Overview

This feature, sometimes called RustAutoOpaque throughout the documentation, allows arbitrary Rust type to be used without manual intervention, by representing arbitrary Rust object as (smart) pointers in Dart.

Different from non-opaque types, opaque types are not copied/moved/reconstructed at all. For example, if you pass around RwLock<Mutex<ArbitraryData> in arguments and return values, you will get the exact same RwLock<ArbitraryData> object.

Example

Suppose you have a type that is not encodable:

pub struct MyNonEncodableType {
    // e.g., a temporary directory, a file descriptor, a native resource, a lock, a channel, ...
    sample_non_encodable_field: tempdir::TempDir,
}

Then you can have Rust functions and methods on it. Most, if not all, features of flutter_rust_bridge are supported, and here are a few examples:

pub fn create() -> MyNonEncodableType { ... }

pub fn consume(obj: MyNonEncodableType) { ... }

pub fn borrow(obj: &MyNonEncodableType) { ... }

pub fn mutable_borrow(obj: &mut MyNonEncodableType) { ... }

impl MyNonEncodableType {
    // Or `self`, `&mut self`
    pub fn methods_on_it(&self) { ... }
}

They can be called in Dart:

var object = await create();
await borrow(object);
await mutable_borrow(object);
await consume(object);

P.S. As for how it is implemented as well as the design towards safety, please refer to this doc