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# Objective - Fix a type inference regression introduced by #3001 - Make read only bounds on world queries more user friendly ptrification required you to write `Q::Fetch: ReadOnlyFetch` as `for<'w> QueryFetch<'w, Q>: ReadOnlyFetch` which has the same type inference problem as `for<'w> QueryFetch<'w, Q>: FilterFetch<'w>` had, i.e. the following code would error: ```rust #[derive(Component)] struct Foo; fn bar(a: Query<(&Foo, Without<Foo>)>) { foo(a); } fn foo<Q: WorldQuery>(a: Query<Q, ()>) where for<'w> QueryFetch<'w, Q>: ReadOnlyFetch, { } ``` `for<..>` bounds are also rather user unfriendly.. ## Solution Remove the `ReadOnlyFetch` trait in favour of a `ReadOnlyWorldQuery` trait, and remove `WorldQueryGats::ReadOnlyFetch` in favor of `WorldQuery::ReadOnly` allowing the previous code snippet to be written as: ```rust #[derive(Component)] struct Foo; fn bar(a: Query<(&Foo, Without<Foo>)>) { foo(a); } fn foo<Q: ReadOnlyWorldQuery>(a: Query<Q, ()>) {} ``` This avoids the `for<...>` bound which makes the code simpler and also fixes the type inference issue. The reason for moving the two functions out of `FetchState` and into `WorldQuery` is to allow the world query `&mut T` to share a `State` with the `&T` world query so that it can have `type ReadOnly = &T`. Presumably it would be possible to instead have a `ReadOnlyRefMut<T>` world query and then do `type ReadOnly = ReadOnlyRefMut<T>` much like how (before this PR) we had a `ReadOnlyWriteFetch<T>`. A side benefit of the current solution in this PR is that it will likely make it easier in the future to support an API such as `Query<&mut T> -> Query<&T>`. The primary benefit IMO is just that `ReadOnlyRefMut<T>` and its associated fetch would have to reimplement all of the logic that the `&T` world query impl does but this solution avoids that :) --- ## Changelog/Migration Guide The trait `ReadOnlyFetch` has been replaced with `ReadOnlyWorldQuery` along with the `WorldQueryGats::ReadOnlyFetch` assoc type which has been replaced with `<WorldQuery::ReadOnly as WorldQueryGats>::Fetch` - Any where clauses such as `QueryFetch<Q>: ReadOnlyFetch` should be replaced with `Q: ReadOnlyWorldQuery`. - Any custom world query impls should implement `ReadOnlyWorldQuery` insead of `ReadOnlyFetch` Functions `update_component_access` and `update_archetype_component_access` have been moved from the `FetchState` trait to `WorldQuery` - Any callers should now call `Q::update_component_access(state` instead of `state.update_component_access` (and `update_archetype_component_access` respectively) - Any custom world query impls should move the functions from the `FetchState` impl to `WorldQuery` impl `WorldQuery` has been made an `unsafe trait`, `FetchState` has been made a safe `trait`. (I think this is how it should have always been, but regardless this is _definitely_ necessary now that the two functions have been moved to `WorldQuery`) - If you have a custom `FetchState` impl make it a normal `impl` instead of `unsafe impl` - If you have a custom `WorldQuery` impl make it an `unsafe impl`, if your code was sound before it is going to still be sound |
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What is Bevy?
Bevy is a refreshingly simple data-driven game engine built in Rust. It is free and open-source forever!
WARNING
Bevy is still in the very early stages of development. APIs can and will change (now is the time to make suggestions!). Important features are missing. Documentation is sparse. Please don't build any serious projects in Bevy unless you are prepared to be broken by API changes constantly.
MSRV: Bevy relies heavily on improvements in the Rust language and compiler. As a result, the Minimum Supported Rust Version (MSRV) is "the latest stable release" of Rust.
Design Goals
- Capable: Offer a complete 2D and 3D feature set
- Simple: Easy for newbies to pick up, but infinitely flexible for power users
- Data Focused: Data-oriented architecture using the Entity Component System paradigm
- Modular: Use only what you need. Replace what you don't like
- Fast: App logic should run quickly, and when possible, in parallel
- Productive: Changes should compile quickly ... waiting isn't fun
About
- Features: A quick overview of Bevy's features.
- News: A development blog that covers our progress, plans and shiny new features.
Docs
- The Bevy Book: Bevy's official documentation. The best place to start learning Bevy.
- Bevy Rust API Docs: Bevy's Rust API docs, which are automatically generated from the doc comments in this repo.
- Official Examples: Bevy's dedicated, runnable examples, which are great for digging into specific concepts.
- Community-Made Learning Resources: More tutorials, documentation, and examples made by the Bevy community.
Community
Before contributing or participating in discussions with the community, you should familiarize yourself with our Code of Conduct.
- Discord: Bevy's official discord server.
- Reddit: Bevy's official subreddit.
- GitHub Discussions: The best place for questions about Bevy, answered right here!
- Bevy Assets: A collection of awesome Bevy projects, tools, plugins and learning materials.
If you'd like to help build Bevy, check out the Contributor's Guide. For simple problems, feel free to open an issue or PR and tackle it yourself!
For more complex architecture decisions and experimental mad science, please open an RFC (Request For Comments) so we can brainstorm together effectively!
Getting Started
We recommend checking out The Bevy Book for a full tutorial.
Follow the Setup guide to ensure your development environment is set up correctly. Once set up, you can quickly try out the examples by cloning this repo and running the following commands:
# Switch to the correct version (latest release, default is main development branch)
git checkout latest
# Runs the "breakout" example
cargo run --example breakout
Fast Compiles
Bevy can be built just fine using default configuration on stable Rust. However for really fast iterative compiles, you should enable the "fast compiles" setup by following the instructions here.
Libraries Used
Bevy is only possible because of the hard work put into these foundational technologies:
- wgpu: modern / low-level / cross-platform graphics library inspired by Vulkan
- glam-rs: a simple and fast 3D math library for games and graphics
- winit: cross-platform window creation and management in Rust
- spirv-reflect: Reflection API in rust for SPIR-V shader byte code
Bevy Cargo Features
This list outlines the different cargo features supported by Bevy. These allow you to customize the Bevy feature set for your use-case.
Third Party Plugins
Plugins are very welcome to extend Bevy's features. Guidelines are available to help integration and usage.
Thanks and Alternatives
Additionally, we would like to thank the Amethyst, macroquad, coffee, ggez, Fyrox, and Piston projects for providing solid examples of game engine development in Rust. If you are looking for a Rust game engine, it is worth considering all of your options. Each engine has different design goals, and some will likely resonate with you more than others.
License
Bevy is free and open source! All code in this repository is dual-licensed under either:
- MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
at your option. This means you can select the license you prefer! This dual-licensing approach is the de-facto standard in the Rust ecosystem and there are very good reasons to include both.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.