Overall design

This doc is still WIP. Tracking issue: https://github.com/fzyzcjy/flutter_rust_bridge/issues/593

Folder structure

• frb_codegen: Code generator. It inputs api.rs and outputs Rust and Dart code files.
• frb_example: Examples.
  • pure_dart: Not only an example, but, more importantly, serves as end-to-end tests.
  • with_flutter: Example with integration into Flutter.
  • pure_dart_multi: Demonstrate multi-file usage.
• frb_dart: Support library for Dart - to be imported by users.
• frb_rust: Support library for Rust - to be imported by users.
• frb_macros: Indeed part of frb_rust. It is a separate package simply because limitation of proc macros.
• book: The documentation.
• .github: GitHub-related.
  • workflows/ci.yaml: Definition of CI workflows.

Terminology

Rust IO Wire types refers to the C types the Dart VM uses to communicate with the Rust library.

Dart IO Wire types are the Dart counterpart of Rust IO wire types, but in the *.io.dart files. Both Rust and Dart wire types communicate using the vocabulary of C types, aka primitives, structs, unions and pointers.

Rust JS Wire types are the WASM equivalent of Rust IO wire types, many of which are distinct from their C siblings. In addition, these types may also take the form of the catch-all JsValue.

Dart JS Wire types are the WASM equivalent of Dart IO wire types, but unlike Rust JS wire types, most of these types remain identical to their real API counterparts. Similar to the the relationship between Rust IO and Dart IO wire types, Rust JS and Dart JS wire types use the vocabulary of JavaScript types, aka primitives, arrays, typed arrays and objects.

Code-generator structure

The pipeline is as follows:

flowchart LR
api.rs -- src/parser --> src/ir
src/ir -- src/generator --> rd[Rust & Dart]

• The input, api.rs in the figure, is the user-provided handwritten Rust code.
• The parser (src/parser) converts the input code (indeed syn tree) into IR.
• IR (src/ir), or internal representation, is a data structure that represents the information of the code that we are interested in.
• The generator (src/generator) converts the IR into final outputs. More specifcially, as you can probably guess, src/generator/dart generates Dart code, src/generator/rust is for Rust code, and src/generator/c is for (a bit of) C code.
• The outputs (Rust & Dart in the figure) are written to corresponding files.

Data flow

Let us see what happens when a function is called.

Suppose a user calls a (generated) Dart function func({required String str}). Then, the following happens:

1. The generated Dart function, func({required String str}), convert "Dart api data" (i.e. the data that user really provides) into "Dart wire data" (i.e. the data that will really pass between Dart and Rust). More specifically, it calls _api2wire_String(str) and get a ffi.Pointer<wire_uint_8_list> (because Strings use pub struct wire_uint_8_list { ptr: *mut u8, len: i32 } under the hood).
2. Now we call the Dart version of wire_func, with low-level data like wire_uint_8_list. We have used our codegen to create a Rust wire_func function, and use cbindgen to generate the corresponding C function, and use ffigen to get the corresponding Dart function. Here, we call the Dart version of wire_func. Since Dart FFI and Rust FFI is C-compatible, it seamlessly calls the Rust version of wire_func. Notice that, since we are utilizing C-compatible functions (and it is the only feasible way), we can only pass around low-level things like pointers, instead of high-level and safe things.
3. Surely, the Rust wire_func is called. The function uses .wire2api() to convert "Rust wire data" (wire_uint_8_list here) into "Rust api data" (String here, i.e. data that users really use).
4. The FLUTTER_RUST_BRIDGE_HANDLER is called with "Rust api data". That handler is user-customizable, so users may provide their own implementation other than the default thread-pool, etc. By default, we use a thread pool, and we call the user-written func Rust function in api.rs.
5. The user-written fn func(str: String) -> String { ... } is called, and we get a return value.
6. The return value, a String, is posted to the Dart side. It is done by the Dart-provided API, Dart_PostCObject, which let us provide C structs and it will automatically become Dart data on the other side. We use the Rust-safe wrapper allo-isolate for it. We deliberately choose this, because this enables Dart code to be async instead of sync.
7. On the Dart side, we now see some Dart objects (indeed "Dart wire data"). We use functions like _wire2api_SomeType to convert it to the final "Dart api data". Notice this "wire2api" is on Dart side, so it means "Dart wire data to Dart api data", and is different from the one above which is for Rust. For example, since Dart_PostCObject does not provide a way to construct arbitrary structs(classes), we have to pass Rust structs as lists, and use the wire2api to convert them to corresponding Dart classes.
8. The final result value is provided as return value of the Dart function, func, that the user called just now. A function call finishes!

Type Mappings

Unless otherwise noted, T refers to a type from the same column or the generic type. Does not include delegated types.

Memory safety

How is memory safety implemented? This is a case-by-case problem. For example, suppose we want to see how a String is safely passed from Dart to Rust. Then, we need to examine the Dart _api2wire_String and the Rust .wire2api() for it.

Indeed String is implemented by delegating to Vec<u8>, so we need to see code related to String as well as Vec<u8>. By simply clicking a few times and jump around code, we will see that:

ffi.Pointer<wire_uint_8_list> _api2wire_String(String raw) {
  return _api2wire_uint_8_list(utf8.encoder.convert(raw));
}

ffi.Pointer<wire_uint_8_list> _api2wire_uint_8_list(Uint8List raw) {
  final ans = inner.new_uint_8_list_0(raw.length);
  ans.ref.ptr.asTypedList(raw.length).setAll(0, raw);
  return ans;
}

and

impl Wire2Api<Vec<u8>> for *mut wire_uint_8_list {
    fn wire2api(self) -> Vec<u8> {
        unsafe {
            let wrap = support::box_from_leak_ptr(self);
            support::vec_from_leak_ptr(wrap.ptr, wrap.len)
        }
    }
}

impl Wire2Api<String> for *mut wire_uint_8_list {
    fn wire2api(self) -> String {
        let vec: Vec<u8> = self.wire2api();
        String::from_utf8_lossy(&vec).into_owned()
    }
}

pub struct wire_uint_8_list {
    ptr: *mut u8,
    len: i32,
}

In other words, String (or Vec<u8>) is converted to a raw struct with pointer and length field. The memory is manipulated carefully so there is no leak or double free.

We use Valgrind to check as well, and I use it in production environment without problems, so no worries about memory problems :)

Dart bridge hierarchy

A bridge module consists of several classes:

• One _Impl class implementing the wire functions and common helpers; and
• One or more _Platform classes implementing the platform-specific helpers.

The implementor class takes a platform class as a private attribute, and the platform class exposes all of its members decorated with @protected. The specific platform class to be used is gated by conditional imports.

Cross-scope communication in the browser

On Web platforms, for lack of a proper SendPort there exists replacements from dart:html.

MessagePort replaces dart:ffi's SendPort and is created from MessageChannel. The Dart thread creates a channel, keeps the receive port and transfers the send port to the workers.

sequenceDiagram
Dart ->> Rust: port2
Rust ->> Rust Worker: port2
Rust Worker ->> Dart: port2.postMessage

BroadcastChannel replaces dart:ffi's SendPort for StreamSinks, due to the fact that wasm_bindgen keeps the ports in a JS-local scope that cannot be shared with other threads. A broadcast channel is created by Dart, then passed to the main Rust thread. Rust then transfers its name to the workers. When other workers refer to a StreamSink from another worker, e.g. if the sink was put in a static variable, a new BroadcastChannel will be created from its name.

BroadcastChannels are guaranteed to be unique for each invocation.⁵

sequenceDiagram
Dart ->> Rust: channel
Rust ->> Rust Worker 1: channel.name
Rust Worker 1 ->> Dart: channel.postMessage
Rust ->> Rust Worker 2: channel.name
Rust Worker 2 ->> Dart: channel.postMessage

It is theoretically possible to have a one-to-one implementation of Isolate using only web primitives, BroadcastChannels and Workers, but it remains to be seen how practical such an approach would be.

OptionalList

Per the implementation, most IRs are also accompanied by a List type (GeneralList, PrimitiveList, StringList etc.) each of which handles lists in different ways. When Optional was first implemented, it relied on GeneralList since the underlying assumption that Optional already boxed stack values should allow for seamless interaction. Howver, this became an issue later because other IRs would have to accommodate for Optionals instead of being perfectly encapsulated, leading to ugly hacks. #1388 introduced OptionalList to bring Optional in line with other IRs, and is implemented as a list of maybe-null pointers. It does highlight several drawbacks to this approach to IRs where specializations shine compared to GeneralList.

1. GeneralList requires a fully-allocated list and asks the Dart side to fill in the blanks via api_fill functions, but these are not implemented by any delegates since they all have their own special lists (StringList, TimeList, Uuids). This renders types like List<String?> difficult to implement without hacks.
2. OptionalList's inner pointer is a *mut *mut T, which without significant refactoring would be difficult to represent with GeneralList, and whose typical usage doesn't really require double indirection often enough to justify it.
3. OptionalList enables future optimizations, for example the case when sizeof(T) <= sizeof(usize), which would certainly be difficult to accomplish with GeneralList.

Want to know more? Tell me

What do you want to know? Feel free to create an issue in GitHub, and I will tell more :)