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# Objective Rework / build on #17043 to simplify the implementation. #17043 should be merged first, and the diff from this PR will get much nicer after it is merged (this PR is net negative LOC). ## Solution 1. Command and EntityCommand have been vastly simplified. No more marker components. Just one function. 2. Command and EntityCommand are now generic on the return type. This enables result-less commands to exist, and allows us to statically distinguish between fallible and infallible commands, which allows us to skip the "error handling overhead" for cases that don't need it. 3. There are now only two command queue variants: `queue` and `queue_fallible`. `queue` accepts commands with no return type. `queue_fallible` accepts commands that return a Result (specifically, one that returns an error that can convert to `bevy_ecs::result::Error`). 4. I've added the concept of the "default error handler", which is used by `queue_fallible`. This is a simple direct call to the `panic()` error handler by default. Users that want to override this can enable the `configurable_error_handler` cargo feature, then initialize the GLOBAL_ERROR_HANDLER value on startup. This is behind a flag because there might be minor overhead with `OnceLock` and I'm guessing this will be a niche feature. We can also do perf testing with OnceLock if someone really wants it to be used unconditionally, but I don't personally feel the need to do that. 5. I removed the "temporary error handler" on Commands (and all code associated with it). It added more branching, made Commands bigger / more expensive to initialize (note that we construct it at high frequencies / treat it like a pointer type), made the code harder to follow, and introduced a bunch of additional functions. We instead rely on the new default error handler used in `queue_fallible` for most things. In the event that a custom handler is required, `handle_error_with` can be used. 6. EntityCommand now _only_ supports functions that take `EntityWorldMut` (and all existing entity commands have been ported). Removing the marker component from EntityCommand hinged on this change, but I strongly believe this is for the best anyway, as this sets the stage for more efficient batched entity commands. 7. I added `EntityWorldMut::resource` and the other variants for more ergonomic resource access on `EntityWorldMut` (removes the need for entity.world_scope, which also incurs entity-lookup overhead). ## Open Questions 1. I believe we could merge `queue` and `queue_fallible` into a single `queue` which accepts both fallible and infallible commands (via the introduction of a `QueueCommand` trait). Is this desirable? |
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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.