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+# Entity
+
+In ECS architecture, an Entity is the basic object in the game world. An entity itself does not contain game logic or data - it's just a container that combines different components to achieve various functionalities.
+
+## Basic Concepts
+
+An entity is a lightweight object mainly used for:
+- Serving as a container for components
+- Providing a unique identifier (ID)
+- Managing component lifecycle
+
+::: tip About Parent-Child Hierarchy
+Parent-child hierarchy relationships between entities are managed through `HierarchyComponent` and `HierarchySystem`, not built-in Entity properties. This design follows ECS composition principles - only entities that need hierarchy relationships add this component.
+
+See [Hierarchy System](./hierarchy.md) documentation.
+:::
+
+## Creating Entities
+
+**Important: Entities must be created through Scene, manual creation is not supported!**
+
+Entities must be created through the scene's `createEntity()` method to ensure:
+- Entity is properly added to the scene's entity management system
+- Entity is added to the query system for system use
+- Entity gets the correct scene reference
+- Related lifecycle events are triggered
+
+```typescript
+// Correct way: create entity through scene
+const player = scene.createEntity("Player");
+
+// Wrong way: manually create entity
+// const entity = new Entity("MyEntity", 1); // System cannot manage such entities
+```
+
+## Adding Components
+
+Entities gain functionality by adding components:
+
+```typescript
+import { Component, ECSComponent } from '@esengine/ecs-framework';
+
+// Define position component
+@ECSComponent('Position')
+class Position extends Component {
+  x: number = 0;
+  y: number = 0;
+
+  constructor(x: number = 0, y: number = 0) {
+    super();
+    this.x = x;
+    this.y = y;
+  }
+}
+
+// Define health component
+@ECSComponent('Health')
+class Health extends Component {
+  current: number = 100;
+  max: number = 100;
+
+  constructor(max: number = 100) {
+    super();
+    this.max = max;
+    this.current = max;
+  }
+}
+
+// Add components to entity
+const player = scene.createEntity("Player");
+player.addComponent(new Position(100, 200));
+player.addComponent(new Health(150));
+```
+
+## Getting Components
+
+```typescript
+// Get component (pass component class, not instance)
+const position = player.getComponent(Position);  // Returns Position | null
+const health = player.getComponent(Health);      // Returns Health | null
+
+// Check if component exists
+if (position) {
+  console.log(`Player position: x=${position.x}, y=${position.y}`);
+}
+
+// Check if entity has a component
+if (player.hasComponent(Position)) {
+  console.log("Player has position component");
+}
+
+// Get all component instances (read-only property)
+const allComponents = player.components;  // readonly Component[]
+
+// Get all components of specified type (supports multiple components of same type)
+const allHealthComponents = player.getComponents(Health);  // Health[]
+
+// Get or create component (creates automatically if not exists)
+const position = player.getOrCreateComponent(Position, 0, 0);  // Pass constructor arguments
+const health = player.getOrCreateComponent(Health, 100);       // Returns existing if present, creates new if not
+```
+
+## Removing Components
+
+```typescript
+// Method 1: Remove by component type
+const removedHealth = player.removeComponentByType(Health);
+if (removedHealth) {
+  console.log("Health component removed");
+}
+
+// Method 2: Remove by component instance
+const healthComponent = player.getComponent(Health);
+if (healthComponent) {
+  player.removeComponent(healthComponent);
+}
+
+// Batch remove multiple component types
+const removedComponents = player.removeComponentsByTypes([Position, Health]);
+
+// Check if component was removed
+if (!player.hasComponent(Health)) {
+  console.log("Health component has been removed");
+}
+```
+
+## Finding Entities
+
+Scene provides multiple ways to find entities:
+
+### Find by Name
+
+```typescript
+// Find single entity
+const player = scene.findEntity("Player");
+// Or use alias method
+const player2 = scene.getEntityByName("Player");
+
+if (player) {
+  console.log("Found player entity");
+}
+```
+
+### Find by ID
+
+```typescript
+// Find by entity ID
+const entity = scene.findEntityById(123);
+```
+
+### Find by Tag
+
+Entities support a tag system for quick categorization and lookup:
+
+```typescript
+// Set tags
+player.tag = 1; // Player tag
+enemy.tag = 2;  // Enemy tag
+
+// Find all entities by tag
+const players = scene.findEntitiesByTag(1);
+const enemies = scene.findEntitiesByTag(2);
+// Or use alias method
+const allPlayers = scene.getEntitiesByTag(1);
+```
+
+## Entity Lifecycle
+
+```typescript
+// Destroy entity
+player.destroy();
+
+// Check if entity is destroyed
+if (player.isDestroyed) {
+  console.log("Entity has been destroyed");
+}
+```
+
+## Entity Events
+
+Component changes on entities trigger events:
+
+```typescript
+// Listen for component added event
+scene.eventSystem.on('component:added', (data) => {
+  console.log('Component added:', data);
+});
+
+// Listen for entity created event
+scene.eventSystem.on('entity:created', (data) => {
+  console.log('Entity created:', data.entityName);
+});
+```
+
+## Performance Optimization
+
+### Batch Entity Creation
+
+The framework provides high-performance batch creation methods:
+
+```typescript
+// Batch create 100 bullet entities (high-performance version)
+const bullets = scene.createEntities(100, "Bullet");
+
+// Add components to each bullet
+bullets.forEach((bullet, index) => {
+  bullet.addComponent(new Position(Math.random() * 800, Math.random() * 600));
+  bullet.addComponent(new Velocity(Math.random() * 100 - 50, Math.random() * 100 - 50));
+});
+```
+
+`createEntities()` method will:
+- Batch allocate entity IDs
+- Batch add to entity list
+- Optimize query system updates
+- Reduce system cache clearing times
+
+## Best Practices
+
+### 1. Appropriate Component Granularity
+
+```typescript
+// Good practice: single-purpose components
+@ECSComponent('Position')
+class Position extends Component {
+  x: number = 0;
+  y: number = 0;
+}
+
+@ECSComponent('Velocity')
+class Velocity extends Component {
+  dx: number = 0;
+  dy: number = 0;
+}
+
+// Avoid: overly complex components
+@ECSComponent('Player')
+class Player extends Component {
+  // Avoid including too many unrelated properties in one component
+  x: number;
+  y: number;
+  health: number;
+  inventory: Item[];
+  skills: Skill[];
+}
+```
+
+### 2. Use Decorators
+
+Always use `@ECSComponent` decorator:
+
+```typescript
+@ECSComponent('Transform')
+class Transform extends Component {
+  // Component implementation
+}
+```
+
+### 3. Proper Naming
+
+```typescript
+// Clear entity naming
+const mainCharacter = scene.createEntity("MainCharacter");
+const enemy1 = scene.createEntity("Goblin_001");
+const collectible = scene.createEntity("HealthPotion");
+```
+
+### 4. Timely Cleanup
+
+```typescript
+// Destroy entities that are no longer needed
+if (enemy.getComponent(Health).current <= 0) {
+  enemy.destroy();
+}
+```
+
+## Debugging Entities
+
+The framework provides debugging features to help development:
+
+```typescript
+// Get entity debug info
+const debugInfo = entity.getDebugInfo();
+console.log('Entity info:', debugInfo);
+
+// List all components of entity
+entity.components.forEach(component => {
+  console.log('Component:', component.constructor.name);
+});
+```
+
+Entities are one of the core concepts in ECS architecture. Understanding how to use entities correctly will help you build efficient, maintainable game code.
+
+## Entity Handle (EntityHandle)
+
+Entity handles provide a safe way to reference entities, solving the "referencing destroyed entity" problem.
+
+### Problem Scenario
+
+Suppose your AI system needs to track a target enemy:
+
+```typescript
+// Wrong approach: directly store entity reference
+class AISystem extends EntitySystem {
+    private targetEnemy: Entity | null = null;
+
+    setTarget(enemy: Entity) {
+        this.targetEnemy = enemy;
+    }
+
+    process() {
+        if (this.targetEnemy) {
+            // Dangerous! Enemy might be destroyed, but reference still exists
+            // Worse: this memory location might be reused by a new entity
+            const health = this.targetEnemy.getComponent(Health);
+            // Might operate on the wrong entity!
+        }
+    }
+}
+```
+
+### Correct Approach Using Handles
+
+Each entity is automatically assigned a handle when created, accessible via `entity.handle`:
+
+```typescript
+import { EntityHandle, NULL_HANDLE, isValidHandle } from '@esengine/ecs-framework';
+
+class AISystem extends EntitySystem {
+    // Store handle instead of entity reference
+    private targetHandle: EntityHandle = NULL_HANDLE;
+
+    setTarget(enemy: Entity) {
+        // Save enemy's handle
+        this.targetHandle = enemy.handle;
+    }
+
+    process() {
+        if (!isValidHandle(this.targetHandle)) {
+            return; // No target
+        }
+
+        // Get entity through handle (automatically checks validity)
+        const enemy = this.scene.findEntityByHandle(this.targetHandle);
+
+        if (!enemy) {
+            // Enemy was destroyed, clear reference
+            this.targetHandle = NULL_HANDLE;
+            return;
+        }
+
+        // Safe operation
+        const health = enemy.getComponent(Health);
+    }
+}
+```
+
+### Complete Example: Skill Target Locking
+
+```typescript
+import {
+    EntitySystem, Entity, EntityHandle, NULL_HANDLE, isValidHandle
+} from '@esengine/ecs-framework';
+
+@ECSSystem('SkillTargeting')
+class SkillTargetingSystem extends EntitySystem {
+    // Store handles for multiple targets
+    private lockedTargets: Map<Entity, EntityHandle> = new Map();
+
+    // Lock target
+    lockTarget(caster: Entity, target: Entity) {
+        this.lockedTargets.set(caster, target.handle);
+    }
+
+    // Get locked target
+    getLockedTarget(caster: Entity): Entity | null {
+        const handle = this.lockedTargets.get(caster);
+
+        if (!handle || !isValidHandle(handle)) {
+            return null;
+        }
+
+        // findEntityByHandle checks if handle is valid
+        const target = this.scene.findEntityByHandle(handle);
+
+        if (!target) {
+            // Target died, clear lock
+            this.lockedTargets.delete(caster);
+        }
+
+        return target;
+    }
+
+    // Cast skill
+    castSkill(caster: Entity) {
+        const target = this.getLockedTarget(caster);
+
+        if (!target) {
+            console.log('Target lost, skill cancelled');
+            return;
+        }
+
+        // Safely deal damage to target
+        const health = target.getComponent(Health);
+        if (health) {
+            health.current -= 10;
+        }
+    }
+}
+```
+
+### Handle vs Entity Reference
+
+| Scenario | Recommended Approach |
+|----------|---------------------|
+| Temporary use within same frame | Use `Entity` reference directly |
+| Cross-frame storage (e.g., AI target, skill target) | Use `EntityHandle` |
+| Needs serialization | Use `EntityHandle` (numeric type) |
+| Network synchronization | Use `EntityHandle` (can be transmitted directly) |
+
+### API Quick Reference
+
+```typescript
+// Get entity's handle
+const handle = entity.handle;
+
+// Check if handle is non-null
+if (isValidHandle(handle)) { ... }
+
+// Get entity through handle (automatically checks validity)
+const entity = scene.findEntityByHandle(handle);
+
+// Check if entity corresponding to handle is alive
+const alive = scene.handleManager.isAlive(handle);
+
+// Null handle constant
+const emptyHandle = NULL_HANDLE;
+```
+
+## Next Steps
+
+- Learn about [Hierarchy System](./hierarchy.md) to establish parent-child relationships
+- Learn about [Component System](./component.md) to add functionality to entities
+- Learn about [Scene Management](./scene.md) to organize and manage entities