layout: Load Web fonts asynchronously.

Improves page load times significantly.

Closes #7343.

components/script/dom/window.rs

@@ -100,6 +100,7 @@ pub enum ReflowReason {
     DocumentLoaded,
     ImageLoaded,
     RequestAnimationFrame,
+    WebFontLoaded,
 }
 
 pub type ScrollPoint = Point2D<Au>;
@@ -1378,6 +1379,7 @@ fn debug_reflow_events(goal: &ReflowGoal, query_type: &ReflowQueryType, reason:
         ReflowReason::DocumentLoaded => "\tDocumentLoaded",
         ReflowReason::ImageLoaded => "\tImageLoaded",
         ReflowReason::RequestAnimationFrame => "\tRequestAnimationFrame",
+        ReflowReason::WebFontLoaded => "\tWebFontLoaded",
     });
 
     println!("{}", debug_msg);

components/script/layout_interface.rs

@@ -51,6 +51,9 @@ pub enum Msg {
     /// Requests that the layout task render the next frame of all animations.
     TickAnimations,
 
+    /// Requests that the layout task reflow with a newly-loaded Web font.
+    ReflowWithNewlyLoadedWebFont,
+
     /// Updates the layout visible rects, affecting the area that display lists will be constructed
     /// for.
     SetVisibleRects(Vec<(LayerId, Rect<Au>)>),
@@ -76,6 +79,10 @@ pub enum Msg {
     /// Get the last epoch counter for this layout task.
     GetCurrentEpoch(IpcSender<Epoch>),
 
+    /// Asks the layout task whether any Web fonts have yet to load (if true, loads are pending;
+    /// false otherwise).
+    GetWebFontLoadState(IpcSender<bool>),
+
     /// Creates a new layout task.
     ///
     /// This basically exists to keep the script-layout dependency one-way.

components/script/script_task.rs

@@ -942,6 +942,8 @@ impl ScriptTask {
                 self.handle_webdriver_msg(pipeline_id, msg),
             ConstellationControlMsg::TickAllAnimations(pipeline_id) =>
                 self.handle_tick_all_animations(pipeline_id),
+            ConstellationControlMsg::WebFontLoaded(pipeline_id) =>
+                self.handle_web_font_loaded(pipeline_id),
             ConstellationControlMsg::StylesheetLoadComplete(id, url, responder) => {
                 responder.respond();
                 self.handle_resource_loaded(id, LoadType::Stylesheet(url));
@@ -1478,6 +1480,15 @@ impl ScriptTask {
         document.r().run_the_animation_frame_callbacks();
     }
 
+    /// Handles a Web font being loaded. Does nothing if the page no longer exists.
+    fn handle_web_font_loaded(&self, pipeline_id: PipelineId) {
+        if let Some(page) = self.page.borrow().as_ref() {
+            if let Some(page) = page.find(pipeline_id) {
+                self.rebuild_and_force_reflow(&*page, ReflowReason::WebFontLoaded);
+            }
+        }
+    }
+
     /// The entry point to document loading. Defines bindings, sets up the window and document
     /// objects, parses HTML and CSS, and kicks off initial layout.
     fn load(&self, metadata: Metadata, incomplete: InProgressLoad) -> Root<ServoHTMLParser> {