High-level memory-safe binding generator for Flutter/Dart <-> Rust #

Want to combine the best between Flutter, a cross-platform hot-reload rapid-development UI toolkit, and Rust, a language empowering everyone to build reliable and efficient software? Here it comes!

🚀 Advantages #

• Memory-safe: Never need to think about malloc/free.
• Type support: Unlike low-level binding generator which only provide primitives and pointers, this package provides things like Vec<u8>(Uint8List), Vec<T>(List<T>), any custom struct(class)s, and even recursive structs (e.g. a tree node).
• Zero-copy: Pass big array of bytes from Rust to Dart without any memory copies.
• Async programming: Simply call functions directly in main isolate (thread) of Dart/Flutter, and Rust code will not block the Flutter UI.
• Easy to use: All you need to do is write down your Rust code. The code generator will do everything and expose an API in Dart/Flutter's style.
• Lightweight: This is not a huge framework that includes everything, so you are free to use your favorite Flutter and Rust libraries. _{For example, state-management with Flutter library (e.g. MobX) can be elegant and simple (contrary to implementing in Rust); implementing a photo manipulation algorithm in Rust will be fast and safe (countrary to implementing in Flutter).}
• Easy to code-review & convince yourself: This package simply simulates how human beings write down boilerplate code. If you want to convince yourself (or your team) that it is safe, there is not much code for you to look at. The runtime is only hundreds of loc, and the generated code follows simple patterns. No magic at all! (More about safety concerns.)
• Pure-Dart compatible: Despite the name, this package is 100% compatible with pure Dart. It does not require anything specific to Flutter. See this pure-Dart example.

🧭 Show me the code #

What you write down (in Rust):

pub fn my_function(a: MyTreeNode, b: SomeOtherStruct) -> Result<Vec<u8>> {
    ... do my heavy computations ...
}

// you can use structs (even recursive)
pub struct TreeNode { pub value: i32, pub children: Vec<MyTreeNode> }

With bindings automatically generated, you can simply use the following API in Flutter/Dart. Nothing more.

Future<Uint8List> myFunction(MyTreeNode a, SomeOtherStruct b);

_{Remark: Why Future in Flutter: Flutter is single-threaded. If not using future, just like what you do with plain-old Flutter bindings, your UI will be stuck as long as your Rust code is executing. If your Rust code run for a second, your UI will fully freeze for one second.}

💡 Quickstart #

Install #

• Install dependency cbindgen: cargo install cbindgen
• Install dependency ffigen: dart pub global activate ffigen, and install LLVM.
• Install this code generator binary by cargo install flutter_rust_bridge_codegen.
• Add flutter_rust_bridge = "1.0" (where 1.0 should be the latest version) to Rust's Cargo.toml.
• Add flutter_rust_bridge: ^1.0 (same as above, should be latest version) to Flutter/Dart's pubspec.yaml under the section of dependencies.

Run #

flutter_rust_bridge_codegen --rust-input path/to/your/api.rs --dart-output path/to/file/being/bridge_generated.dart

(For more options, use --help; To see what types and function signatures can you write in Rust, have a look at this example.) (For Windows, you may need \\ instead of / for paths.)

Enjoy #

Use the class in the generated .dart file, as if it is a normal Flutter/Dart class! (The abstract class at the top of the generated file.)

_{Remark: If you are interested, why abstractclass can be used - it is because of the factory language feature.}

📚 Tutorial: A Flutter+Rust app #

Get example code #

Please install Flutter, install Rust, and have some familiarity with them. Then run git clone https://github.com/fzyzcjy/flutter_rust_bridge && cd frb_example/with_flutter to get my example.

Or you can use your own code (if you find this Quickstart section too brief, have a look at the later Usage section).

(Optional) Run code generator #

Remark: I have generated the source code already (in quickstart), so this step is optional. Even if you do it, you should not see anything changed.

Install it: cargo install flutter_rust_bridge_codegen.

Run it: flutter_rust_bridge_codegen --rust-input frb_example/with_flutter/rust/src/api.rs --dart-output frb_example/with_flutter/lib/bridge_generated.dart --c-output frb_example/with_flutter/ios/Runner/bridge_generated.h. (See CI workflow as a reference.) (For Windows, you may need \\ instead of / for paths.)

Run "Flutter+Rust" app #

If Android

Run cargo ndk -o ../android/app/src/main/jniLibs build. Then run the Flutter app normally as is taught in official tutorial. For example, flutter run.

Remark: Since my quickstart app is so baremetal, I do not integrate the Rust building process into Flutter building process. But you can look at this tutorial to easily do that.

If iOS

Modify Cargo.toml to change cdylib to staticlib. (Again, this is baremetal example so it is done manually. For your project, you can automate it.)

Run cargo lipo && cp target/universal/debug/libflutter_rust_bridge_example.a ../ios/Runner to build Rust and copy the static library. Then run the Flutter app normally as is taught in official tutorial. For example, flutter run. (Similarly, this tutorial can automate the process.)

(Optional) See more types that this library can generate #

Have a look at the function arguments and return types in this file: api.rs. With this library, we have a generated API that resides at generated_api.dart (of course, that is auto generated, and you can use it in other Dart code).

(Optional) Remarks #

The mod

If you are adding this lib to your own existing code, please put mod generated_wire; (where generated_wire is the name of the wire file that you choose) into your lib.rs or main.rs. Only by doing this, Rust can understand that this generated file is a part of your project.

Version

Dart SDK >=2.14.0 is needed not by this library, but by the latest version of the ffigen tool. Therefore, write sdk: ">=2.14.0 <3.0.0" in the environment section of pubspec.yaml. If you do not want that, consider installing a older version of the ffigen tool.

📪 Safety #

This library has CI that runs Valgrind automatically on the setup that a Dart program calls a Rust program using this package, so memory problems should be found by Valgrind. (Notice that, even when running a simple hello-world Dart program, Valgrind will report hundreds of errors. See this Dart lang issue for more details. Therefore, I both look at "definitely lost" in Valgrind, and manually search things related to this library - if all reported errors are unrelated to this library then we are safe.)

In addition, Flutter integration tests are also done in CI. This ensures a real Flutter application using this library does not suffer from problems.

Most of the code are written in safe Rust. The unsafe code mainly comes from support::box_from_leak_ptr and support::vec_from_leak_ptr. They are used for pointers and arrays, and I follow the high-upvoted answers and official doc when writing down that few lines of code.

I use this library heavily in my own Flutter project (yplusplus, or why++). That app is in production and it works quite well. If I observe any problems, I will fix it in this library.

The CI also runs the run_codegen workflow, which ensure that the code generator can compile and generate desired results. Lastly, the CI also runs formatters and linters (fmt, clippy, dart analyze, dart format), and linters can also catch some common problems.

Command line arguments #

Simply add --help to see full documentation.

flutter_rust_bridge_codegen 1.0.0

USAGE:
    flutter_rust_bridge_codegen [OPTIONS] --dart-output <dart-output> --rust-input <rust-input>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -r, --rust-input <rust-input>                              Path of input Rust code
    -d, --dart-output <dart-output>                            Path of output generated Dart code
    -c, --c-output <c-output>                                  Path of output generated C header
        --rust-crate-dir <rust-crate-dir>                      Crate directory for your Rust project
        --rust-output <rust-output>                            Path of output generated Rust code
        --class-name <class-name>                              Generated class name
        --dart-format-line-length <dart-format-line-length>    Line length for dart formatting

What this library is & isn't #

This library is nothing but a code generator that helps your Flutter/Dart functions call Rust functions. Therefore, you may refer to external materials to learn Flutter, learn Rust, learn Flutter FFI (Dart FFI) and so on. With material on the Internet, you will know how to create a mobile application using Flutter, and how that app can call Rust functions via Dart FFI (in the C ABI). Then this package comes in, and ease you from the burden to write down tons of boilerplate code ;)

Advanced #

Using your own executor #

DefaultExecutor: When Dart calls Rust, the DefaultExecutor use a simple thread pool to execute the real Rust functions. By doing this, Rust function that needs to run for a long time (more than a few frames) will never make the UI stuck.

However, you can implement your own Executor doing whatever you want. In order to do this, implement the Executor trait, and call set_executor to set your own executor.

Appendix: Set up Flutter/Dart+Rust support #

I suggest that you can start with the Flutter example first, and modify it to satisfy your needs. It can serve as a template for new projects. It is run against CI so we are sure it works.

Indeed, this library is nothing but a code generator that helps your Flutter/Dart functions call Rust functions. Therefore, "how to create a Flutter app that can run Rust code" is actually out of the scope of this library, and there are already several tutorials on the Internet.

However, I can sketch the outline of what to do if you want to set up a new Flutter+Rust project as follows.

Step 1: Create a new Flutter project (or use an existing one). The Dart SDK should be >=2.14.0 if you want to use the latest ffigen tool.

Step 2: Create a new Rust project, say, at directory rust under the Flutter project.

Step 3: Edit Cargo.toml and add:

[lib]
name = "flutter_rust_bridge_example" # whatever you like
crate-type = ["cdylib"] # <-- notice this type. `cdylib` for android, and `staticlib` for iOS. I write down a script to change it before build.

Step 4: Follow the standard steps of "how iOS uses static libraries". For example, in XCode, edit Strip Style in Build Settings to Debugging Symbols. Also, add your libyour_generate_file.a to Link Binary With Libraries in Build Phases. Add binding.h to Copy Bundle Resources. Add #import "binding.h" to Runner-Bridging-Header. Last but not least, add a never-to-be-executed dummy function in Swift that calls any of the generated C bindings, such as func dummyMethodToAvoidSymbolStripping() { wire_passing_complex_structs(42, nil) }, and this will prevent symbol stripping.

Lastly, in order to build Rust automatically when you are building Flutter, follow this tutorial.

Appendix: Future work #

I plan to support the following features. Of course, if you want to have other features, feel free to make an issue or PR.

• Support async in Rust (currently only async in Dart). Should be quite easy to implement; I have not done it because my use case currently does not includ that, but feel free to PR.
• Support Streams, which is a powerful abstraction. Should also be easy to implement.
• Beautify the generated code, possibly making the cases (camel/snake/...) consistent with the language guide.
• Make the code generator more robust to invalid inputs.

Appendix: Contributing #

Firstly, welcome, and thanks for your contributions! If you want to contribute, feel free to create a Pull Request. If you need some ideas of what to contribute, have a look at the Issues section of this repository. The code is covered by CI, and please ensure the CI passes - which often catches bugs. To release a new version, bump several versions, and write down a changelog: vim frb_codegen/Cargo.toml && vim frb_rust/Cargo.toml && vim frb_dart/pubspec.yaml && vim frb_dart/CHANGELOG.md