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# Objective As discussed in #14275, Bevy is currently too prone to panic, and makes the easy / beginner-friendly way to do a large number of operations just to panic on failure. This is seriously frustrating in library code, but also slows down development, as many of the `Query::single` panics can actually safely be an early return (these panics are often due to a small ordering issue or a change in game state. More critically, in most "finished" products, panics are unacceptable: any unexpected failures should be handled elsewhere. That's where the new With the advent of good system error handling, we can now remove this. Note: I was instrumental in a) introducing this idea in the first place and b) pushing to make the panicking variant the default. The introduction of both `let else` statements in Rust and the fancy system error handling work in 0.16 have changed my mind on the right balance here. ## Solution 1. Make `Query::single` and `Query::single_mut` (and other random related methods) return a `Result`. 2. Handle all of Bevy's internal usage of these APIs. 3. Deprecate `Query::get_single` and friends, since we've moved their functionality to the nice names. 4. Add detailed advice on how to best handle these errors. Generally I like the diff here, although `get_single().unwrap()` in tests is a bit of a downgrade. ## Testing I've done a global search for `.single` to track down any missed deprecated usages. As to whether or not all the migrations were successful, that's what CI is for :) ## Future work ~~Rename `Query::get_single` and friends to `Query::single`!~~ ~~I've opted not to do this in this PR, and smear it across two releases in order to ease the migration. Successive deprecations are much easier to manage than the semantics and types shifting under your feet.~~ Cart has convinced me to change my mind on this; see https://github.com/bevyengine/bevy/pull/18082#discussion_r1974536085. ## Migration guide `Query::single`, `Query::single_mut` and their `QueryState` equivalents now return a `Result`. Generally, you'll want to: 1. Use Bevy 0.16's system error handling to return a `Result` using the `?` operator. 2. Use a `let else Ok(data)` block to early return if it's an expected failure. 3. Use `unwrap()` or `Ok` destructuring inside of tests. The old `Query::get_single` (etc) methods which did this have been deprecated. |
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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.