Speculatively evaluate paint functions during style.

components/layout/context.rs

@@ -4,7 +4,6 @@
 
 //! Data needed by the layout thread.
 
-use fnv::FnvHashMap;
 use fnv::FnvHasher;
 use gfx::display_list::{WebRenderImageInfo, OpaqueNode};
 use gfx::font_cache_thread::FontCacheThread;
@@ -25,8 +24,8 @@ use std::collections::HashMap;
 use std::hash::BuildHasherDefault;
 use std::sync::{Arc, Mutex};
 use std::thread;
+use style::context::RegisteredSpeculativePainter;
 use style::context::SharedStyleContext;
-use style::properties::PropertyId;
 
 thread_local!(static FONT_CONTEXT_KEY: RefCell<Option<FontContext>> = RefCell::new(None));
 
@@ -73,7 +72,7 @@ pub struct LayoutContext<'a> {
                                                   BuildHasherDefault<FnvHasher>>>>,
 
     /// Paint worklets
-    pub registered_painters: Arc<RwLock<FnvHashMap<Atom, RegisteredPainter>>>,
+    pub registered_painters: &'a RegisteredPainters,
 
     /// A list of in-progress image loads to be shared with the script thread.
     /// A None value means that this layout was not initiated by the script thread.
@@ -179,9 +178,11 @@ impl<'a> LayoutContext<'a> {
     }
 }
 
-/// A registered paint worklet.
-pub struct RegisteredPainter {
-    pub name: Atom,
-    pub properties: FnvHashMap<Atom, PropertyId>,
-    pub painter: Arc<Painter>,
+/// A registered painter
+pub trait RegisteredPainter: RegisteredSpeculativePainter + Painter {}
+
+/// A set of registered painters
+pub trait RegisteredPainters: Sync {
+    /// Look up a painter
+    fn get(&self, name: &Atom) -> Option<&RegisteredPainter>;
 }

components/layout/display_list_builder.rs

@@ -1181,24 +1181,18 @@ impl FragmentDisplayListBuilding for Fragment {
             .map(|argument| argument.to_css_string())
             .collect();
 
-        // Get the painter, and the computed values for its properties.
-        // TODO: less copying.
-        let (properties, painter) = match state.layout_context.registered_painters.read().get(&name) {
-            Some(registered_painter) => (
-                registered_painter.properties
-                    .iter()
+        let mut draw_result = match state.layout_context.registered_painters.get(&name) {
+            Some(painter) => {
+                debug!("Drawing a paint image {}({},{}).", name, size_in_px.width, size_in_px.height);
+                let properties = painter.properties().iter()
                     .filter_map(|(name, id)| id.as_shorthand().err().map(|id| (name, id)))
                     .map(|(name, id)| (name.clone(), style.computed_value_to_string(id)))
-                    .collect(),
-                registered_painter.painter.clone()
-            ),
+                    .collect();
+                painter.draw_a_paint_image(size_in_px, device_pixel_ratio, properties, arguments)
+            },
             None => return debug!("Worklet {} called before registration.", name),
         };
 
-        // TODO: add a one-place cache to avoid drawing the paint image every time.
-        // https://github.com/servo/servo/issues/17369
-        debug!("Drawing a paint image {}({},{}).", name, size_in_px.width, size_in_px.height);
-        let mut draw_result = painter.draw_a_paint_image(size_in_px, device_pixel_ratio, properties, arguments);
         let webrender_image = WebRenderImageInfo {
             width: draw_result.width,
             height: draw_result.height,