Extra Dart code

Arbitrary extra Dart code can be inserted into auto-generated Dart classes, by using the attribute #[frb(dart_code = ...)] syntax.

Example

#[frb(dart_code = "
    int extraMethod() => a * 2;
"
)]
pub struct MyStruct {
    ...
}

Then the generated Dart class will look like:

class MyStruct {
  ... // other auto-generated code
  
  int extraMethod() => a * 2; // The extra code
}

How to import things?

In order to import things, simply write down the import statements besides normal code, and it will be automatically recognized and pasted.

Where to annotate the attribute?

Currently, you can annotate the attribute only at rust structs and enums. It doesn't work on functions, mods or other elements, which are not translated into dart classes.

The dart code is copied into the generated dart class, inside the scope of the rust element, which was annotated.

For example, if you annotate a struct MyStruct in the rust file minimal.rs, then the dart code will be inserted into the generated dart class class MyStruct in the generated file minimal.dart, inside this class definition.

Impossible Dart Code

Because the dart_code is inserted into the generated dart code from the rust element you annotated, the result must be valid dart code.

Thus a pure statement like int a = 10; will not work as expected if inserted into a struct, it needs to be a member field, like a variable or method/function.

For the same reason extend doesn't work neither.

Possible Dart Code

Any valid Dart Code for a class body is allowed in the #[frb(dart_code =...)].

Works only if rust code is callable

As usual, if a struct is not used by any functions, flutter_rust_bridge will automatically ignore it, avoiding generating useless structs. This means that, at the current stage, your annotated rust struct needs to be callable by Dart - otherwise no code will be generated. This may be improved in the future, i.e. auto detect such scenario and generate code for it (feel free to create an issue if this is needed for you!).

You can achieve this in two ways: Having an additional rust function inside your struct:

#[frb(dart_code = "static void dartSay() => print('Dart_code at struct');")]
pub struct DartCodeStruct {}

impl DartCodeStruct {
    pub fn noop() {}
}

or having another rust function using your rust struct as a parameter:

#[frb(dart_code = "static void dartSay() => print('Dart_code at struct');")]
pub struct DartCodeStruct {}

pub fn noop(dcs: DartCodeStruct) {}

Troubleshooting

You can see if the dart_code was inserted into the generated code (e.g. myRustCode.dart) by looking into this file. If you see the comment

// These types are ignored because they are not used by any `pub` functions: `MyStruct`

then your rust code is not generated, thus no dart_code was inserted.

Another error could be that your dart_code is invalid. If so you will get the error message: stderr=Could not format because the source could not be parsed: when running the code generation.