spans now good (other than direction) + refactor

src/layout/painter.rs

@@ -0,0 +1,44 @@
+use crate::{
+    UIRegion, WidgetId, Widgets,
+    primitive::{PrimitiveData, PrimitiveInstance, Primitives},
+};
+
+pub struct Painter<'a> {
+    nodes: &'a Widgets,
+    primitives: Primitives,
+    pub region: UIRegion,
+}
+
+impl<'a> Painter<'a> {
+    pub fn new(nodes: &'a Widgets) -> Self {
+        Self {
+            nodes,
+            primitives: Primitives::default(),
+            region: UIRegion::full(),
+        }
+    }
+    pub fn write<Data: PrimitiveData>(&mut self, data: Data) {
+        let ptr = self.primitives.data.len() as u32;
+        let region = self.region;
+        self.primitives
+            .instances
+            .push(PrimitiveInstance { region, ptr });
+        self.primitives.data.push(Data::DISCRIM);
+        self.primitives
+            .data
+            .extend_from_slice(bytemuck::cast_slice::<_, u32>(&[data]));
+    }
+    pub fn draw(&mut self, node: &WidgetId) {
+        self.nodes.get(node).draw(self);
+    }
+    pub fn draw_within(&mut self, node: &WidgetId, region: UIRegion) {
+        let old = self.region;
+        self.region.select(&region);
+        self.draw(node);
+        self.region = old;
+    }
+
+    pub fn finish(self) -> Primitives {
+        self.primitives
+    }
+}