Attempt to get reference to resource data while it is…

Error message

Attempt to get reference to resource data while it is unloaded!

What it means

DerefMut on Resource<T> panics unless the resource is in the Ok state: Unloaded, Pending, and LoadError states have no (valid) mutable data to expose. Unlike the immutable deref, these panics carry no type/path details. Mutable access additionally requires exclusive access to the resource.

Solutions

Check resource state is Ok (state().is_ok()) before any mutable dereference.
Await load completion (or use try_data_ref_mut-style access if available) and handle the error case explicitly.
Avoid mutating loaded resource data at all; build data up front or use separate runtime buffers instead of editing shared resources.

Example fix

// before
let tex = rm.request::<Texture>(path)?;
tex.data_ref_mut().set_pixel(x, y, color); // panics unless Ok
// after
let tex = rm.request::<Texture>(path)?;
if tex.state().is_ok() {
    tex.data_ref_mut().set_pixel(x, y, color);
}

Defensive patterns

Strategy: type-guard

Validate before calling

if resource.state().is_ok() {
    let data = resource.data_ref_mut();
}

Type guard

fn mutable_ready<T: TypedResourceData>(r: &mut Resource<T>) -> bool { r.state().is_ok() }

Prevention

Gate every data_ref_mut call behind an is_ok state check.
Prefer generating/editeable copies of data over mutating shared loaded resources.
Avoid mutating resources whose reload (re-request) may be in flight.

When it happens

Trigger: Getting a mutable reference (data_ref_mut / &mut through the guard) to a resource before its load completed or after it failed; attempting to mutate resource data on a just-requested handle.

Common situations: Procedurally editing textures/models right after requesting them; retry/migration code that mutates resources on load without checking state first; mutating a resource whose reload is in flight.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of FyroxEngine/Fyrox@76c91aad8e (2026-09-10). Data as JSON: /api/errors/4aec781c37ee7491. Report an issue: GitHub.

Appendix: source

Thrown at fyrox-resource/src/lib.rs:636

                };
                panic!("Attempt to get reference to resource data which failed to load! Type {}. Path: {path}. Error: {error:?}", std::any::type_name::<T>())
            }
            ResourceState::Ok { ref data } => (data.inner_ref() as &dyn Any)
                .downcast_ref()
                .expect("Type mismatch!"),
        }
    }
}

impl<T> DerefMut for ResourceDataRef<'_, T>
where
    T: TypedResourceData,
{
    fn deref_mut(&mut self) -> &mut Self::Target {
        let header = &mut *self.guard;
        match header.state {
            ResourceState::Unloaded => {
                panic!("Attempt to get reference to resource data while it is unloaded!")
            }
            ResourceState::Pending { .. } => {
                panic!("Attempt to get reference to resource data while it is loading!")
            }
            ResourceState::LoadError { .. } => {
                panic!("Attempt to get reference to resource data which failed to load!")
            }
            ResourceState::Ok { ref mut data, .. } => (data.inner_mut() as &mut dyn Any)
                .downcast_mut()
                .expect("Type mismatch!"),
        }
    }
}

/// Collects all resources used by a given entity. Internally, it uses reflection to iterate over
/// each field of every descendant sub-object of the entity. This function could be used to collect
/// all resources used by an object, which could be useful if you're building a resource dependency
/// analyzer.

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