Accessing Members of RustOpaque Objects from Dart

[coder0xff]

Having learned about RustOpaque, I wanted to sandbox a bit and see what it was all about. I wrote api_sandbox.rs.

pub fn f() -> RustOpaque<fn(i32) -> i32> {
    fn g(x: i32) -> i32 { return x*2; }
    return RustOpaque::<fn(i32) -> i32>::new(g);
}

I was hopeful that RustOpaque<fn(i32) -> i32> would result in a callable Dart type.

@sealed
class FnI32I32 extends FrbOpaque {
  final ApiSandbox bridge;
  FnI32I32.fromRaw(int ptr, int size, this.bridge) : super.unsafe(ptr, size);
  @override
  DropFnType get dropFn => bridge.dropOpaqueFnI32I32;

  @override
  ShareFnType get shareFn => bridge.shareOpaqueFnI32I32;

  @override
  OpaqueTypeFinalizer get staticFinalizer => bridge.FnI32I32Finalizer;
}

It doesn't seem to be callable, but hey it's called "Opaque" for a reason. I can write this, however.

pub fn invoke_i32_i32(func: RustOpaque<fn(i32) -> i32>, v: i32) -> i32 {
    (*func)(v)
}

Then in Dart,

var func = await apiSandbox.f();
var result = await apiSandbox.invokeI32I32(func: func, v: 6);
assert(result == 12);

I think flutter_rust_bridge could generate these helpers and make invocation transparent to callers. But why stop there? Since Dart supports getter and setter functions, all the fields and methods could be exposed on a "by reference" object, though probably best kept as a separate feature from RustOpaque. If very tightly integrated, it could do this to all non-serializable values by default, without any explicit encapsulation:

pub fn f() -> fn(i32) -> i32 {
    fn g(x: i32) -> i32 { return x*2; }
    return g;
}

var func = await apiSandbox.f();
var result = await func(6);
assert(result == 12);

I'm thinking of attempting it myself (just fn to start), but I wanted to reach out and ask if this has already been tried and if there's any standing knowledge on the feasibility. Thanks!

[fzyzcjy]

That looks interesting!

I guess the more general form of this proposal is like, "exposing methods defined on opaque types".

We already understand methods defined on non-opaque types https://cjycode.com/flutter_rust_bridge/feature/lang_methods.html. So does this work on opaque types? (or with minor modifications?)

cc @rogurotus who implemented the great opaque types