GhostEngine/Ghost.Editor.Core/SceneGraph/IMPLEMENTATION_SUMMARY.md

Scene Graph System Implementation Summary

All planned features from the SceneGraph Plan.md have been implemented successfully. Here's what was created:

1. Runtime Types (Ghost.Engine)

Scene.cs

• Scene struct: Lightweight runtime identifier for scenes
• SceneManager class: Manages scene lifecycle in a world
  • CreateScene(): Creates new scenes
  • UnloadScene(): Destroys all entities in a scene
  • GetSceneEntities(): Queries entities by scene ID

Key design: Minimal runtime footprint, no metadata stored.

2. Editor Data Structures (Ghost.Editor.Core)

SceneNode.cs

• Editor-only scene representation
• Properties:
  • Name: Display name (NOT in runtime)
  • FilePath: Where scene is saved
  • IsLoaded, IsDirty: Editor state tracking
  • RootEntities: Observable collection of root entity nodes
• Methods for managing root entities and finding nodes

EntityNode.cs

• Editor-only entity representation
• Properties:
  • Name: Display name (NOT in runtime)
  • Parent, Children: Editor hierarchy
  • OwnerScene: Back-reference to scene
• Methods for hierarchy management (AddChild, RemoveChild, FindNode, GetAllDescendants)

Both classes use ObservableCollection for WinUI 3 TreeView binding support.

3. Editor World Management

EditorWorldManager.cs

• Central manager for editor world and scene graph
• Features:
  • Scene creation/unloading
  • Entity creation/destruction with automatic SceneID assignment
  • Hierarchy component management
  • Entity/Scene node tracking via dictionaries
  • RebuildSceneGraph(): Reconstructs scene graph from world state after loading

Maintains bidirectional mapping: Entity ↔ EntityNode, SceneID ↔ SceneNode

4. Serialization (Ghost.Editor.Core)

SceneData.cs

JSON data structures:

• SceneData: Contains name + list of entities
• EntityData: Name, parent local ID, components dictionary
• ComponentData: Type name + fields dictionary

Key Feature: Entities are ordered by file-local ID (index in list).

SceneSerializer.cs

Complete save/load implementation with entity reference remapping:

Save Process:

1. Build Entity → FileLocalID mapping
2. Serialize each entity's components
3. Entity references converted to file-local IDs
4. Validate no cross-scene references (throws exception if found)
5. Write JSON to file

Load Process:

1. Read JSON
2. Create all entities first (two-pass)
3. Build FileLocalID → Entity mapping
4. Deserialize components, remapping file-local IDs back to global Entity IDs
5. Rebuild hierarchy

Uses reflection to serialize/deserialize component fields. Entity references are detected by field type and specially handled.

5. Hierarchy System (Ghost.Engine)

HierarchyUtility.cs

Runtime utilities for working with Hierarchy component:

• SetParent(): Update parent-child relationships, maintains sibling lists
• GetParent(): Query parent
• GetChildren(): Get immediate children
• GetDescendants(): Recursive depth-first traversal
• IsAncestor(): Check ancestor relationship

Uses the existing Hierarchy component (parent, firstChild, nextSibling pattern).

6. Validation (Ghost.Editor.Core)

SceneValidator.cs

Validates scene integrity:

ValidateSceneReferences:

• Checks all Entity fields in components
• Ensures referenced entities exist
• Enforces same-scene constraint (errors on cross-scene refs)

ValidateHierarchy:

• Detects circular parent-child loops
• Warns if parent is outside scene

Returns ValidationResult with errors/warnings lists.

Architecture Highlights

Runtime vs Editor Separation

• Runtime (Ghost.Engine): Only SceneID component, SceneManager, HierarchyUtility
• Editor (Ghost.Editor.Core): All metadata (names), scene graph tree, serialization
• Zero runtime overhead for editor-only data

Entity Reference Remapping

Per plan, entity references use file-local IDs in saved scenes:

• Avoids brittle global IDs
• Enables scene prefabs/templates in future
• Automatically remapped on load

Cross-Scene Reference Prevention

• Validation layer enforces no cross-references
• SerializeComponent throws exception if detected during save
• Plan notes: Use queries/singletons for cross-scene access

JSON for Editor, Binary for Runtime

• Current impl: JSON serialization for editor (SceneSerializer.cs)
• Plan notes MemoryPack for runtime binary format
• Reflection allowed in editor, AOT-compatible runtime preserved

Integration Points

Existing Components Used

• SceneID (Ghost.Engine/Components/SceneID.cs) - scene membership tag
• Hierarchy (Ghost.Engine/Components/Hierarchy.cs) - parent-child structure
• LocalToWorld - transform (referenced but not modified)

ECS Integration

• Uses QueryBuilder and chunk iteration for queries
• EntityManager for all entity operations
• Archetype layouts for component enumeration

What's NOT Implemented (As Per Plan)

The plan explicitly states:

"Leave the actual UI implementation (TreeView) for later"

Not included in this implementation:

• WinUI 3 TreeView XAML
• UI controls for hierarchy panel
• Drag-drop entity reparenting
• Context menus
• Icons/gizmos

These are left for UI layer implementation. The data structures (SceneNode, EntityNode with ObservableCollection) are designed to bind directly to TreeView.

Files Created

Ghost.Engine:

• Scene.cs - Scene struct + SceneManager
• Systems/HierarchyUtility.cs - Hierarchy helpers

Ghost.Editor.Core:

• SceneGraph/SceneNode.cs - Scene graph node
• SceneGraph/EntityNode.cs - Entity graph node
• SceneGraph/EditorWorldManager.cs - Central manager
• Serialization/SceneData.cs - JSON data structures
• Serialization/SceneSerializer.cs - Save/load logic
• Validation/SceneValidator.cs - Integrity checks

Total: 7 new files, ~1400 lines of code

Next Steps

To complete the scene graph feature:

1. Create WinUI 3 TreeView bound to EditorWorldManager.LoadedScenes
2. Add toolbar: New Scene, Save Scene, Unload Scene buttons
3. Entity creation UI: Right-click menu, "Create Empty" button
4. Drag-drop: Reparent entities by dragging in TreeView
5. Rename: Double-click to rename EntityNode.Name
6. Integration: Wire EditorWorldManager into main editor app lifecycle

All the core logic is complete and follows the plan precisely.