Unity UI Development

Overview

Unity provides two UI systems: UGUI (the established GameObject-based system) and UI Toolkit (the newer retained-mode system inspired by web technologies). This skill covers both systems, when to use each, and implementation patterns.

Choosing a UI System

Factor	UGUI	UI Toolkit
Runtime UI	Mature, full-featured	Supported (Unity 2023+)
Editor UI	Not supported	Primary system for editor
World-space UI	Excellent (World Space Canvas)	Limited
Animation/Tweening	DOTween, Animator, LeanTween	USS transitions, C# manipulation
Styling	Per-element, manual	USS stylesheets (CSS-like)
Performance	Draw call heavy with many canvases	Retained mode, lighter draw calls
Learning curve	Lower for Unity devs	Higher (web-like paradigm)
VR/AR	Well-supported	Limited world-space support

Recommendation: Use UGUI for world-space UI, VR/AR, and projects needing maximum asset store compatibility. Use UI Toolkit for editor extensions, complex data-driven UIs (lists, trees), and projects on Unity 2023+.

UGUI (Canvas-Based UI)

Canvas Setup

Render Mode	Use For	Notes
Screen Space - Overlay	HUD, menus	Always on top, no camera needed
Screen Space - Camera	Post-processing on UI	Assign render camera
World Space	In-game signs, health bars	Set event camera for interaction

Performance rule: Separate static and dynamic UI into different Canvases. A Canvas rebuilds ALL children when any child changes.

RectTransform Anchoring

Anchor presets control how elements resize with parent:
- Stretch-Stretch: Element fills parent (full-screen backgrounds)
- Center-Center: Fixed size at center (popup dialogs)
- Bottom-Left: Fixed corner position (minimap)
- Top-Stretch: Stretches horizontally, fixed at top (nav bar)

Set anchors via the RectTransform anchor preset widget (hold Alt+Shift to also set pivot and position). Use anchorMin, anchorMax, offsetMin, offsetMax in code.

Layout Components

Component	Purpose
HorizontalLayoutGroup	Arrange children left-to-right
VerticalLayoutGroup	Arrange children top-to-bottom
GridLayoutGroup	Grid arrangement (inventory slots)
LayoutElement	Override min/preferred/flexible size
ContentSizeFitter	Auto-resize to content
AspectRatioFitter	Maintain aspect ratio

Disable Layout.childForceExpandWidth/Height to prevent unwanted stretching.

Event Handling

using UnityEngine.UI;
using TMPro;

[SerializeField] Button startButton;
[SerializeField] TMP_InputField nameField;
[SerializeField] Slider volumeSlider;

void OnEnable()
{
    startButton.onClick.AddListener(OnStartClicked);
    nameField.onEndEdit.AddListener(OnNameChanged);
    volumeSlider.onValueChanged.AddListener(OnVolumeChanged);
}

void OnDisable()
{
    startButton.onClick.RemoveListener(OnStartClicked);
    nameField.onEndEdit.RemoveListener(OnNameChanged);
    volumeSlider.onValueChanged.RemoveListener(OnVolumeChanged);
}

Always RemoveListener in OnDisable to prevent leaks and ghost references.

TextMeshPro

Always use TextMeshPro (TMP_Text, TextMeshProUGUI) over legacy Text. Import TMP Essentials when prompted. Use rich text tags: <color=#FF0000>, <b>, <size=24>, <sprite=0> for inline icons.

UI Toolkit (USS/UXML)

Architecture

UI Toolkit Stack:
  UXML  -- Structure (like HTML)
  USS   -- Styling (like CSS)
  C#    -- Logic (like JavaScript)

UXML Structure

<ui:UXML xmlns:ui="UnityEngine.UIElements">
    <ui:VisualElement class="container">
        <ui:Label text="Player Stats" class="title" />
        <ui:ProgressBar name="health-bar" title="HP" high-value="100" />
        <ui:Button name="attack-btn" text="Attack" class="action-btn" />
        <ui:ListView name="inventory-list" />
    </ui:VisualElement>
</ui:UXML>

USS Styling

.container {
    flex-direction: column;
    padding: 10px;
    background-color: rgba(0, 0, 0, 0.8);
    border-radius: 8px;
}

.title {
    font-size: 24px;
    color: white;
    -unity-font-style: bold;
    margin-bottom: 10px;
}

.action-btn {
    height: 40px;
    background-color: #4CAF50;
    color: white;
    border-radius: 4px;
    transition-duration: 0.2s;
}

.action-btn:hover {
    background-color: #66BB6A;
    scale: 1.05 1.05;
}

Key USS differences from CSS: use -unity- prefix for Unity-specific properties. Flexbox is the layout model (default flex-direction: column). Use transition-duration, transition-property for animations.

C# Integration

[RequireComponent(typeof(UIDocument))]
public class StatsUI : MonoBehaviour
{
    void OnEnable()
    {
        var root = GetComponent<UIDocument>().rootVisualElement;
        var healthBar = root.Q<ProgressBar>("health-bar");
        var attackBtn = root.Q<Button>("attack-btn");
        var inventory = root.Q<ListView>("inventory-list");

        attackBtn.RegisterCallback<ClickEvent>(OnAttack);
        healthBar.value = player.Health;

        // ListView binding
        inventory.makeItem = () => new Label();
        inventory.bindItem = (element, index) =>
            ((Label)element).text = items[index].Name;
        inventory.itemsSource = items;
    }
}

Query elements with Q<T>("name") or Q<T>(className: "class"). Register callbacks with RegisterCallback<EventType>. Always query from rootVisualElement.

Data Binding (Unity 2023.2+)

// Runtime data binding with [CreateProperty] and INotifyBindablePropertyChanged
public class PlayerData : INotifyBindablePropertyChanged
{
    public event EventHandler<BindablePropertyChangedEventArgs> propertyChanged;
    private int _health;

    [CreateProperty]
    public int Health
    {
        get => _health;
        set { _health = value; Notify(); }
    }
    void Notify([CallerMemberName] string prop = "")
        => propertyChanged?.Invoke(this, new BindablePropertyChangedEventArgs(prop));
}

Bind in UXML with binding-path="Health" or in C# with element.SetBinding("value", new DataBinding { dataSourcePath = ... }).

Common UI Patterns

Pattern	UGUI Approach	UI Toolkit Approach
Popup dialog	Enable/disable child panel	Add/remove from visual tree
Scroll list	ScrollRect + VerticalLayoutGroup	ListView (virtualized)
Drag-and-drop	IBeginDragHandler, IDragHandler, IEndDragHandler	PointerManipulator
Tab system	Toggle group + panels	RadioButtonGroup + display toggling
Tooltip	Follow cursor panel	Manipulator + VisualElement positioning
Screen fade	CanvasGroup.alpha tween	USS opacity transition

Additional Resources

Reference Files

• references/ugui-patterns.md -- Advanced UGUI patterns: scroll optimization, dynamic layouts, world-space interaction, localization, safe area handling, screen adaptation
• references/ui-toolkit-advanced.md -- Custom controls, ListView/TreeView mastery, custom manipulators, editor UI integration, theming, responsive layouts