Returning a reference

[devongovett]

I'm looking into implementing the CSS Typed OM API for Parcel CSS. This has a complex hierarchy of objects, with fields that include other objects, etc. One thing I ran into is that it currently doesn't seem easy (or possible?) to have a getter or method return the same instance of an object each time it is called.

I tried returning a reference, but this seems to currently panic:

napi-rs/crates/backend/src/codegen/struct.rs

Lines 102 to 112 in 4f422b6

	impl napi::NapiRaw for &#name {
	unsafe fn raw(&self) -> napi::sys::napi_value {
	unreachable!()
	}
	}
	impl napi::NapiRaw for &mut #name {
	unsafe fn raw(&self) -> napi::sys::napi_value {
	unreachable!()
	}
	}

I saw the Reference API in a recent release, but I'm not sure if this is related or not.

Say I have the following structs:

#[napi]
struct Foo {
  bar: Bar
}

#[napi]
struct Bar {
  baz: u32
}

#[napi]
impl Foo {
  #[napi(getter)]
  pub fn bar(&self) -> ??? {
    self.bar
  }
}

How can I make bar return the same instance of Bar each time it is called?

[Brooooooklyn]

The 2.3.0 released a new feature: Reference API.
I think it's perfectly suitable for your scenario. I haven't had time to write the documentation for this feature but you can see the real-world example: https://github.com/Brooooooklyn/simple-git/blob/main/src/repo.rs#L211.

@devongovett

[devongovett]

Thanks for this example! It looks like I cannot return a SharedReference from an napi function though. I get "the trait bound napi::bindgen_prelude::SharedReference<stylesheet::CSSStyleSheet, stylesheet::CSSRuleList>: NapiRaw is not satisfied". In your example, it looks like it's returning a new instance of a wrapper object pointing to the inner reference each time. So, running a small example:

import { Repository } from '@napi-rs/simple-git'
const repo = new Repository('/path/to/repo') // Open an existed repo
repo.head() == repo.head() // false

Is it possible to somehow make this return true?

[devongovett]

Here's one way I came up with, but it's a bit manual and required some unsafe code...

struct CSSStyleSheet {
  stylesheet: Rc<RefCell<OwnedStyleSheet>>,
  rules: Option<Ref<()>>,
}

#[napi]
impl CSSStyleSheet {
  #[napi(getter)]
  pub fn css_rules(&mut self, env: Env) -> Result<JsUnknown> {
    if let Some(rules) = &self.rules {
      return env.get_reference_value(rules);
    }

    let rules = CSSRuleList {
      stylesheet: self.stylesheet.clone(),
      rules: Vec::new(),
    };

    let value = unsafe {
      let napi_value = napi::bindgen_prelude::ToNapiValue::to_napi_value(env.raw(), rules)?;
      napi::JsUnknown::from_napi_value(env.raw(), napi_value)?
    };

    self.rules = Some(env.create_reference_with_refcount(&value, 0)?);
    Ok(value)
  }
}

It seems to work ok, but maybe something like that could be wrapped up a little nicer.

[Brooooooklyn]

Does this meet your requirement? https://github.com/napi-rs/napi-rs/blob/into-reference/examples/napi/src/reference.rs#L60-L101

[devongovett]

Yeah that looks really nice. Thank you!