Use intersection observer to control frame throttling

third_party/WebKit/Source/core/dom/IntersectionObserver.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IntersectionObserver_h
 #define IntersectionObserver_h
 
 #include "bindings/core/v8/ExceptionStatePlaceholder.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/dom/IntersectionObservation.h"
@@ skipping 41 matching lines @@
     const Length& leftMargin() const { return m_leftMargin; }
     void computeIntersectionObservations();
     void enqueueIntersectionObserverEntry(IntersectionObserverEntry&);
     void applyRootMargin(LayoutRect&) const;
     unsigned firstThresholdGreaterThan(float ratio) const;
     void deliver();
     void removeObservation(IntersectionObservation&);
     bool hasEntries() const { return m_entries.size(); }
     const HeapLinkedHashSet<WeakMember<IntersectionObservation>>& observations() const { return m_observations; }
 
+    // A low latency intersection observer delivers events with an immediately
+    // posted task instead of waiting for an idle period (or up to 100ms).
+    // TODO(skyostil): Figure out how we want to expose low latency mode for web
+    // authors and adjust/remove this API accordingly.

[ojan, 2016/09/06 21:16:35]

FWIW, it looks like we're just going to change it to always postTask, but you
don't need to be on the hook for that, so a TODO is still good. Me or Stefan
will fix.

Or, if you're willing to wait, you can wait till we fix it. Either way is fine w
me.

+    void setLowLatency(bool lowLatency) { m_lowLatency = lowLatency; }
+    bool lowLatency() const { return m_lowLatency; }
+
     DECLARE_TRACE();
 
 private:
     explicit IntersectionObserver(IntersectionObserverCallback&, Node&, const Vector<Length>& rootMargin, const Vector<float>& thresholds);
     void clearWeakMembers(Visitor*);
 
     Member<IntersectionObserverCallback> m_callback;
     WeakMember<Node> m_root;
     HeapLinkedHashSet<WeakMember<IntersectionObservation>> m_observations;
     HeapVector<Member<IntersectionObserverEntry>> m_entries;
     Vector<float> m_thresholds;
     Length m_topMargin;
     Length m_rightMargin;
     Length m_bottomMargin;
     Length m_leftMargin;
+    bool m_lowLatency;
 };
 
 } // namespace blink
 
 #endif // IntersectionObserver_h