4 files changed, 25 insertions, 2 deletions
diff --git a/components/compositing/Cargo.toml b/components/compositing/Cargo.toml
index 426746838b3..c07e7365ad9 100644
--- a/components/compositing/Cargo.toml
+++ b/components/compositing/Cargo.toml
@@ -39,6 +39,7 @@ style_traits = {path = "../style_traits"}
 time = "0.1.17"
 webrender = {git = "https://github.com/servo/webrender", features = ["capture"]}
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}
+webvr_traits = {path = "../webvr_traits"}
 webvr = {path = "../webvr"}
 
 [build-dependencies]
diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 253fc1cb925..9ff74668e0d 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -43,6 +43,7 @@ use style_traits::{CSSPixel, DevicePixel, PinchZoomFactor};
 use time::{now, precise_time_ns, precise_time_s};
 use webrender_api::{self, DeviceIntPoint, DevicePoint, HitTestFlags, HitTestResult};
 use webrender_api::{LayoutVector2D, ScrollLocation};
+use webvr_traits::WebVRMainThreadHeartbeat;
 
 #[derive(Debug, PartialEq)]
 enum UnableToComposite {
@@ -176,6 +177,9 @@ pub struct IOCompositor<Window: WindowMethods> {
     /// The webrender interface, if enabled.
     webrender_api: webrender_api::RenderApi,
 
+    /// Some VR displays want to be sent a heartbeat from the main thread.
+    webvr_heartbeats: Vec<Box<dyn WebVRMainThreadHeartbeat>>,
+
     /// Map of the pending paint metrics per layout thread.
     /// The layout thread for each specific pipeline expects the compositor to
     /// paint frames with specific given IDs (epoch). Once the compositor paints
@@ -283,6 +287,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             webrender: state.webrender,
             webrender_document: state.webrender_document,
             webrender_api: state.webrender_api,
+            webvr_heartbeats: state.webvr_heartbeats,
             pending_paint_metrics: HashMap::new(),
         }
     }
@@ -919,7 +924,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 pipeline_ids.push(*pipeline_id);
             }
         }
-        let animation_state = if pipeline_ids.is_empty() {
+        let animation_state = if pipeline_ids.is_empty() && !self.webvr_heartbeats_racing() {
             windowing::AnimationState::Idle
         } else {
             windowing::AnimationState::Animating
@@ -930,6 +935,10 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         }
     }
 
+    fn webvr_heartbeats_racing(&self) -> bool {
+        self.webvr_heartbeats.iter().any(|hb| hb.heart_racing())
+    }
+
     fn tick_animations_for_pipeline(&mut self, pipeline_id: PipelineId) {
         let animation_callbacks_running = self
             .pipeline_details(pipeline_id)
@@ -1345,6 +1354,11 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             CompositionRequest::CompositeNow(_) => self.composite(),
         }
 
+        // Send every VR display that wants one a main-thread heartbeat
+        for webvr_heartbeat in &mut self.webvr_heartbeats {
+            webvr_heartbeat.heartbeat();
+        }
+
         if !self.pending_scroll_zoom_events.is_empty() && !self.waiting_for_results_of_scroll {
             self.process_pending_scroll_events()
         }
diff --git a/components/compositing/compositor_thread.rs b/components/compositing/compositor_thread.rs
index e4dee4f595a..1a7ee9309cb 100644
--- a/components/compositing/compositor_thread.rs
+++ b/components/compositing/compositor_thread.rs
@@ -18,6 +18,7 @@ use script_traits::{AnimationState, ConstellationMsg, EventResult};
 use std::fmt::{Debug, Error, Formatter};
 use style_traits::viewport::ViewportConstraints;
 use webrender_api::{self, DeviceIntPoint, DeviceIntSize};
+use webvr_traits::WebVRMainThreadHeartbeat;
 
 /// Sends messages to the compositor.
 pub struct CompositorProxy {
@@ -153,4 +154,5 @@ pub struct InitialCompositorState {
     pub webrender: webrender::Renderer,
     pub webrender_document: webrender_api::DocumentId,
     pub webrender_api: webrender_api::RenderApi,
+    pub webvr_heartbeats: Vec<Box<WebVRMainThreadHeartbeat>>,
 }
diff --git a/components/compositing/windowing.rs b/components/compositing/windowing.rs
index 091eb661a5e..6c4ba4b3cee 100644
--- a/components/compositing/windowing.rs
+++ b/components/compositing/windowing.rs
@@ -19,6 +19,7 @@ use std::rc::Rc;
 use style_traits::DevicePixel;
 use webrender_api::{DeviceIntPoint, DeviceIntRect, DeviceIntSize, DevicePoint, ScrollLocation};
 use webvr::VRServiceManager;
+use webvr_traits::WebVRMainThreadHeartbeat;
 
 #[derive(Clone)]
 pub enum MouseWindowEvent {
@@ -147,7 +148,12 @@ pub trait WindowMethods {
     /// run the event loop at the vsync interval.
     fn set_animation_state(&self, _state: AnimationState);
     /// Register services with a VRServiceManager.
-    fn register_vr_services(&self, _: &mut VRServiceManager) {}
+    fn register_vr_services(
+        &self,
+        _: &mut VRServiceManager,
+        _: &mut Vec<Box<WebVRMainThreadHeartbeat>>,
+    ) {
+    }
 }
 
 #[derive(Clone, Copy, Debug)]