![]() ## Objective A major critique of Bevy at the moment is how boilerplatey it is to compose (and read) entity hierarchies: ```rust commands .spawn(Foo) .with_children(|p| { p.spawn(Bar).with_children(|p| { p.spawn(Baz); }); p.spawn(Bar).with_children(|p| { p.spawn(Baz); }); }); ``` There is also currently no good way to statically define and return an entity hierarchy from a function. Instead, people often do this "internally" with a Commands function that returns nothing, making it impossible to spawn the hierarchy in other cases (direct World spawns, ChildSpawner, etc). Additionally, because this style of API results in creating the hierarchy bits _after_ the initial spawn of a bundle, it causes ECS archetype changes (and often expensive table moves). Because children are initialized after the fact, we also can't count them to pre-allocate space. This means each time a child inserts itself, it has a high chance of overflowing the currently allocated capacity in the `RelationshipTarget` collection, causing literal worst-case reallocations. We can do better! ## Solution The Bundle trait has been extended to support an optional `BundleEffect`. This is applied directly to World immediately _after_ the Bundle has fully inserted. Note that this is [intentionally](https://github.com/bevyengine/bevy/discussions/16920) _not done via a deferred Command_, which would require repeatedly copying each remaining subtree of the hierarchy to a new command as we walk down the tree (_not_ good performance). This allows us to implement the new `SpawnRelated` trait for all `RelationshipTarget` impls, which looks like this in practice: ```rust world.spawn(( Foo, Children::spawn(( Spawn(( Bar, Children::spawn(Spawn(Baz)), )), Spawn(( Bar, Children::spawn(Spawn(Baz)), )), )) )) ``` `Children::spawn` returns `SpawnRelatedBundle<Children, L: SpawnableList>`, which is a `Bundle` that inserts `Children` (preallocated to the size of the `SpawnableList::size_hint()`). `Spawn<B: Bundle>(pub B)` implements `SpawnableList` with a size of 1. `SpawnableList` is also implemented for tuples of `SpawnableList` (same general pattern as the Bundle impl). There are currently three built-in `SpawnableList` implementations: ```rust world.spawn(( Foo, Children::spawn(( Spawn(Name::new("Child1")), SpawnIter(["Child2", "Child3"].into_iter().map(Name::new), SpawnWith(|parent: &mut ChildSpawner| { parent.spawn(Name::new("Child4")); parent.spawn(Name::new("Child5")); }) )), )) ``` We get the benefits of "structured init", but we have nice flexibility where it is required! Some readers' first instinct might be to try to remove the need for the `Spawn` wrapper. This is impossible in the Rust type system, as a tuple of "child Bundles to be spawned" and a "tuple of Components to be added via a single Bundle" is ambiguous in the Rust type system. There are two ways to resolve that ambiguity: 1. By adding support for variadics to the Rust type system (removing the need for nested bundles). This is out of scope for this PR :) 2. Using wrapper types to resolve the ambiguity (this is what I did in this PR). For the single-entity spawn cases, `Children::spawn_one` does also exist, which removes the need for the wrapper: ```rust world.spawn(( Foo, Children::spawn_one(Bar), )) ``` ## This works for all Relationships This API isn't just for `Children` / `ChildOf` relationships. It works for any relationship type, and they can be mixed and matched! ```rust world.spawn(( Foo, Observers::spawn(( Spawn(Observer::new(|trigger: Trigger<FuseLit>| {})), Spawn(Observer::new(|trigger: Trigger<Exploded>| {})), )), OwnerOf::spawn(Spawn(Bar)) Children::spawn(Spawn(Baz)) )) ``` ## Macros While `Spawn` is necessary to satisfy the type system, we _can_ remove the need to express it via macros. The example above can be expressed more succinctly using the new `children![X]` macro, which internally produces `Children::spawn(Spawn(X))`: ```rust world.spawn(( Foo, children![ ( Bar, children![Baz], ), ( Bar, children![Baz], ), ] )) ``` There is also a `related!` macro, which is a generic version of the `children!` macro that supports any relationship type: ```rust world.spawn(( Foo, related!(Children[ ( Bar, related!(Children[Baz]), ), ( Bar, related!(Children[Baz]), ), ]) )) ``` ## Returning Hierarchies from Functions Thanks to these changes, the following pattern is now possible: ```rust fn button(text: &str, color: Color) -> impl Bundle { ( Node { width: Val::Px(300.), height: Val::Px(100.), ..default() }, BackgroundColor(color), children![ Text::new(text), ] ) } fn ui() -> impl Bundle { ( Node { width: Val::Percent(100.0), height: Val::Percent(100.0), ..default(), }, children![ button("hello", BLUE), button("world", RED), ] ) } // spawn from a system fn system(mut commands: Commands) { commands.spawn(ui()); } // spawn directly on World world.spawn(ui()); ``` ## Additional Changes and Notes * `Bundle::from_components` has been split out into `BundleFromComponents::from_components`, enabling us to implement `Bundle` for types that cannot be "taken" from the ECS (such as the new `SpawnRelatedBundle`). * The `NoBundleEffect` trait (which implements `BundleEffect`) is implemented for empty tuples (and tuples of empty tuples), which allows us to constrain APIs to only accept bundles that do not have effects. This is critical because the current batch spawn APIs cannot efficiently apply BundleEffects in their current form (as doing so in-place could invalidate the cached raw pointers). We could consider allocating a buffer of the effects to be applied later, but that does have performance implications that could offset the balance and value of the batched APIs (and would likely require some refactors to the underlying code). I've decided to be conservative here. We can consider relaxing that requirement on those APIs later, but that should be done in a followup imo. * I've ported a few examples to illustrate real-world usage. I think in a followup we should port all examples to the `children!` form whenever possible (and for cases that require things like SpawnIter, use the raw APIs). * Some may ask "why not use the `Relationship` to spawn (ex: `ChildOf::spawn(Foo)`) instead of the `RelationshipTarget` (ex: `Children::spawn(Spawn(Foo))`)?". That _would_ allow us to remove the `Spawn` wrapper. I've explicitly chosen to disallow this pattern. `Bundle::Effect` has the ability to create _significant_ weirdness. Things in `Bundle` position look like components. For example `world.spawn((Foo, ChildOf::spawn(Bar)))` _looks and reads_ like Foo is a child of Bar. `ChildOf` is in Foo's "component position" but it is not a component on Foo. This is a huge problem. Now that `Bundle::Effect` exists, we should be _very_ principled about keeping the "weird and unintuitive behavior" to a minimum. Things that read like components _should be the components they appear to be". ## Remaining Work * The macros are currently trivially implemented using macro_rules and are currently limited to the max tuple length. They will require a proc_macro implementation to work around the tuple length limit. ## Next Steps * Port the remaining examples to use `children!` where possible and raw `Spawn` / `SpawnIter` / `SpawnWith` where the flexibility of the raw API is required. ## Migration Guide Existing spawn patterns will continue to work as expected. Manual Bundle implementations now require a `BundleEffect` associated type. Exisiting bundles would have no bundle effect, so use `()`. Additionally `Bundle::from_components` has been moved to the new `BundleFromComponents` trait. ```rust // Before unsafe impl Bundle for X { unsafe fn from_components<T, F>(ctx: &mut T, func: &mut F) -> Self { } /* remaining bundle impl here */ } // After unsafe impl Bundle for X { type Effect = (); /* remaining bundle impl here */ } unsafe impl BundleFromComponents for X { unsafe fn from_components<T, F>(ctx: &mut T, func: &mut F) -> Self { } } ``` --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: Gino Valente <49806985+MrGVSV@users.noreply.github.com> Co-authored-by: Emerson Coskey <emerson@coskey.dev> |
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benches | ||
crates | ||
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docs-template | ||
errors | ||
examples | ||
src | ||
tests | ||
tests-integration | ||
tools | ||
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Cargo.toml | ||
clippy.toml | ||
CODE_OF_CONDUCT.md | ||
CONTRIBUTING.md | ||
CREDITS.md | ||
deny.toml | ||
LICENSE-APACHE | ||
LICENSE-MIT | ||
README.md | ||
rustfmt.toml | ||
typos.toml |
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.