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88 lines
4.3 KiB
Markdown
88 lines
4.3 KiB
Markdown
# hecs
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[](https://docs.rs/hecs/)
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[](https://crates.io/crates/hecs)
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[](LICENSE-APACHE)
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hecs provides a high-performance, minimalist entity-component-system (ECS)
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world. It is a library, not a framework. In place of an explicit "System"
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abstraction, a `World`'s entities are easily queried from regular code. Organize
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your application however you like!
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### Bevy Fork Information
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This is the Bevy project's fork of hecs with changes that accommodate the needs of the Bevy game engine. Some notable changes:
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* Entity indices are now queryable and are not returned in queries by default. This both improves ergonomics and significantly boosts performance in some cases.
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* Expose more interfaces as public so that we can build higher-level apis on top of the core hecs codebase (multithreading, functions-as-systems, world builders, schedules, etc)
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* Change Tracking
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### Why ECS?
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Entity-component-system architecture makes it easy to compose loosely-coupled
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state and behavior. An ECS world consists of:
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- any number of **entities**, which represent distinct objects
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- a collection of **component** data associated with each entity, where each
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entity has at most one component of any type, and two entities may have
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different components
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That world is then manipulated by **systems**, each of which accesses all
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entities having a particular set of component types. Systems implement
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self-contained behavior like physics (e.g. by accessing "position", "velocity",
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and "collision" components) or rendering (e.g. by accessing "position" and
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"sprite" components).
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New components and systems can be added to a complex application without
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interfering with existing logic, making the ECS paradigm well suited to
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applications where many layers of overlapping behavior will be defined on the
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same set of objects, particularly if new behaviors will be added in the
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future. This flexibility sets it apart from traditional approaches based on
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heterogeneous collections of explicitly defined object types, where implementing
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new combinations of behaviors (e.g. a vehicle which is also a questgiver) can
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require far-reaching changes.
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#### Performance
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In addition to having excellent composability, the ECS paradigm can also provide
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exceptional speed and cache locality. `hecs` internally tracks groups of
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entities which all have the same components. Each group has a dense, contiguous
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array for each type of component. When a system accesses all entities with a
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certain set of components, a fast linear traversal can be made through each
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group having a superset of those components. This is effectively a columnar
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database, and has the same benefits: the CPU can accurately predict memory
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accesses, bypassing unneeded data, maximizing cache use and minimizing latency.
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### Why Not ECS?
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An ECS world is not a be-all end-all data structure. Most games will store
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significant amounts of state in other structures. For example, many games
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maintain a spatial index structure (e.g. a tile map or bounding volume
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hierarchy) used to find entities and obstacles near a certain location for
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efficient collision detection without searching the entire world.
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If you need to search for specific entities using criteria other than the types
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of their components, consider maintaining a specialized index beside your world,
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storing `Entity` handles and whatever other data is necessary. Insert into the
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index when spawning relevant entities, and include a component with that allows
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efficiently removing them from the index when despawning.
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### Other Libraries
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hecs would not exist if not for the great work done by others to introduce and
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develop the ECS paradigm in the Rust ecosystem. In particular:
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- [specs] played a key role in popularizing ECS in Rust
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- [legion] reduced boilerplate and improved cache locality with sparse
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components
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hecs builds on these successes by focusing on further simplification, boiling
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the paradigm down to a minimal, light-weight and ergonomic core, without
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compromising on performance or flexibility.
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### Disclaimer
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This is not an official Google product (experimental or otherwise), it is just
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code that happens to be owned by Google.
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[specs]: https://github.com/amethyst/specs
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[legion]: https://github.com/TomGillen/legion
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