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Rust runtime

Customize handler​

By default, a FLUTTER_RUST_BRIDGE_HANDLER instance is automatically generated (by the auto-generated flutter_rust_bridge::frb_generated_default_handler!() macro), and you can see it at frb_generated.rs. However, you can provide your own FLUTTER_RUST_BRIDGE_HANDLER instance. If your instance is detected, the generator will not generate one, but will use your version.

As for how to write a custom handler, often copy-paste-modify the code in flutter_rust_bridge::frb_generated_default_handler!() is a good idea.

The handler is the central entrypoint to handle calls between Rust and Dart, therefore please visit the API of the Handler trait for more details.

Some typical scenarios are:

The API of handler may have breaking change across minor version bumps, which is unlike most parts of flutter_rust_bridge which follows semantics versioning. However, usually it can be easily migrated by adding the corresponding parameters in your code.

States in handler​

If you are only using the provided handler and executor (with your own arguments), you can ignore this section. Only read it when you want to create your brand new handler.

It is usually a good idea to avoid coupling the handler with the specific Dart side isolate. For example, suppose whenever we call RustLib.init() on Dart side, we create a Dart isolate port and let the Rust side save and use it. Then, during flutter tests, multiple Dart test files will be executed concurrently, but (from my experiments) they all share one single Rust side. Then, the one Rust side will be initialized multiple times with different Dart ports, and it causes confusion and bugs. Similar things may happen if you are using multiple isolates in your Dart program, or when you hot-restart the Dart side.

Customize WASM initialization code​

By default, this library injects its own initialization code to facilitate panic information recovery using console_error_panic_hook. If you would like to run some initialization code for WASM, e.g. to set up logging libraries, specify default-features = false in Cargo.toml:

flutter_rust_bridge = { version = "..", default-features = false, features = [..] }

The wasm-start feature governs this behavior and is enabled by default.

Feature flags​

The full list of feature flags can be seen here (you may need to switch to corresponding version).

For example, suppose you need to use chrono types, then add that feature flag. As another example, suppose you never use async code, then the rust-async feature flag can be removed.