mirror of
https://github.com/bevyengine/bevy
synced 2024-12-26 21:13:09 +00:00
88 lines
4.3 KiB
Markdown
88 lines
4.3 KiB
Markdown
# hecs
|
|
|
|
[](https://docs.rs/hecs/)
|
|
[](https://crates.io/crates/hecs)
|
|
[](LICENSE-APACHE)
|
|
|
|
hecs provides a high-performance, minimalist entity-component-system (ECS)
|
|
world. It is a library, not a framework. In place of an explicit "System"
|
|
abstraction, a `World`'s entities are easily queried from regular code. Organize
|
|
your application however you like!
|
|
|
|
### Bevy Fork Information
|
|
|
|
This is the Bevy project's fork of hecs with changes that accommodate the needs of the Bevy game engine. Some notable changes:
|
|
* Entity indices are now queryable and are not returned in queries by default. This both improves ergonomics and significantly boosts performance in some cases.
|
|
* 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)
|
|
* Change Tracking
|
|
|
|
### Why ECS?
|
|
|
|
Entity-component-system architecture makes it easy to compose loosely-coupled
|
|
state and behavior. An ECS world consists of:
|
|
|
|
- any number of **entities**, which represent distinct objects
|
|
- a collection of **component** data associated with each entity, where each
|
|
entity has at most one component of any type, and two entities may have
|
|
different components
|
|
|
|
That world is then manipulated by **systems**, each of which accesses all
|
|
entities having a particular set of component types. Systems implement
|
|
self-contained behavior like physics (e.g. by accessing "position", "velocity",
|
|
and "collision" components) or rendering (e.g. by accessing "position" and
|
|
"sprite" components).
|
|
|
|
New components and systems can be added to a complex application without
|
|
interfering with existing logic, making the ECS paradigm well suited to
|
|
applications where many layers of overlapping behavior will be defined on the
|
|
same set of objects, particularly if new behaviors will be added in the
|
|
future. This flexibility sets it apart from traditional approaches based on
|
|
heterogeneous collections of explicitly defined object types, where implementing
|
|
new combinations of behaviors (e.g. a vehicle which is also a questgiver) can
|
|
require far-reaching changes.
|
|
|
|
#### Performance
|
|
|
|
In addition to having excellent composability, the ECS paradigm can also provide
|
|
exceptional speed and cache locality. `hecs` internally tracks groups of
|
|
entities which all have the same components. Each group has a dense, contiguous
|
|
array for each type of component. When a system accesses all entities with a
|
|
certain set of components, a fast linear traversal can be made through each
|
|
group having a superset of those components. This is effectively a columnar
|
|
database, and has the same benefits: the CPU can accurately predict memory
|
|
accesses, bypassing unneeded data, maximizing cache use and minimizing latency.
|
|
|
|
### Why Not ECS?
|
|
|
|
An ECS world is not a be-all end-all data structure. Most games will store
|
|
significant amounts of state in other structures. For example, many games
|
|
maintain a spatial index structure (e.g. a tile map or bounding volume
|
|
hierarchy) used to find entities and obstacles near a certain location for
|
|
efficient collision detection without searching the entire world.
|
|
|
|
If you need to search for specific entities using criteria other than the types
|
|
of their components, consider maintaining a specialized index beside your world,
|
|
storing `Entity` handles and whatever other data is necessary. Insert into the
|
|
index when spawning relevant entities, and include a component with that allows
|
|
efficiently removing them from the index when despawning.
|
|
|
|
### Other Libraries
|
|
|
|
hecs would not exist if not for the great work done by others to introduce and
|
|
develop the ECS paradigm in the Rust ecosystem. In particular:
|
|
|
|
- [specs] played a key role in popularizing ECS in Rust
|
|
- [legion] reduced boilerplate and improved cache locality with sparse
|
|
components
|
|
|
|
hecs builds on these successes by focusing on further simplification, boiling
|
|
the paradigm down to a minimal, light-weight and ergonomic core, without
|
|
compromising on performance or flexibility.
|
|
|
|
### Disclaimer
|
|
|
|
This is not an official Google product (experimental or otherwise), it is just
|
|
code that happens to be owned by Google.
|
|
|
|
[specs]: https://github.com/amethyst/specs
|
|
[legion]: https://github.com/TomGillen/legion
|