6: Pre-completion Checklist

Before returning code to the user, verify:

• [ ] UI type matches need: Client-only Screen has no player param; Menu UI has player param
• [ ] Data bindings are correct: Server data uses DataBindingBuilder.xxx().build() + .bind(); client-only data uses bindDataSource/bindObserver
• [ ] Factory is registered: Menu UIs need a factory registration (Block/Item/Player) or manual MenuType
• [ ] KubeJS script placement: LDLib2UI.* handlers must run on both sides — prefer startup_scripts/
• [ ] XML files are placed under assets/<modid>/ui/ and loaded via ResourceLocation
• [ ] Stylesheet applied: Use StylesheetManager.GDP, .MC, or .MODERN for built-in themes, or provide custom LSS
• [ ] Output format matches request: Return ModularUI for full UIs, UIElement for reusable components