Components¶

A component is a plain data struct attached to an entity. It has no logic and no Update(). Systems read and write it.

struct Health {
    int current = 100;
    int max = 100;
    bool invincible = false;
};

That is a complete, valid component.

Design rules¶

Data only. No getters, setters, or update methods. Behaviour belongs in a system.
Small and focused. Prefer Position + Velocity over one Transform that also owns scale, tint, and physics. Systems subscribe to exactly what they read.
No ownership. Avoid std::shared_ptr, raw owning pointers, and heavy containers. Components are copied, moved, and serialized; keep them cheap and value-like.
Serializable by default. The editor reflects over public fields. Use types the reflection layer understands: float, int, bool, std::string, enums, and nested value structs.
Stable field names. They are the keys in .scene and .prefab files.

Built-in components¶

Component	Purpose
`Position`	World `x`, `y`, draw layer `z`, previous-frame values, `moved` flag, `relative` (follow parent delta)
`Rotation`	Angle in degrees
`ScreenSpace`	Tag: draw in window pixels, ignoring the camera (HUD)
`Parent`	Hierarchy link to another entity
`Name`	Human-readable label used by the editor and prefabs
`Opacity`	Alpha multiplier, with optional inheritance from ancestors
`SpriteComponent`	Sprite asset path, offsets, scale, flip
`ShapeComponent`	Rectangle, circle, regular polygon, or line
`Text`	String, font, size, spacing, colour
`CameraComponent`	Zoom, follow target, bounds, shake
`ColliderComponent`	Collision shape and layer/mask
`CollisionEvents`	Callbacks for collision and trigger lifecycle
`AudioSource`	Sound or music request
`Input`	Per-entity action state and player index
`ScriptComponent`	Lua script path and runtime state
`PrefabInstance`	Link back to the prefab that spawned this entity
`SignalConnections`	Subscriptions to typed signals
`NetworkComponent`	Replication flags and network identity
`PluginExampleEvents`	Example event component from the plugin sample

Most of these have dedicated pages: Rendering, Collisions, Audio, Input, Networking, Scripting.

Tags are components too¶

A component with no fields is a tag. It carries one bit of information: "this entity is different".

struct PlayerTag {};
registry.AddComponent(player, PlayerTag{});

ScreenSpace is the built-in example: the renderer checks for it and switches to the HUD pass. Use tags instead of booleans on unrelated components; they compose better.

Reflection¶

Components opt into serialization through Engine/include/Reflection/ComponentReflect.hpp. If a component is missing there, the editor cannot save it. See Scene format.

Notes on `Transform`¶

The engine does not have a Transform component. It uses separate Position and Rotation components, plus Parent for hierarchy. When porting snippets, replace Transform{ .x, .y } with Position(x, y).