entitas

Entitas is an open-source Entity-Component-System (ECS) framework for building games and other performance-sensitive software in C#, with particular adoption in Unity projects. It emphasizes data-oriented design and code generation to produce a fast, strongly typed API for components, contexts, and systems.

In Entitas, an entity is a lightweight container that holds components, which are plain data structures containing state. Systems operate on entities that match specific component compositions, enabling clear separation of concerns and predictable performance. Core concepts include contexts (also called pools), entities, components, and matchers. A matcher defines the set of components an entity must possess to be included in a system’s processing. Entitas uses a generator to create typed code from component definitions, avoiding runtime reflection and reducing allocations.

Typical workflow involves defining components as simple data classes or structs, running the generator to produce the API, creating and configuring contexts, and implementing systems (such as Execute, Initialize, or Reactive systems) that query entities via matchers. Changes to components on entities drive system execution, while Unity’s update loop can drive the overall processing.

History and ecosystem: Entitas emerged from the Unity community in the early 2010s and became one of the most widely used ECS frameworks for Unity. The project has inspired forks and related projects that adapt the pattern to different needs.

Strengths and considerations: Entitas offers high performance for large numbers of entities and clear data-oriented architecture, at the cost of added complexity and a build-time code-generation step. It is commonly cited as a practical entry point to ECS for Unity developers seeking scalable, maintainable game logic. Related concepts include the broader ECS pattern and data-oriented design.