It's like a nice pair of specs, except much more uncomfortable and inconvenient

This crate is a heavily modified, stripped down version of the specs ECS library. It is less of a framework for doing a specific style of ECS as easily as possible and more of a DIY library for doing ECS that allows you to adapt it to your own needs.

goggles may not be as straightforward to use as other ECS libraries, if something like specs or legion already fits your needs then you should probably use it.

The basic data structure design is nearly exactly the same as specs, it uses the same basic data structures that specs does to store components and do joins. Just like specs, it stores components in separate storages and records their presence with a hierarchal bitset type from hibitset, and uses that same hierarchal bitset to do joins.

On top of this, however, is a more minimal, piecemeal API than even what specs provides. It is somewhat opinionated in what it removes: everything that I felt was extraneous or magical, and only tries to handle what is very hard to do otherwise or is unsafe, and mostly leaves the easier parts to the user to design themselves.

The library contains a set of more or less independent parts, you can stop at whatever level of abstraction is appropriate:

1. The par_seq module is a completely independent way of setting up and running generic parallel and sequential systems. It can be compared to shred, but it contains a much more generic System trait, and does not include functionality for locking and reading resources. It simply allows you to define a set of resources in Systems, combine those systems using Par and Seq, make sure that the result doesn't have resource conflicts, then run it.

2. The system_data module defines a SystemData trait for statically defined resources for the par_seq module, and provides a SystemData implementation for tuples of SystemData.

3. The resource_set module defines a ResourceSet which is similar to an AnyMap with values stored in a RwLock. Unlike a set of RwLocks though it doesn't ever block, instead it simply panics when aliasing rules are violated. It is designed so that you can use the par_seq module to build systems that operate over the defined resources. It also includes convenient types for defining and requesting read / write handles to resources which implement SystemData, so they can be used in tuples like (Read<ResourceA>, Write<ResourceB>). It is very similar to the World type in shred.

4. The join module contains a definition for a Join trait for data stored by u32 indexes and tracked by a BitSet with fast, unsafe access. It provides the ability to iterate safely over a Join sequentially and in parallel, and provides means to join multiple Join instances together. It is similar to the Join trait in specs, but redesigned for a bit more safety.

5. The storage module contains the RawStorage trait as well as 3 useful implementations: VecStorage, DenseVecStorage, and HashMapStorage. This is an abstract, unsafe trait for storing values associated with indexes. It is extremely similar to the equivalent functionality in specs.

6. The tracked module contains a RawStorage wrapper that keeps track of component changes. Unlike specs, this is pretty minimal and only optionally sets a flag in an AtomicBitSet on mutable access.

7. The masked module contains the MaskedStorage struct which safely wraps a RawStorage together with a BitSet to keep track of which components are present. MaskedStorage is also join-able.

8. The entity module contains an atomic generational index allocator that also uses hibitset types to track which indexes are alive. It also allows you to join on the allocator to output live Entity's.

9. The world module ties all of this together into something with a recognizable ECS API. If you want to understand how this works, or want to build your own abstractions instead of what's provided in this module, start here. Many of the changes to specs have been made so that the world module contains only safe code.

Here is an incomplete list of the important things removed from shred and specs:

1. No automatic setup, you must insert / register all resource and component types.

2. No saving / loading support, you should handle this yourself.

3. No automatic system scheduling / dispatching / batching, you must design your execution strategy yourself with par and seq and then it can be checked for conflicts at startup.

4. No lazy updates, you probably want to handle this specifically for your application. There is not always a universally best way to do this.

And here is an incomplete list of the important changes from specs for functionality that is present in both this library and specs:

1. Does not require T: Sync for mutable access to resources / components (only requires T: Send).

2. Redesigned Join trait for soundness and a bit more safety. There is an additional IntoJoin trait that allows you to participate in the convenient tuple join syntax without having to write unsafe code.

3. Component RawStorage impls require UnsafeCell for soundness

4. Component RawStorage impls always assume disjoint storages that are safe to iterate over in parallel.

5. Simplified component modification tracking.

6. Removes some features of specs which are known to be unsound such as index component access through iterators.

7. The individual parts of the library go out of their way to be more loosely coupled, sometimes at the cost of extra code or user convenience.

8. Nearly all of the internals are public in case you need to build a different abstraction.

I'd been working on a project for a while that used specs proper, and I kept needing to redesign small pieces of it. Much to specs credit, it is already a library that you can use in a very piecemeal, independent way, and that's exactly what I did. At some point I looked up and realized that I only used maybe a core 20% of what specs provided, and that was around the time that I started needing to use messy extension traits to go further. I decided to re-implement the core part of specs that I still used, package it up with some of the other things I made that were more flexible (but arguably harder to use) than what shred / specs offered, and put it here.

There's a good chance you don't need or want this crate. Still, if you find yourself needing to break up specs into its constituent parts and build your own APIs on top of it, this is here if you need it.

This project is directly derived from specs so most of the credit goes to specs' creators. Anything at all in this crate that is especially clever is probably theirs.

This is derived from specs, so similarly it is dual-licensed under Apache-2.0 / MIT.