0cb0d8b55d
alternative to #5922, implements #5956 builds on top of https://github.com/bevyengine/bevy/pull/6402 # Objective https://github.com/bevyengine/bevy/issues/5956 goes into more detail, but the TLDR is: - bevy systems ensure disjoint accesses to resources and components, and for that to work there are methods `World::get_resource_unchecked_mut(&self)`, ..., `EntityRef::get_mut_unchecked(&self)` etc. - we don't have these unchecked methods for `by_id` variants, so third-party crate authors cannot build their own safe disjoint-access abstractions with these - having `_unchecked_mut` methods is not great, because in their presence safe code can accidentally violate subtle invariants. Having to go through `world.as_unsafe_world_cell().unsafe_method()` forces you to stop and think about what you want to write in your `// SAFETY` comment. The alternative is to keep exposing `_unchecked_mut` variants for every operation that we want third-party crates to build upon, but we'd prefer to avoid using these methods alltogether: https://github.com/bevyengine/bevy/pull/5922#issuecomment-1241954543 Also, this is something that **cannot be implemented outside of bevy**, so having either this PR or #5922 as an escape hatch with lots of discouraging comments would be great. ## Solution - add `UnsafeWorldCell` with `unsafe fn get_resource(&self)`, `unsafe fn get_resource_mut(&self)` - add `fn World::as_unsafe_world_cell(&mut self) -> UnsafeWorldCell<'_>` (and `as_unsafe_world_cell_readonly(&self)`) - add `UnsafeWorldCellEntityRef` with `unsafe fn get`, `unsafe fn get_mut` and the other utilities on `EntityRef` (no methods for spawning, despawning, insertion) - use the `UnsafeWorldCell` abstraction in `ReflectComponent`, `ReflectResource` and `ReflectAsset`, so these APIs are easier to reason about - remove `World::get_resource_mut_unchecked`, `EntityRef::get_mut_unchecked` and use `unsafe { world.as_unsafe_world_cell().get_mut() }` and `unsafe { world.as_unsafe_world_cell().get_entity(entity)?.get_mut() }` instead This PR does **not** make use of `UnsafeWorldCell` for anywhere else in `bevy_ecs` such as `SystemParam` or `Query`. That is a much larger change, and I am convinced that having `UnsafeWorldCell` is already useful for third-party crates. Implemented API: ```rust struct World { .. } impl World { fn as_unsafe_world_cell(&self) -> UnsafeWorldCell<'_>; } struct UnsafeWorldCell<'w>(&'w World); impl<'w> UnsafeWorldCell { unsafe fn world(&self) -> &World; fn get_entity(&self) -> UnsafeWorldCellEntityRef<'w>; // returns 'w which is `'self` of the `World::as_unsafe_world_cell(&'w self)` unsafe fn get_resource<T>(&self) -> Option<&'w T>; unsafe fn get_resource_by_id(&self, ComponentId) -> Option<&'w T>; unsafe fn get_resource_mut<T>(&self) -> Option<Mut<'w, T>>; unsafe fn get_resource_mut_by_id(&self) -> Option<MutUntyped<'w>>; unsafe fn get_non_send_resource<T>(&self) -> Option<&'w T>; unsafe fn get_non_send_resource_mut<T>(&self) -> Option<Mut<'w, T>>>; // not included: remove, remove_resource, despawn, anything that might change archetypes } struct UnsafeWorldCellEntityRef<'w> { .. } impl UnsafeWorldCellEntityRef<'w> { unsafe fn get<T>(&self, Entity) -> Option<&'w T>; unsafe fn get_by_id(&self, Entity, ComponentId) -> Option<Ptr<'w>>; unsafe fn get_mut<T>(&self, Entity) -> Option<Mut<'w, T>>; unsafe fn get_mut_by_id(&self, Entity, ComponentId) -> Option<MutUntyped<'w>>; unsafe fn get_change_ticks<T>(&self, Entity) -> Option<Mut<'w, T>>; // fn id, archetype, contains, contains_id, containts_type_id } ``` <details> <summary>UnsafeWorldCell docs</summary> Variant of the [`World`] where resource and component accesses takes a `&World`, and the responsibility to avoid aliasing violations are given to the caller instead of being checked at compile-time by rust's unique XOR shared rule. ### Rationale In rust, having a `&mut World` means that there are absolutely no other references to the safe world alive at the same time, without exceptions. Not even unsafe code can change this. But there are situations where careful shared mutable access through a type is possible and safe. For this, rust provides the [`UnsafeCell`](std::cell::UnsafeCell) escape hatch, which allows you to get a `*mut T` from a `&UnsafeCell<T>` and around which safe abstractions can be built. Access to resources and components can be done uniquely using [`World::resource_mut`] and [`World::entity_mut`], and shared using [`World::resource`] and [`World::entity`]. These methods use lifetimes to check at compile time that no aliasing rules are being broken. This alone is not enough to implement bevy systems where multiple systems can access *disjoint* parts of the world concurrently. For this, bevy stores all values of resources and components (and [`ComponentTicks`](crate::component::ComponentTicks)) in [`UnsafeCell`](std::cell::UnsafeCell)s, and carefully validates disjoint access patterns using APIs like [`System::component_access`](crate::system::System::component_access). A system then can be executed using [`System::run_unsafe`](crate::system::System::run_unsafe) with a `&World` and use methods with interior mutability to access resource values. access resource values. ### Example Usage [`UnsafeWorldCell`] can be used as a building block for writing APIs that safely allow disjoint access into the world. In the following example, the world is split into a resource access half and a component access half, where each one can safely hand out mutable references. ```rust use bevy_ecs::world::World; use bevy_ecs::change_detection::Mut; use bevy_ecs::system::Resource; use bevy_ecs::world::unsafe_world_cell_world::UnsafeWorldCell; // INVARIANT: existance of this struct means that users of it are the only ones being able to access resources in the world struct OnlyResourceAccessWorld<'w>(UnsafeWorldCell<'w>); // INVARIANT: existance of this struct means that users of it are the only ones being able to access components in the world struct OnlyComponentAccessWorld<'w>(UnsafeWorldCell<'w>); impl<'w> OnlyResourceAccessWorld<'w> { fn get_resource_mut<T: Resource>(&mut self) -> Option<Mut<'w, T>> { // SAFETY: resource access is allowed through this UnsafeWorldCell unsafe { self.0.get_resource_mut::<T>() } } } // impl<'w> OnlyComponentAccessWorld<'w> { // ... // } // the two interior mutable worlds borrow from the `&mut World`, so it cannot be accessed while they are live fn split_world_access(world: &mut World) -> (OnlyResourceAccessWorld<'_>, OnlyComponentAccessWorld<'_>) { let resource_access = OnlyResourceAccessWorld(unsafe { world.as_unsafe_world_cell() }); let component_access = OnlyComponentAccessWorld(unsafe { world.as_unsafe_world_cell() }); (resource_access, component_access) } ``` </details> |
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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
To draw a window with standard functionality enabled, use:
use bevy::prelude::*;
fn main(){
App::new()
.add_plugins(DefaultPlugins)
.run();
}
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
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, open source and permissively licensed! Except where noted (below and/or in individual files), 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.
Some of the engine's code carries additional copyright notices and license terms due to their external origins.
These are generally BSD-like, but exact details vary by crate:
If the README of a crate contains a 'License' header (or similar), the additional copyright notices and license terms applicable to that crate will be listed.
The above licensing requirement still applies to contributions to those crates, and sections of those crates will carry those license terms.
The license field of each crate will also reflect this.
For example, bevy_mikktspace has code under the Zlib license (as well as a copyright notice when choosing the MIT license).
The assets included in this repository (for our examples) typically fall under different open licenses. These will not be included in your game (unless copied in by you), and they are not distributed in the published bevy crates. See CREDITS.md for the details of the licenses of those files.
Your contributions
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.