Use a struct for the children cache

Author: nicoburns

src/tree/taffy_tree/tree.rs

@@ -26,6 +26,21 @@ impl Default for TaffyConfig {
     }
 }
 
+/// Used to cache the resolved children of a node (taking into account `Display::contents`) during layout
+/// so that repeated calls to the children method don't need to re-resolve the list.
+struct ChildrenCache {
+    /// The NodeId of the node whose children we are caching (the cache key)
+    node_id: NodeId,
+    /// The actual list of child ids
+    children: Vec<NodeId>,
+}
+impl ChildrenCache {
+    /// Create a new empty cache
+    fn new() -> ChildrenCache {
+        ChildrenCache { node_id: NodeId::new(0), children: Vec::new() }
+    }
+}
+
 /// A tree of UI nodes suitable for UI layout
 pub struct Taffy {
     /// The [`NodeData`] for each node stored in this tree
@@ -54,7 +69,7 @@ pub struct Taffy {
 
     /// Used to cache the resolved children of a node (taking into account `Display::contents`) during layout
     /// so that repeated calls to the children method don't need to re-resolve the list.
-    node_children_cache: RefCell<(NodeId, Vec<NodeId>)>,
+    node_children_cache: RefCell<ChildrenCache>,
 }
 
 impl Default for Taffy {
@@ -99,12 +114,17 @@ impl LayoutTree for Taffy {
     #[inline(always)]
     fn children(&self, node: NodeId) -> Self::ChildIter<'_> {
         let mut cache = self.node_children_cache.borrow_mut();
-        if cache.0 != node {
-            cache.1.clear();
-            cache.0 = node;
-            find_children_recursive(&mut cache.1, self, node);
+
+        // If the cache key does not match the requested node_id, then recompute the children for
+        // the requested node and update the cache in-place.
+        if cache.node_id != node {
+            cache.node_id = node;
+            cache.children.clear();
+            find_children_recursive(&mut cache.children, self, node);
         }
-        RefCellVecIter { children: RefMut::map(cache, |c| &mut c.1), index: 0 }
+
+        // In all cases, return a reference into the cache
+        RefCellVecIter { children: RefMut::map(cache, |c| &mut c.children), index: 0 }
     }
 
     #[inline(always)]
@@ -205,7 +225,7 @@ impl Taffy {
             measure_funcs: SparseSecondaryMap::with_capacity(capacity),
             config: TaffyConfig::default(),
             is_layouting: false,
-            node_children_cache: RefCell::new((NodeId::new(0), Vec::new())),
+            node_children_cache: RefCell::new(ChildrenCache::new()),
         }
     }