UIDocument (MonoBehaviour on GameObject)
  --> VisualTreeAsset (.uxml)  -- defines structure
  --> StyleSheet (.uss)         -- defines appearance
  --> C# script                 -- defines behavior

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements">
    <ui:Style src="MainMenu.uss" />
    <ui:VisualElement name="root-container" class="container">
        <ui:Label text="Game Menu" class="title" />
        <ui:Button text="Play" name="play-button" class="menu-btn" />
        <ui:Button text="Settings" name="settings-button" class="menu-btn" />
        <ui:Toggle label="Fullscreen" name="fullscreen-toggle" />
        <ui:Slider label="Volume" low-value="0" high-value="100" name="volume-slider" />
        <ui:TextField label="Player Name" name="player-name" />
    </ui:VisualElement>
</ui:UXML>

/* Type selector */
Button {
    background-color: #2D2D2D;
    border-radius: 4px;
    padding: 8px 16px;
    -unity-font-style: bold;
}

/* Class selector */
.menu-btn {
    width: 200px;
    height: 40px;
    margin: 4px 0;
    font-size: 16px;
    color: #FFFFFF;
}

/* Name selector */
#play-button {
    background-color: #4CAF50;
}

/* Pseudo-class */
.menu-btn:hover {
    background-color: #555555;
    scale: 1.05 1.05;
}

.menu-btn:active {
    background-color: #333333;
}

.menu-btn:disabled {
    opacity: 0.5;
}

/* Descendant selector */
.container > Label {
    -unity-text-align: middle-center;
}

/* USS variables */
:root {
    --primary-color: #4CAF50;
    --font-large: 24px;
}

.title {
    color: var(--primary-color);
    font-size: var(--font-large);
}

using UnityEngine;
using UnityEngine.UIElements;

public class MainMenuController : MonoBehaviour
{
    [SerializeField] private UIDocument uiDocument;

    private Button playButton;
    private Button settingsButton;
    private Toggle fullscreenToggle;
    private Slider volumeSlider;
    private TextField playerNameField;

    private void OnEnable()
    {
        var root = uiDocument.rootVisualElement;

        // Query single elements by name
        playButton = root.Q<Button>("play-button");
        settingsButton = root.Q<Button>("settings-button");
        fullscreenToggle = root.Q<Toggle>("fullscreen-toggle");
        volumeSlider = root.Q<Slider>("volume-slider");
        playerNameField = root.Q<TextField>("player-name");

        // Register click callbacks
        playButton.RegisterCallback<ClickEvent>(OnPlayClicked);
        settingsButton.RegisterCallback<ClickEvent>(OnSettingsClicked);

        // Register value change callbacks
        fullscreenToggle.RegisterValueChangedCallback(OnFullscreenChanged);
        volumeSlider.RegisterValueChangedCallback(OnVolumeChanged);

        // Query multiple elements by class
        var allButtons = root.Query<Button>(className: "menu-btn").ToList();
    }

    private void OnDisable()
    {
        playButton.UnregisterCallback<ClickEvent>(OnPlayClicked);
        settingsButton.UnregisterCallback<ClickEvent>(OnSettingsClicked);
        fullscreenToggle.UnregisterValueChangedCallback(OnFullscreenChanged);
        volumeSlider.UnregisterValueChangedCallback(OnVolumeChanged);
    }

    private void OnPlayClicked(ClickEvent evt) => Debug.Log("Play clicked");
    private void OnSettingsClicked(ClickEvent evt) => Debug.Log("Settings clicked");

    private void OnFullscreenChanged(ChangeEvent<bool> evt)
    {
        Screen.fullScreen = evt.newValue;
    }

    private void OnVolumeChanged(ChangeEvent<float> evt)
    {
        AudioListener.volume = evt.newValue / 100f;
    }
}

private void CreateUIFromCode()
{
    var root = uiDocument.rootVisualElement;

    var container = new VisualElement();
    container.AddToClassList("container");
    root.Add(container);

    var label = new Label("Created from C#");
    container.Add(label);

    var button = new Button(() => Debug.Log("Clicked")) { text = "Click Me" };
    button.name = "dynamic-button";
    container.Add(button);
}

// Default: bubble-up phase
element.RegisterCallback<PointerDownEvent>(OnPointerDown);

// Trickle-down phase (parent reacts before children)
element.RegisterCallback<PointerDownEvent>(OnPointerDown, TrickleDown.TrickleDown);

// Pass custom data to callbacks
element.RegisterCallback<ClickEvent, string>(OnClickWithData, "my-data");

// Set value without triggering ChangeEvent
myControl.SetValueWithoutNotify(newValue);

// In UXML: <ui:IntegerField binding-path="m_Health" label="Health" />
// In C#:
var healthField = new IntegerField("Health") { bindingPath = "m_Health" };
root.Add(healthField);
root.Bind(new SerializedObject(targetComponent));

public class DragManipulator : PointerManipulator
{
    private Vector3 startPosition;
    private bool isDragging;

    public DragManipulator(VisualElement target)
    {
        this.target = target;
    }

    protected override void RegisterCallbacksOnTarget()
    {
        target.RegisterCallback<PointerDownEvent>(OnPointerDown);
        target.RegisterCallback<PointerMoveEvent>(OnPointerMove);
        target.RegisterCallback<PointerUpEvent>(OnPointerUp);
    }

    protected override void UnregisterCallbacksFromTarget()
    {
        target.UnregisterCallback<PointerDownEvent>(OnPointerDown);
        target.UnregisterCallback<PointerMoveEvent>(OnPointerMove);
        target.UnregisterCallback<PointerUpEvent>(OnPointerUp);
    }

    private void OnPointerDown(PointerDownEvent evt)
    {
        startPosition = evt.position;
        isDragging = true;
        target.CapturePointer(evt.pointerId);
        evt.StopPropagation();
    }

    private void OnPointerMove(PointerMoveEvent evt)
    {
        if (!isDragging) return;
        var delta = evt.position - startPosition;
        target.transform.position += (Vector3)delta;
        startPosition = evt.position;
    }

    private void OnPointerUp(PointerUpEvent evt)
    {
        isDragging = false;
        target.ReleasePointer(evt.pointerId);
        evt.StopPropagation();
    }
}

// Usage:
myElement.AddManipulator(new DragManipulator(myElement));

// Unity 6+ recommended pattern: [UxmlElement] attribute (replaces deprecated UxmlFactory/UxmlTraits)
[UxmlElement]
public partial class HealthBar : VisualElement
{
    [UxmlAttribute]
    public float MaxHealth { get; set; } = 100f;

    private VisualElement fillBar;
    private float currentHealth;

    public float CurrentHealth
    {
        get => currentHealth;
        set
        {
            currentHealth = Mathf.Clamp(value, 0, MaxHealth);
            fillBar.style.width = Length.Percent(currentHealth / MaxHealth * 100f);
        }
    }

    public HealthBar()
    {
        AddToClassList("health-bar");
        fillBar = new VisualElement();
        fillBar.AddToClassList("health-bar__fill");
        Add(fillBar);
    }
}

using UnityEngine;
using UnityEngine.UI;

public class MenuManager : MonoBehaviour
{
    [SerializeField] private Button playButton;
    [SerializeField] private Slider volumeSlider;
    [SerializeField] private Toggle muteToggle;

    private void OnEnable()
    {
        playButton.onClick.AddListener(OnPlayClicked);
        volumeSlider.onValueChanged.AddListener(OnVolumeChanged);
        muteToggle.onValueChanged.AddListener(OnMuteToggled);
    }

    private void OnDisable()
    {
        playButton.onClick.RemoveListener(OnPlayClicked);
        volumeSlider.onValueChanged.RemoveListener(OnVolumeChanged);
        muteToggle.onValueChanged.RemoveListener(OnMuteToggled);
    }

    private void OnPlayClicked() => Debug.Log("Play");
    private void OnVolumeChanged(float value) => AudioListener.volume = value;
    private void OnMuteToggled(bool muted) => AudioListener.pause = muted;
}