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..3a090e25ef8d830e61d27903431df88367b59068 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 {
 
@@ -19,8 +21,10 @@ IntersectionObserverController* IntersectionObserverController::create(Document*
 
 IntersectionObserverController::IntersectionObserverController(Document* document)
     : ActiveDOMObject(document)
+    , m_lowLatencyNotificationTask(CancellableTaskFactory::create(this, &IntersectionObserverController::deliverLowLatencyNotifications))
     , m_callbackID(0)
     , m_callbackFiredWhileSuspended(false)
+    , m_lowLatencyCallbackFiredWhileSuspended(false)

[szager1, 2016/08/30 23:57:11]

I don't think you need m_lowLatencyCallbacksFiredWhileSuspended.  Instead,
resume() should call deliverLowLatencyNotifications() unconditionally, or post a
task to do so.

[Sami, 2016/08/31 11:08:55]

Done.

 {
 }
 
@@ -28,6 +32,12 @@ IntersectionObserverController::~IntersectionObserverController() { }
 
 void IntersectionObserverController::scheduleIntersectionObserverForDelivery(IntersectionObserver& observer)
 {
+    if (observer.lowLatency()) {

[szager1, 2016/08/30 23:57:11]

I think this would read better if you refactor it to call into one of two new
methods:

void
IntersectionObserverController::scheduleIntersectionObserverForDelivery(IntersectionObserver&
observer)
{
  if (observer.lowLatency())
    scheduleLowLatencyNotification(observer);
  else
    scheduleIdleNotification(observer);
}

[Sami, 2016/08/31 11:08:55]

Good idea, done.

+        m_pendingLowLatencyIntersectionObservers.add(&observer);
+        if (!m_lowLatencyNotificationTask->isPending())
+            TaskRunnerHelper::get(TaskType::Unthrottled, getExecutionContext())->postTask(BLINK_FROM_HERE, m_lowLatencyNotificationTask->cancelAndCreate());
+        return;
+    }
     m_pendingIntersectionObservers.add(&observer);
     if (m_callbackID)
         return;
@@ -44,6 +54,10 @@ 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).
+    if (m_lowLatencyCallbackFiredWhileSuspended) {
+        m_lowLatencyCallbackFiredWhileSuspended = false;
+        deliverLowLatencyNotifications();
+    }
     if (m_callbackFiredWhileSuspended) {
         m_callbackFiredWhileSuspended = false;
         deliverIntersectionObservations();
@@ -74,6 +88,23 @@ void IntersectionObserverController::deliverIntersectionObservations()
         observer->deliver();
 }
 
+void IntersectionObserverController::deliverLowLatencyNotifications()
+{
+    ExecutionContext* context = getExecutionContext();
+    if (!context) {
+        m_pendingLowLatencyIntersectionObservers.clear();
+        return;
+    }
+    if (context->activeDOMObjectsAreSuspended()) {
+        m_lowLatencyCallbackFiredWhileSuspended = true;
+        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 +134,7 @@ DEFINE_TRACE(IntersectionObserverController)
 {
     visitor->trace(m_trackedIntersectionObservers);
     visitor->trace(m_pendingIntersectionObservers);
+    visitor->trace(m_pendingLowLatencyIntersectionObservers);
     ActiveDOMObject::trace(visitor);
     IdleRequestCallback::trace(visitor);
 }