Widget `handle_os_event` method is already registered!

Error message

Widget {node_handle} `handle_os_event` method is already registered!

What it means

During widget registration the registry inserts the node handle into the handle_os_event set when the widget's handle_os_events flag is true. If the handle is already in the set the widget is being registered a second time, and this warning is logged. It indicates duplicate registration of a widget that handles OS events; behavior is otherwise unaffected.

Solutions

Ensure register() is called only once per widget instance.
Call unregister() on widget removal so re-adds don't collide with stale set entries.
Deduplicate your registration code path (e.g. use a HashSet or builder state to check before registering).
Ignore if benign, but resolve to keep method routing correct.

Example fix

// before
fn on_rebuild(&mut self, ui) {
    self.register(ui); // runs every rebuild for an already-registered widget
}
// after
fn on_rebuild(&mut self, ui) {
    if !self.is_registered {
        self.register(ui);
        self.is_registered = true;
    }
}

Defensive patterns

Strategy: validation

Validate before calling

let newly = self.registered_os_event_handles.insert(node.handle());
if newly { ui.register_widget(node); }

Prevention

Keep registration idempotent in rebuild/reload paths.
Unregister widgets before re-registering.
Only enable handle_os_events on widgets that truly need it.
Audit custom Control impls for double registration calls.

When it happens

Trigger: Calling WidgetMethodsRegistry::register (via the UI's node registration path) twice for the same widget whose Control::handle_os_events returns true — e.g. re-adding a node or double-invoked registration code.

Common situations: Custom widgets that respond to keyboard/mouse OS events being re-registered by plugin reload or builder misuse; adding the same node handle to the UI graph twice.

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/b14d6e89fdc0256e. Report an issue: GitHub.

Appendix: source

Thrown at fyrox-ui/src/lib.rs:668

#[derive(Default, PartialEq, Debug, Clone)]
struct WidgetMethodsRegistry {
    preview_message: FxHashSet<Handle<UiNode>>,
    on_update: FxHashSet<Handle<UiNode>>,
    handle_os_event: FxHashSet<Handle<UiNode>>,
}

impl WidgetMethodsRegistry {
    fn register<T: Control + ?Sized>(&mut self, node: &T) {
        let node_handle = node.handle();

        if node.preview_messages && !self.preview_message.insert(node_handle) {
            Log::warn(format!(
                "Widget {node_handle} `preview_message` method is already registered!"
            ));
        }
        if node.handle_os_events && !self.handle_os_event.insert(node_handle) {
            Log::warn(format!(
                "Widget {node_handle} `handle_os_event` method is already registered!"
            ));
        }
        if node.need_update && !self.on_update.insert(node_handle) {
            Log::warn(format!(
                "Widget {node_handle} `on_update` method is already registered!"
            ));
        }
    }

    fn unregister<T: Control + ?Sized>(&mut self, node: &T) {
        let node_handle = node.handle();

        self.preview_message.remove(&node_handle);
        self.on_update.remove(&node_handle);
        self.handle_os_event.remove(&node_handle);
    }
}

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