55dddaf72e
# Objective - Fixes #8074 - Adopts / Supersedes #8104 ## Solution Adapted from #8104 and affords the same benefits. **Additions** - [x] Update scrolling on relayout (height of node or contents may have changed) - [x] Make ScrollPosition component optional for ui nodes to avoid checking every node on scroll - [x] Nested scrollviews **Omissions** - Removed input handling for scrolling from `bevy_ui`. Users should update `ScrollPosition` directly. ### Implementation Adds a new `ScrollPosition` component. Updating this component on a `Node` with an overflow axis set to `OverflowAxis::Scroll` will reposition its children by that amount when calculating node transforms. As before, no impact on the underlying Taffy layout. Calculating this correctly is trickier than it was in #8104 due to `"Update scrolling on relayout"`. **Background** When `ScrollPosition` is updated directly by the user, it can be trivially handled in-engine by adding the parent's scroll position to the final location of each child node. However, _other layout actions_ may result in a situation where `ScrollPosition` needs to be updated. Consider a 1000 pixel tall vertically scrolling list of 100 elements, each 100 pixels tall. Scrolled to the bottom, the `ScrollPosition.offset_y` is 9000, just enough to display the last element in the list. When removing an element from that list, the new desired `ScrollPosition.offset_y` is 8900, but, critically, that is not known until after the sizes and positions of the children of the scrollable node are resolved. All user scrolling code today handles this by delaying the resolution by one frame. One notable disadvantage of this is the inability to support `WinitSettings::desktop_app()`, since there would need to be an input AFTER the layout change that caused the scroll position to update for the results of the scroll position update to render visually. I propose the alternative in this PR, which allows for same-frame resolution of scrolling layout. **Resolution** _Edit: Below resolution is outdated, and replaced with the simpler usage of taffy's `Layout::content_size`._ When recursively iterating the children of a node, each child now returns a `Vec2` representing the location of their own bottom right corner. Then, `[[0,0, [x,y]]` represents a bounding box containing the scrollable area filled by that child. Scrollable parents aggregate those areas into the bounding box of _all_ children, then consider that result against `ScrollPosition` to ensure its validity. In the event that resolution of the layout of the children invalidates the `ScrollPosition` (e.g. scrolled further than there were children to scroll to), _all_ children of that node must be recursively repositioned. The position of each child must change as a result of the change in scroll position. Therefore, this implementation takes care to only spend the cost of the "second layout pass" when a specific node actually had a `ScrollPosition` forcibly updated by the layout of its children. ## Testing Examples in `ui/scroll.rs`. There may be more complex node/style interactions that were unconsidered. --- ## Showcase  ## Alternatives - `bevy_ui` doesn't support scrolling. - `bevy_ui` implements scrolling with a one-frame delay on reactions to layout changes. |
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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.