generalize Layers

crates/tek_core/src/space.rs

@@ -145,6 +145,32 @@ impl<
     }
 }
 
+impl<E: Engine, F> Widget for Layers<E, F>
+where
+    F: Send + Sync + Fn(&mut dyn FnMut(&dyn Widget<Engine = E>)->Usually<()>)->Usually<()>
+{
+    type Engine = E;
+    fn layout (&self, area: E::Size) -> Perhaps<E::Size> {
+        let mut w: E::Unit = 0.into();
+        let mut h: E::Unit = 0.into();
+        (self.0)(&mut |layer| {
+            if let Some(layer_area) = layer.layout(area)? {
+                w = w.max(layer_area.w());
+                h = h.max(layer_area.h());
+            }
+            Ok(())
+        })?;
+        Ok(Some([w, h].into()))
+    }
+    fn render (&self, to: &mut E::Output) -> Usually<()> {
+        if let Some(size) = self.layout(to.area().wh().into())? {
+            (self.0)(&mut |layer|to.render_in(to.area().clip(size).into(), &layer))
+        } else {
+            Ok(())
+        }
+    }
+}
+
 //pub fn collect <'a, E: Engine, const N: usize> (
     //items: &'a [&'a dyn Widget<Engine = E>;N]
 //) -> impl Send + Sync + Fn(&'a mut dyn FnMut(&'a dyn Widget<Engine = E>)->Usually<()>)->Usually<()> + '_ {