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# Objective Fixes #15367. Currently, required components can only be defined through the `require` macro attribute. While this should be used in most cases, there are also several instances where you may want to define requirements at runtime, commonly in plugins. Example use cases: - Require components only if the relevant optional plugins are enabled. For example, a `SleepTimer` component (for physics) is only relevant if the `SleepPlugin` is enabled. - Third party crates can define their own requirements for first party types. For example, "each `Handle<Mesh>` should require my custom rendering data components". This also gets around the orphan rule. - Generic plugins that add marker components based on the existence of other components, like a generic `ColliderPlugin<C: AnyCollider>` that wants to add a `ColliderMarker` component for all types of colliders. - This is currently relevant for the retained render world in #15320. The `ExtractComponentPlugin<C>` should add `SyncToRenderWorld` to all components that should be extracted. This is currently done with observers, which is more expensive than required components, and causes archetype moves. - Replace some built-in components with custom versions. For example, if `GlobalTransform` required `Transform` through `TransformPlugin`, but we wanted to use a `CustomTransform` type, we could replace `TransformPlugin` with our own plugin. (This specific example isn't good, but there are likely better use cases where this may be useful) See #15367 for more in-depth reasoning. ## Solution Add `register_required_components::<T, R>` and `register_required_components_with::<T, R>` methods for `Default` and custom constructors respectively. These methods exist on `App` and `World`. ```rust struct BirdPlugin; impl Plugin for BirdPlugin { fn plugin(app: &mut App) { // Make `Bird` require `Wings` with a `Default` constructor. app.register_required_components::<Bird, Wings>(); // Make `Wings` require `FlapSpeed` with a custom constructor. // Fun fact: Some hummingbirds can flutter their wings 80 times per second! app.register_required_components_with::<Wings, FlapSpeed>(|| FlapSpeed::from_duration(1.0 / 80.0)); } } ``` The custom constructor is a function pointer to match the `require` API, though it could take a raw value too. Requirement inheritance works similarly as with the `require` attribute. If `Bird` required `FlapSpeed` directly, it would take precedence over indirectly requiring it through `Wings`. The same logic applies to all levels of the inheritance tree. Note that registering the same component requirement more than once will panic, similarly to trying to add multiple component hooks of the same type to the same component. This avoids constructor conflicts and confusing ordering issues. ### Implementation Runtime requirements have two additional challenges in comparison to the `require` attribute. 1. The `require` attribute uses recursion and macros with clever ordering to populate hash maps of required components for each component type. The expected semantics are that "more specific" requirements override ones deeper in the inheritance tree. However, at runtime, there is no representation of how "specific" each requirement is. 2. If you first register the requirement `X -> Y`, and later register `Y -> Z`, then `X` should also indirectly require `Z`. However, `Y` itself doesn't know that it is required by `X`, so it's not aware that it should update the list of required components for `X`. My solutions to these problems are: 1. Store the depth in the inheritance tree for each entry of a given component's `RequiredComponents`. This is used to determine how "specific" each requirement is. For `require`-based registration, these depths are computed as part of the recursion. 2. Store and maintain a `required_by` list in each component's `ComponentInfo`, next to `required_components`. For `require`-based registration, these are also added after each registration, as part of the recursion. When calling `register_required_components`, it works as follows: 1. Get the required components of `Foo`, and check that `Bar` isn't already a *direct* requirement. 3. Register `Bar` as a required component for `Foo`, and add `Foo` to the `required_by` list for `Bar`. 4. Find and register all indirect requirements inherited from `Bar`, adding `Foo` to the `required_by` list for each component. 5. Iterate through components that require `Foo`, registering the new inherited requires for them as indirect requirements. The runtime registration is likely slightly more expensive than the `require` version, but it is a one-time cost, and quite negligible in practice, unless projects have hundreds or thousands of runtime requirements. I have not benchmarked this however. This does also add a small amount of extra cost to the `require` attribute for updating `required_by` lists, but I expect it to be very minor. ## Testing I added some tests that are copies of the `require` versions, as well as some tests that are more specific to the runtime implementation. I might add a few more tests though. ## Discussion - Is `register_required_components` a good name? Originally I went for `register_component_requirement` to be consistent with `register_component_hooks`, but the general feature is often referred to as "required components", which is why I changed it to `register_required_components`. - Should we *not* panic for duplicate requirements? If so, should they just be ignored, or should the latest registration overwrite earlier ones? - If we do want to panic for duplicate, conflicting registrations, should we at least not panic if the registrations are *exactly* the same, i.e. same component and same constructor? The current implementation panics for all duplicate direct registrations regardless of the constructor. ## Next Steps - Allow `register_required_components` to take a `Bundle` instead of a single required component. - I could also try to do it in this PR if that would be preferable. - Not directly related, but archetype invariants? |
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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 early stages of development. Important features are missing. Documentation is sparse. A new version of Bevy containing breaking changes to the API is released approximately once every 3 months. We provide migration guides, but we can't guarantee migrations will always be easy. Use only if you are willing to work in this environment.
MSRV: Bevy relies heavily on improvements in the Rust language and compiler. As a result, the Minimum Supported Rust Version (MSRV) is generally close to "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
- Quick Start Guide: Bevy's official Quick Start Guide. 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.
Contributing
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 Quick Start Guide for a brief introduction.
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.
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.
Thanks
Bevy is the result of the hard work of many people. A huge thanks to all Bevy contributors, the many open source projects that have come before us, the Rust gamedev ecosystem, and the many libraries we build on.
A huge thanks to Bevy's generous sponsors. Bevy will always be free and open source, but it isn't free to make. Please consider sponsoring our work if you like what we're building.
This project is tested with BrowserStack.
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.