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Contributing to Ruffle

🎉 Thanks for your interest in Ruffle! Contributions of all kinds are welcome.

This document serves as a general guide for contributing to Ruffle. Follow your best judgement in following these guidelines.

Table of Contents

Getting Started

The Ruffle wiki is a great way to familiarize yourself with the project. It contains info on how to build Ruffle, using Ruffle, and links to helpful documentation about the Flash format.

Feel free to ask questions in our Discord server.

Ways to Contribute

We love new contributors! You can contribute to Ruffle in several ways:

Test your favorite Flash content

Try out your favorite SWF content in Ruffle and see how it works! Follow the instructions on the Using Ruffle page to get Ruffle for your desired platform. You can run the SWF through the desktop player, the web demo, or try the extension on live websites.

If you encounter specific issues with content, please follow the guidelines on filing an issue.

Improve documentation

Improving documentation is a great way to learn the codebase. Adding documentation to both the wiki and the code eases the learning curve for both end users and new contributors.

For documentation in the code, we follow the rustdoc guidelines.

Fix interesting issues

Try your hand at fixing issues that are interesting to you. Follow the instructions on building Ruffle, familiarize yourself with the project layout, and use SWF resources and decompilers to help debug the issue.

You can also ask for mentoring on our Discord server.

Implement missing Flash functionality

Ruffle is a young project, and there is still much Flash functionality that is unimplemented. Check for the "unimplemented" in issues.

Debugging ActionScript Content

To enable debug logging, set RUST_LOG=warn,ruffle_core=debug,avm_trace=trace and run Ruffle from the command line. This will also enable printing trace() statements.

Additionally, if you build Ruffle with --features avm_debug then you will activate a few more built-in debugging utilities inside Ruffle, listed below.

Warnings and Errors

All AVM errors and warnings will print their stack trace so that you can view where they are in relation to the ActionScript inside the movie.

Step-By-Step Output

The hotkey Ctrl+Alt+D toggles verbose AVM debugging output on and off (default off). You will be able to follow the flow of ActionScript inside of a SWF movie, as each action is performed. Please note that this will likely slow down Ruffle, and it may significantly spam output. Please use sparingly.

When paired with a tool such as JPEXS, you can compare the ActionScript you see being executed in Ruffle with the actual ActionScript inside of a SWF movie, and attempt to find whatever problem it is that you're looking for.

Complete Variable Dumping

The hotkey Ctrl+Alt+V dumps every variable inside the AVM at the moment you press it. This can be very useful to inspect the internal state of games and see, for example, if a coordinate is NaN, your lives are negative, or maybe an important object just didn't get initialized.

Reporting Bugs

Issue reports and feature requests are encouraged, and are a great way to measure our progress!

When filing an issue, if possible, please include:

  • A clear description of the problem.
  • The platform you are testing on (web, desktop, OS).
  • A link/attachment to the SWF demonstrating the issue, if possible.
  • Screenshots if the issue is a visible problem.
  • Bonus points for including the correct output from the official Flash Player.

These types of focused issues are helpful:

  • Tracking issues for specific Flash features (ActionScript 3.0, drawing API, etc.)
  • Bug reports for specific content that works but isn't quite right (art not looking correct, etc.)
  • Platform-specific issues
  • Enhancement requests to improve user experience

The project is still in the early stages, so many Flash features are unimplemented and not yet expected to work. Please avoid filing generic issues such as:

  • A "this SWF doesn't work at all" report (what about it doesn't work?).
  • Duplicate issues for each piece of content using an unimplemented feature.
  • Asking for dates when a feature will be implemented.

Code Guidelines

Ruffle is built using the latest stable version of the Rust compiler. Nightly and unstable features should be avoided.

The Rust code in Ruffle strives to be idiomatic. The Rust compiler should emit no warnings when building the project. Additionally, all code should be formatted using rustfmt and linted using clippy. You can install these tools using rustup:

rustup component add rustfmt
rustup component add clippy

You can auto-format your changes with rustfmt:

cargo fmt --all

and you can run the clippy lints:

cargo clippy --all --tests

Specific warnings and clippy lints can be allowed when appropriate using attributes, such as:


Test Guidelines

Heavily algorithmic code may benefit from unit tests in Rust: create a module mod tests conditionally compiled with #[cfg(test)], and add your tests in there.

Most tests are SWF-based, with the SWFs stored in tests/tests/swfs/. They are configured in tests/tests/ To add a new test, create an .swf that runs trace() statements. You can do this in several ways, listed below.

Once you have an .swf, run it in Flash Player and create a file output.txt with the contents of the trace statements. Add the output.txt, test.swf and either the or test.fla file to a directory under tests/tests/swfs/avm1 (or avm2) named after what your test tests, and add a line in to have Ruffle run it.

Running cargo test [your test] will run the .swf in Ruffle and compare the trace() output against output.txt.

Flash authoring tool

Create a new ActionScript project. Save the .fla file and export an .swf (File -> Export -> Export Movie...).

Adobe Flash Professional CS6 is the most recent version to support both ActionScript 2 and 3. Newer versions support ActionScript 3 only.

Motion-Twin ActionScript 2 Compiler

This is a free and open source command-line ActionScript 2 compiler. It can be downloaded from here.

Create a file in a text editor, per the following template:

class Test {
    static function main() {
        // Your test here.
        trace("Hello World!");

Then compile it using:

mtasc -main -header 200:150:30 -swf test.swf

Apache Flex SDK

This is a free and open source SDK capable of compiling ActionScript 3 code. It can be set up as follows:

  1. Download a release for your platform from here, and extract the files somewhere.
  2. Add the <sdk-root>/bin directory to your PATH. After that, the command-line compiler mxmlc (among other tools) should be available.
  3. mxmlc needs a playerglobal.swc in order to work, which can be grabbed from here. Place it in <sdk-root>/frameworks/libs/player/32.0/playerglobal.swc, while creating intermediate player and 32.0 directories.
  4. Define the FLEX_HOME and PLAYERGLOBAL_HOME environment variables to the path of the extracted SDK root, and the path of the <sdk-root>/frameworks/libs/player subdirectory, respectively.
  5. Edit <sdk-root>/frameworks/flex-config.xml and change <target-player>27.0</target-player> to <target-player>32.0</target-player>.

After mxmlc is set up, create a file in a text editor, per the following template:

package {
    public class Test {}

// Your test here.
trace("Hello World!");

Then compile it using:

mxmlc -output test.swf -compiler.debug=true

You may want to use Docker instead - something like docker run -it --rm -v ${PWD}:/src jeko/airbuild mxmlc -output test.swf -compiler.debug=true works well.


RABCDAsm allows writing AVM2 bytecode sequences directly, without intermediate AS3 code, which is primarily useful for testing opcodes that aren't generated by the above-mentioned AS3 compilers. However it cannot generate SWF files from scratch. Instead, you must first generate a SWF from the above mentioned methods, then extract and disassemble its ABC with abcexport and rabcdasm. Once you have modified your bytecode, you must reassemble and inject it into the movie with rabcasm and abcreplace. If you are adding a new test, commit both your SWF source (.fla and/or .as files) as well as the modified bytecode (.abc files and test-0 folder).

Commit Message Guidelines

Here is a sample commit message:

web: Fix incorrect rendering of gradients (close #23)
  • If applicable, prefix the first line with a tag indicating the relevant area of changes:
    • core:
    • desktop:
    • web:
    • avm1:
    • avm2:
    • docs:
    • chore:
    • tests:
  • Capitalize the first letter following the tag.
  • Limit line length to 72 characters.
  • Use the present tense and imperative mood ("fix", not "fixed" nor "fixes").
  • Reference any PRs or issues in the first line.
  • Use keywords to close/address issues when applicable ("close #23").
  • Write more detailed info on following lines when applicable.

Pull Requests

Pull requests are the primary way to contribute code to Ruffle. Pull requests should be made against the latest master branch. Your pull request should not contain merges; you should always rebase when bringing the latest changes into your branch from the master branch. If there are merge conflicts, or if your commit history is messy, please rebase onto the latest master. git rebase -i is a great way to clean up your pull request.

When you make a pull request, our CI will build your changes and run them through all tests and style checks. All of these tests should pass before your pull request can be accepted.

One of our regular contributors will review your changes and try their best to helpfully suggest any changes. If all goes well, your PR should be merged without much delay. We use both standard merge commits and fast-forward merges depending on the size of the changes. Thanks for your contribution!