style: Introduce a generic way of gathering information from the cascade, and use it for viewport units.

components/style/cascade_info.rs

@@ -0,0 +1,78 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use dom::TNode;
+use properties::{DeclaredValue, PropertyDeclaration};
+use values::HasViewportPercentage;
+
+/// A structure to collect information about the cascade.
+///
+/// This is useful to gather information about what an element is affected by,
+/// and can be used in the future to track optimisations like when a
+/// non-inherited property is explicitly inherited, in order to cut-off the
+/// traversal.
+pub struct CascadeInfo {
+    pub saw_viewport_units: bool,
+    #[cfg(debug_assertions)]
+    finished: bool,
+}
+
+impl CascadeInfo {
+    #[cfg(debug_assertions)]
+    pub fn new() -> Self {
+        CascadeInfo {
+            saw_viewport_units: false,
+            finished: false,
+        }
+    }
+
+    #[cfg(not(debug_assertions))]
+    pub fn new() -> Self {
+        CascadeInfo {
+            saw_viewport_units: false,
+        }
+    }
+
+    /// Called when a property is cascaded.
+    ///
+    /// NOTE: We can add a vast amount of information here.
+    #[inline]
+    pub fn on_cascade_property<T>(&mut self,
+                                  _property_declaration: &PropertyDeclaration,
+                                  value: &DeclaredValue<T>)
+        where T: HasViewportPercentage
+    {
+        // TODO: we can be smarter and keep a property bitfield to keep track of
+        // the last applying rule.
+        if value.has_viewport_percentage() {
+            self.saw_viewport_units = true;
+        }
+    }
+
+    #[cfg(debug_assertions)]
+    fn mark_as_finished_if_appropriate(&mut self) {
+        self.finished = true;
+    }
+
+    #[cfg(not(debug_assertions))]
+    fn mark_as_finished_if_appropriate(&mut self) {}
+
+    #[allow(unsafe_code)]
+    pub fn finish<N: TNode>(mut self, node: &N) {
+        self.mark_as_finished_if_appropriate();
+
+        if self.saw_viewport_units {
+            unsafe {
+                node.set_dirty_on_viewport_size_changed();
+            }
+        }
+    }
+}
+
+#[cfg(debug_assertions)]
+impl Drop for CascadeInfo {
+    fn drop(&mut self) {
+        debug_assert!(self.finished,
+                      "Didn't use the result of CascadeInfo, if you don't need it, consider passing None");
+    }
+}