perf(doc,engine): rebuild a widget by removing its children in one pass (#1967)

crates/doc/src/lib.rs

@@ -574,6 +574,19 @@ impl Document {
         Ok(())
     }
 
+    /// Remove every child of `id` with its subtree, in one pass over `id`'s child list (removing
+    /// them one by one would search that list once per child).
+    pub fn remove_children(&mut self, id: NodeId) -> DocResult<()> {
+        let kids = std::mem::take(&mut self.get_mut(id)?.children);
+        for c in kids {
+            for d in self.subtree(c) {
+                self.nodes.remove(&d);
+                self.dirty.touch(d);
+            }
+        }
+        Ok(())
+    }
+
     /// Whether sections may go in `parent`: a page or another section (never a frame or a group).
     pub fn takes_sections(&self, parent: NodeId) -> bool {
         self.node(parent).is_some_and(|p| p.is_page() || matches!(p.kind, NodeKind::Section { .. }))

crates/doc/src/tests.rs

@@ -846,3 +846,17 @@ fn the_colour_walker_visits_calculation_literals() {
     let ColorOperand::Literal { literal } = from else { panic!("literal operand") };
     assert_eq!(literal.a, 0.5, "the mix's stored literal was converted like every other colour");
 }
+
+#[test]
+fn remove_children_removes_every_subtree_and_keeps_the_parent() {
+    let mut doc = Document::new("T");
+    let page = doc.first_page().unwrap();
+    let frame = doc.create(page, NodeKind::Frame, "F", 0.0, 0.0, 100.0, 100.0).unwrap();
+    let inner = doc.create(frame, NodeKind::Frame, "Inner", 0.0, 0.0, 10.0, 10.0).unwrap();
+    let leaf = doc.create(inner, NodeKind::Rectangle, "Leaf", 0.0, 0.0, 5.0, 5.0).unwrap();
+    let other = doc.create(page, NodeKind::Rectangle, "Other", 0.0, 0.0, 5.0, 5.0).unwrap();
+    doc.remove_children(frame).unwrap();
+    assert!(doc.contains(frame) && doc.contains(other));
+    assert!(!doc.contains(inner) && !doc.contains(leaf));
+    assert!(doc.get(frame).unwrap().children.is_empty());
+}

crates/engine/src/extensions/widgets.rs

@@ -243,9 +243,7 @@ fn build(s: &mut Session, plugin: &str, w: NodeId, r: &WidgetRender) -> Result<(
     if !is_widget(n) || n.plugin_data.get(PLUGIN).map(String::as_str) != Some(plugin) {
         return Err(bad(cmd, "the layer is no longer this plugin's widget"));
     }
-    for c in n.children.clone() {
-        doc.remove(c)?;
-    }
+    doc.remove_children(w)?;
     crate::cmd::props::apply(cmd, doc, w, &props(&r.root, true))?;
     let n = doc.get_mut(w)?;
     n.locked = false;