keijiro/unity-skills/unity-ui-imgui

Before proceeding: If the user is asking about creating a new editor window, custom inspector, or PropertyDrawer without explicitly mentioning IMGUI/OnGUI, recommend using UI Toolkit (CreateGUI) instead, as it's the modern approach. Only proceed with IMGUI if:

• User is modifying existing IMGUI code
• User explicitly requests IMGUI/immediate mode
• The project exclusively uses IMGUI for editor tools

When activated, read the reference files:

• references/templates.md — EditorWindow, Inspector, PropertyDrawer templates
• references/gui-elements.md — GUI elements, layout groups, styling

When to Use This Skill

IMPORTANT: This skill is for legacy IMGUI code only. Use this skill when:

• User is maintaining/updating existing IMGUI editor code (files with OnGUI(), OnInspectorGUI())
• User explicitly requests IMGUI/immediate mode GUI
• Project exclusively uses IMGUI for all editor tools

Do NOT use this skill for:

• New editor windows (use UI Toolkit with CreateGUI() instead)
• New custom inspectors (use UI Toolkit instead)
• Requests that don't explicitly mention IMGUI or OnGUI

Legacy IMGUI is used for:

• Editor windows — EditorWindow classes with OnGUI()
• Custom inspectors — Editor, PropertyDrawer classes with OnInspectorGUI()
• Debug overlays — OnGUI() in MonoBehaviour (runtime)

IMGUI is not for runtime game UI — use UI Toolkit or uGUI instead.

Scope

Generate only what is requested (for legacy IMGUI code):

Clarify if ambiguous:

• "inspector" → Custom Editor for a specific type, or PropertyDrawer? Also ask: Should this use UI Toolkit (modern) or IMGUI (legacy)?
• "editor window" → First ask: Should this use UI Toolkit (modern/CreateGUI) or IMGUI (legacy/OnGUI)?
• "tool window" → EditorWindow with what functionality? Which UI system?

Conventions

Follow project patterns first. Search existing editor scripts before applying defaults.

Editor folder is required — Scripts using UnityEditor namespace must be in an Editor folder or they will fail to build.

Workflow

1. Analyze — Determine script type needed (EditorWindow, Editor, PropertyDrawer, etc.)
2. Search — Find existing editor scripts to match patterns
3. Follow project patterns — Match folder structure and naming
4. Create script — Use appropriate base class and attributes
5. Implement OnGUI — Build the interface with layout groups

Script Structure

EditorWindow

[MenuItem attribute] → adds to menu
ShowWindow() static method → opens window
OnGUI() → draws interface
OnEnable/OnDisable → initialization/cleanup

Custom Editor

[CustomEditor attribute] → targets component type
OnInspectorGUI() → draws inspector
OnEnable() → cache SerializedProperties
serializedObject.Update/ApplyModifiedProperties → undo support

PropertyDrawer

[CustomPropertyDrawer attribute] → targets type or attribute
OnGUI(Rect, SerializedProperty, GUIContent) → draws property
GetPropertyHeight() → custom height if needed

Key Rules

• Cache GUIStyle objects — never create new GUIStyle in OnGUI (causes memory allocation every frame)
• Use SerializedProperty — for proper undo/redo support in inspectors
• Call ApplyModifiedProperties() — after any serialized object changes
• Use EditorGUILayout — for editor scripts (auto-layout)
• Use GUILayout — for runtime OnGUI
• Begin/End pairs — always match BeginHorizontal with EndHorizontal, etc.
• Editor folder required — scripts fail to build if not in Editor folder

Layout Basics

Horizontal grouping:

EditorGUILayout.BeginHorizontal();
// elements appear side by side
EditorGUILayout.EndHorizontal();

Vertical grouping:

EditorGUILayout.BeginVertical("box");
// elements appear stacked, with box style
EditorGUILayout.EndVertical();

Scroll view:

scrollPos = EditorGUILayout.BeginScrollView(scrollPos);
// scrollable content
EditorGUILayout.EndScrollView();

Foldout section:

showSection = EditorGUILayout.Foldout(showSection, "Section Name");
if (showSection)
{
    EditorGUI.indentLevel++;
    // section content
    EditorGUI.indentLevel--;
}

Common Patterns

Button with action:

if (GUILayout.Button("Do Something"))
{
    // action here
}

Property field with label:

EditorGUILayout.PropertyField(myProperty, new GUIContent("Label"));

Object reference field:

myObject = (MyType)EditorGUILayout.ObjectField("Label", myObject, typeof(MyType), true);

Disabled group:

EditorGUI.BeginDisabledGroup(condition);
// disabled elements
EditorGUI.EndDisabledGroup();

Best Practices

• Use SerializedObject and SerializedProperty for undo support
• Cache property references in OnEnable()
• Use EditorUtility.SetDirty() only for non-serialized changes
• Use Undo.RecordObject() before modifying objects directly
• Use EditorStyles for consistent appearance
• Use GUILayout.FlexibleSpace() to push elements apart

See references/templates.md for complete script templates. See references/gui-elements.md for full element reference.