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# Objective Current `FixedTime` and `Time` have several problems. This pull aims to fix many of them at once. - If there is a longer pause between app updates, time will jump forward a lot at once and fixed time will iterate on `FixedUpdate` for a large number of steps. If the pause is merely seconds, then this will just mean jerkiness and possible unexpected behaviour in gameplay. If the pause is hours/days as with OS suspend, the game will appear to freeze until it has caught up with real time. - If calculating a fixed step takes longer than specified fixed step period, the game will enter a death spiral where rendering each frame takes longer and longer due to more and more fixed step updates being run per frame and the game appears to freeze. - There is no way to see current fixed step elapsed time inside fixed steps. In order to track this, the game designer needs to add a custom system inside `FixedUpdate` that calculates elapsed or step count in a resource. - Access to delta time inside fixed step is `FixedStep::period` rather than `Time::delta`. This, coupled with the issue that `Time::elapsed` isn't available at all for fixed steps, makes it that time requiring systems are either implemented to be run in `FixedUpdate` or `Update`, but rarely work in both. - Fixes #8800 - Fixes #8543 - Fixes #7439 - Fixes #5692 ## Solution - Create a generic `Time<T>` clock that has no processing logic but which can be instantiated for multiple usages. This is also exposed for users to add custom clocks. - Create three standard clocks, `Time<Real>`, `Time<Virtual>` and `Time<Fixed>`, all of which contain their individual logic. - Create one "default" clock, which is just `Time` (or `Time<()>`), which will be overwritten from `Time<Virtual>` on each update, and `Time<Fixed>` inside `FixedUpdate` schedule. This way systems that do not care specifically which time they track can work both in `Update` and `FixedUpdate` without changes and the behaviour is intuitive. - Add `max_delta` to virtual time update, which limits how much can be added to virtual time by a single update. This fixes both the behaviour after a long freeze, and also the death spiral by limiting how many fixed timestep iterations there can be per update. Possible future work could be adding `max_accumulator` to add a sort of "leaky bucket" time processing to possibly smooth out jumps in time while keeping frame rate stable. - Many minor tweaks and clarifications to the time functions and their documentation. ## Changelog - `Time::raw_delta()`, `Time::raw_elapsed()` and related methods are moved to `Time<Real>::delta()` and `Time<Real>::elapsed()` and now match `Time` API - `FixedTime` is now `Time<Fixed>` and matches `Time` API. - `Time<Fixed>` default timestep is now 64 Hz, or 15625 microseconds. - `Time` inside `FixedUpdate` now reflects fixed timestep time, making systems portable between `Update ` and `FixedUpdate`. - `Time::pause()`, `Time::set_relative_speed()` and related methods must now be called as `Time<Virtual>::pause()` etc. - There is a new `max_delta` setting in `Time<Virtual>` that limits how much the clock can jump by a single update. The default value is 0.25 seconds. - Removed `on_fixed_timer()` condition as `on_timer()` does the right thing inside `FixedUpdate` now. ## Migration Guide - Change all `Res<Time>` instances that access `raw_delta()`, `raw_elapsed()` and related methods to `Res<Time<Real>>` and `delta()`, `elapsed()`, etc. - Change access to `period` from `Res<FixedTime>` to `Res<Time<Fixed>>` and use `delta()`. - The default timestep has been changed from 60 Hz to 64 Hz. If you wish to restore the old behaviour, use `app.insert_resource(Time::<Fixed>::from_hz(60.0))`. - Change `app.insert_resource(FixedTime::new(duration))` to `app.insert_resource(Time::<Fixed>::from_duration(duration))` - Change `app.insert_resource(FixedTime::new_from_secs(secs))` to `app.insert_resource(Time::<Fixed>::from_seconds(secs))` - Change `system.on_fixed_timer(duration)` to `system.on_timer(duration)`. Timers in systems placed in `FixedUpdate` schedule automatically use the fixed time clock. - Change `ResMut<Time>` calls to `pause()`, `is_paused()`, `set_relative_speed()` and related methods to `ResMut<Time<Virtual>>` calls. The API is the same, with the exception that `relative_speed()` will return the actual last ste relative speed, while `effective_relative_speed()` returns 0.0 if the time is paused and corresponds to the speed that was set when the update for the current frame started. ## Todo - [x] Update pull name and description - [x] Top level documentation on usage - [x] Fix examples - [x] Decide on default `max_delta` value - [x] Decide naming of the three clocks: is `Real`, `Virtual`, `Fixed` good? - [x] Decide if the three clock inner structures should be in prelude - [x] Decide on best way to configure values at startup: is manually inserting a new clock instance okay, or should there be config struct separately? - [x] Fix links in docs - [x] Decide what should be public and what not - [x] Decide how `wrap_period` should be handled when it is changed - [x] ~~Add toggles to disable setting the clock as default?~~ No, separate pull if needed. - [x] Add tests - [x] Reformat, ensure adheres to conventions etc. - [x] Build documentation and see that it looks correct ## Contributors Huge thanks to @alice-i-cecile and @maniwani while building this pull. It was a shared effort! --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: Cameron <51241057+maniwani@users.noreply.github.com> Co-authored-by: Jerome Humbert <djeedai@gmail.com> |
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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
- 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 based on the WebGPU API.
- 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.
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.