[Help] GodotNative: accessing Rust nodes from Rust nodes - Liftime problem about kohonen

[mlange-42]

@adamreichold Any idea how to solve that? I was hoping to get the result with the lifetime of owner. Thanks!

[adamreichold]

Any idea how to solve that? I was hoping to get the result with the lifetime of owner. Thanks!

I think the first attempt should be to drop the lifetime all together and pass owner by value. It already is a pointer to an object whose life time is managed by Godot. (Which is the reason why all actual accesses are unsafe.)

The second observation would be that you probably have to pass the full Instance<Kohonen> around as the Godot object part of it controls the life time even though you are only interested in the native class part.

(More specifically, the signature fn Instance::map<F, U>(&self, op: F) with F: for<'a>: FnOnce(&'a T, <T as NativeClass>::Base) -> U informs you that you loose any control over the lifetime of the &'a T pass to you and hence its scope is effectively limited to your calling scope.)

Long story short: Rust lifetimes will probably not help much when interacting with objects managed by Godots runtime are they are currently exposed which explains the huge amount of unsafe functions in its API.

[adamreichold]

Meaning that your signature should probably just be fn get_kohonen(owner: gdnative::Node2D) -> Option<Instance<Kohonen>>. (I don't think you actually need &self here?)

[mlange-42]

@adamreichold Thanks!
So far, I solved it by a function taking a closure, so I don't need to care for lifetimes:

trait KohonenUser2D {
    fn with_kohonen<F>(mut owner: gdnative::Control, path: &str, mut fun: F)
    where
        F: FnMut(&mut gdnative::Control, &Kohonen),
    {
        let node = unsafe { owner.get_node(NodePath::from_str(path)) };
        node.and_then(|node| {
            Instance::<Kohonen>::try_from_base(node)
                .map(|inst| inst.map(|koh, _| fun(&mut owner, koh)))
        });
    }
}

[adamreichold]

@adamreichold Thanks!
So far, I solved it by a function taking a closure, so I don't need to care for lifetimes:

Closure-based are often the last resort when fixing lifetime issue due to the implicit HKTB on the closure types FnOnce, Fn and FnMut. Keep in mind that fetch the node via get_node is not particularly cheap and even NodePath::from_str will allocate.