Use intersection observer to control frame throttling

third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp

Index: third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp b/third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp
index dbdb3f937f598f7f37bb4aad6f082e6f24f0c087..9e59de76adad1f0546002607d3debee247514c2b 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp
@@ -6,7 +6,9 @@
 
 #include "core/dom/Document.h"
 #include "core/dom/IdleRequestOptions.h"
+#include "core/dom/TaskRunnerHelper.h"
 #include "platform/TraceEvent.h"
+#include "platform/scheduler/CancellableTaskFactory.h"
 
 namespace blink {
 
@@ -19,6 +21,7 @@ IntersectionObserverController* IntersectionObserverController::create(Document*
 
 IntersectionObserverController::IntersectionObserverController(Document* document)
     : ActiveDOMObject(document)
+    , m_lowLatencyNotificationTask(CancellableTaskFactory::create(this, &IntersectionObserverController::deliverLowLatencyNotifications))
     , m_callbackID(0)
     , m_callbackFiredWhileSuspended(false)
 {
@@ -28,6 +31,15 @@ IntersectionObserverController::~IntersectionObserverController() { }
 
 void IntersectionObserverController::scheduleIntersectionObserverForDelivery(IntersectionObserver& observer)
 {
+    if (observer.lowLatency()) {
+        scheduleLowLatencyNotification(observer);
+    } else {
+        scheduleIdleNotification(observer);
+    }
+}
+
+void IntersectionObserverController::scheduleIdleNotification(IntersectionObserver& observer)
+{
     m_pendingIntersectionObservers.add(&observer);
     if (m_callbackID)
         return;
@@ -40,10 +52,18 @@ void IntersectionObserverController::scheduleIntersectionObserverForDelivery(Int
     m_callbackID = document->requestIdleCallback(this, options);
 }
 
+void IntersectionObserverController::scheduleLowLatencyNotification(IntersectionObserver& observer)
+{
+    m_pendingLowLatencyIntersectionObservers.add(&observer);
+    if (!m_lowLatencyNotificationTask->isPending())
+        TaskRunnerHelper::get(TaskType::Unthrottled, getExecutionContext())->postTask(BLINK_FROM_HERE, m_lowLatencyNotificationTask->cancelAndCreate());
+}
+
 void IntersectionObserverController::resume()
 {
     // If the callback fired while DOM objects were suspended, notifications might be late, so deliver
     // them right away (rather than waiting to fire again).
+    deliverLowLatencyNotifications();
     if (m_callbackFiredWhileSuspended) {
         m_callbackFiredWhileSuspended = false;
         deliverIntersectionObservations();
@@ -74,6 +94,23 @@ void IntersectionObserverController::deliverIntersectionObservations()
         observer->deliver();
 }
 
+void IntersectionObserverController::deliverLowLatencyNotifications()
+{
+    ExecutionContext* context = getExecutionContext();
+    if (!context) {
+        m_pendingLowLatencyIntersectionObservers.clear();
+        return;
+    }
+    if (context->activeDOMObjectsAreSuspended()) {
+        // resume() will deliver low latency notifications directly.
+        return;
+    }
+    HeapHashSet<Member<IntersectionObserver>> observers;
+    m_pendingLowLatencyIntersectionObservers.swap(observers);
+    for (auto& observer : observers)
+        observer->deliver();
+}
+
 void IntersectionObserverController::computeTrackedIntersectionObservations()
 {
     TRACE_EVENT0("blink", "IntersectionObserverController::computeTrackedIntersectionObservations");
@@ -103,6 +140,7 @@ DEFINE_TRACE(IntersectionObserverController)
 {
     visitor->trace(m_trackedIntersectionObservers);
     visitor->trace(m_pendingIntersectionObservers);
+    visitor->trace(m_pendingLowLatencyIntersectionObservers);
     ActiveDOMObject::trace(visitor);
     IdleRequestCallback::trace(visitor);
 }