better global state structure?

core/src/widget/handle.rs

@@ -1,7 +1,7 @@
 use std::{marker::Unsize, mem::MaybeUninit, ops::CoerceUnsized, sync::mpsc::Sender};
 
 use crate::{
-    Widget,
+    HasUi, Widget,
     util::{RefCounter, SlotId},
 };
 
@@ -143,3 +143,11 @@ impl<W> std::fmt::Debug for WidgetHandle<W> {
         self.id.fmt(f)
     }
 }
+
+impl<'a, W: Widget + 'a, State: HasUi> FnOnce<(&'a mut State,)> for WidgetRef<W> {
+    type Output = &'a mut W;
+
+    extern "rust-call" fn call_once(self, args: (&'a mut State,)) -> Self::Output {
+        &mut args.0.ui_mut()[self]
+    }
+}