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# Objective ### TL;DR #14098 added the `FunctionRegistry` but had some last minute complications due to anonymous functions. It ended up going with a "required name" approach to ensure anonymous functions would always have a name. However, this approach isn't ideal for named functions since, by definition, they will always have a name. Therefore, this PR aims to modify function reflection such that we can make function registration easier for named functions, while still allowing anonymous functions to be registered as well. ### Context Function registration (#14098) ran into a little problem: anonymous functions. Anonymous functions, including function pointers, have very non-unique type names. For example, the anonymous function `|a: i32, b: i32| a + b` has the type name of `fn(i32, i32) -> i32`. This obviously means we'd conflict with another function like `|a: i32, b: i32| a - b`. The solution that #14098 landed on was to always require a name during function registration. The downside with this is that named functions (e.g. `fn add(a: i32, b: i32) -> i32 { a + b }`) had to redundantly provide a name. Additionally, manually constructed `DynamicFunction`s also ran into this ergonomics issue. I don't entirely know how the function registry will be used, but I have a strong suspicion that most of its registrations will either be named functions or manually constructed `DynamicFunction`s, with anonymous functions only being used here and there for quick prototyping or adding small functionality. Why then should the API prioritize the anonymous function use case by always requiring a name during registration? #### Telling Functions Apart Rust doesn't provide a lot of out-of-the-box tools for reflecting functions. One of the biggest hurdles in attempting to solve the problem outlined above would be to somehow tell the different kinds of functions apart. Let's briefly recap on the categories of functions in Rust: | Category | Example | | ------------------ | ----------------------------------------- | | Named function | `fn add(a: i32, b: i32) -> i32 { a + b }` | | Closure | `\|a: i32\| a + captured_variable` | | Anonymous function | `\|a: i32, b: i32\| a + b` | | Function pointer | `fn(i32, i32) -> i32` | My first thought was to try and differentiate these categories based on their size. However, we can see that this doesn't quite work: | Category | `size_of` | | ------------------ | --------- | | Named function | 0 | | Closure | 0+ | | Anonymous function | 0 | | Function pointer | 8 | Not only does this not tell anonymous functions from named ones, but it struggles with pretty much all of them. My second then was to differentiate based on type name: | Category | `type_name` | | ------------------ | ----------------------- | | Named function | `foo::bar::baz` | | Closure | `foo::bar::{{closure}}` | | Anonymous function | `fn() -> String` | | Function pointer | `fn() -> String` | This is much better. While it can't distinguish between function pointers and anonymous functions, this doesn't matter too much since we only care about whether we can _name_ the function. So why didn't we implement this in #14098? #### Relying on `type_name` While this solution was known about while working on #14098, it was left out from that PR due to it being potentially controversial. The [docs](https://doc.rust-lang.org/stable/std/any/fn.type_name.html) for `std::any::type_name` state: > The returned string must not be considered to be a unique identifier of a type as multiple types may map to the same type name. Similarly, there is no guarantee that all parts of a type will appear in the returned string: for example, lifetime specifiers are currently not included. In addition, the output may change between versions of the compiler. So that's it then? We can't use `type_name`? Well, this statement isn't so much a rule as it is a guideline. And Bevy is no stranger to bending the rules to make things work or to improve ergonomics. Remember that before `TypePath`, Bevy's scene system was entirely dependent on `type_name`. Not to mention that `type_name` is being used as a key into both the `TypeRegistry` and the `FunctionRegistry`. Bevy's practices aside, can we reliably use `type_name` for this? My answer would be "yes". Anonymous functions are anonymous. They have no name. There's nothing Rust could do to give them a name apart from generating a random string of characters. But remember that this is a diagnostic tool, it doesn't make sense to obfuscate the type by randomizing the output. So changing it to be anything other than what it is now is very unlikely. The only changes that I could potentially see happening are: 1. Closures replace `{{closure}}` with the name of their variable 2. Lifetimes are included in the output I don't think the first is likely to happen, but if it does then it actually works out in our favor: closures are now named! The second point is probably the likeliest. However, adding lifetimes doesn't mean we can't still rely on `type_name` to determine whether or not a function is named. So we should be okay in this case as well. ## Solution Parse the `type_name` of the function in the `TypedFunction` impl to determine if the function is named or anonymous. This once again makes `FunctionInfo::name` optional. For manual constructions of `DynamicFunction`, `FunctionInfo::named` or ``FunctionInfo::anonymous` can be used. The `FunctionRegistry` API has also been reworked to account for this change. `FunctionRegistry::register` no longer takes a name and instead takes it from the supplied function, returning a `FunctionRegistrationError::MissingName` error if the name is `None`. This also doubles as a replacement for the old `FunctionRegistry::register_dynamic` method, which has been removed. To handle anonymous functions, a `FunctionRegistry::register_with_name` method has been added. This works in the same way `FunctionRegistry::register` used to work before this PR. The overwriting methods have been updated in a similar manner, with modifications to `FunctionRegistry::overwrite_registration`, the removal of `FunctionRegistry::overwrite_registration_dynamic`, and the addition of `FunctionRegistry::overwrite_registration_with_name`. This PR also updates the methods on `App` in a similar way: `App::register_function` no longer requires a name argument and `App::register_function_with_name` has been added to handle anonymous functions (and eventually closures). ## Testing You can run the tests locally by running: ``` cargo test --package bevy_reflect --features functions ``` --- ## Internal Migration Guide > [!important] > Function reflection was introduced as part of the 0.15 dev cycle. This migration guide was written for developers relying on `main` during this cycle, and is not a breaking change coming from 0.14. > [!note] > This list is not exhaustive. It only contains some of the most important changes. `FunctionRegistry::register` no longer requires a name string for named functions. Anonymous functions, however, need to be registered using `FunctionRegistry::register_with_name`. ```rust // BEFORE registry .register(std::any::type_name_of_val(&foo), foo)? .register("bar", || println!("Hello world!")); // AFTER registry .register(foo)? .register_with_name("bar", || println!("Hello world!")); ``` `FunctionInfo::name` is now optional. Anonymous functions and closures will now have their name set to `None` by default. Additionally, `FunctionInfo::new` has been renamed to `FunctionInfo::named`. |
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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.