Monitor the intersection of video and viewport.

third_party/WebKit/Source/core/dom/ElementVisibilityObserver.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 ElementVisibilityObserver_h
 #define ElementVisibilityObserver_h
 
 #include "core/CoreExport.h"
 #include "core/dom/IntersectionObserver.h"
 #include "platform/heap/Heap.h"
 #include "platform/heap/Member.h"
 
 namespace blink {
 
 class Element;
 
 // ElementVisibilityObserver is a helper class to be used to track the
 // visibility of an Element in the viewport. Creating an
 // ElementVisibilityObserver is a no-op with regards to CPU cycle. The observing
-// has be started by calling |start()| and can be stopped with |stop()|.
+// has be started by calling |start()| can be stopped with |stop()|.
 // When creating an instance, the caller will have to pass a callback taking
 // a boolean as an argument. The boolean will be the new visibility state.
 // The ElementVisibilityObserver is implemented on top of IntersectionObserver.
 // It is a layer meant to simplify the usage for C++ Blink code checking for the
 // visibility of an element.
-class CORE_EXPORT ElementVisibilityObserver final
+class CORE_EXPORT ElementVisibilityObserver
     : public GarbageCollectedFinalized<ElementVisibilityObserver> {
   WTF_MAKE_NONCOPYABLE(ElementVisibilityObserver);
 
  public:
   using VisibilityCallback = Function<void(bool), WTF::SameThreadAffinity>;
 
   ElementVisibilityObserver(Element*, std::unique_ptr<VisibilityCallback>);
   virtual ~ElementVisibilityObserver();
 
   void start();
   void stop();
 
   void deliverObservationsForTesting();
 
+  float ratio() { return m_ratio; }
+
   DECLARE_VIRTUAL_TRACE();
 
+ protected:
+  void startObserveElementViewportRatio();

[miu, 2016/11/09 22:02:07]

Instead of the separate method, you should make start() a virtual method and
override it in the RatioObserver subclass.

[xjz, 2016/11/11 01:07:29]

Done.

+
  private:
   class ElementVisibilityCallback;
 
   void onVisibilityChanged(
       const HeapVector<Member<IntersectionObserverEntry>>&);
+  void startInternal(bool isObservingElementViewportRatio);
 
   Member<Element> m_element;
   Member<IntersectionObserver> m_intersectionObserver;
   std::unique_ptr<VisibilityCallback> m_callback;
+  float m_ratio = 0;
+};
+
+// ElementViewportRatioObserver monitors both the visibilty of the element and
+// the element/viewport ratio. When creating an instance, the caller will have
+// to pass a callback taking a boolean as an argument. The boolean will be the
+// new visibility state. Caller can call ratio() at any time to get current
+// element/viewport ratio.
+class ElementViewportRatioObserver : public ElementVisibilityObserver {
+  WTF_MAKE_NONCOPYABLE(ElementViewportRatioObserver);
+
+ public:
+  ElementViewportRatioObserver(Element*, std::unique_ptr<VisibilityCallback>);

[miu, 2016/11/09 22:02:07]

Reading this subclass's usage comments, it seems like a lot of burden is being
placed on the client code that uses this class. Therefore, how about we add:

  using ChangeCallback = Function<void(float), WTF::SameThreadAffinity>;

...and the constructor becomes:

  ElementViewportRatioObserver(Element*, std::unique_ptr<ChangeCallback>);

...and that way the client code only has to provide a callback that takes a
float argument (the viewport intersection ratio).

[xjz, 2016/11/11 01:07:29]

Done.

+  void start();
 };
 
 }  // namespace blink
 
 #endif  // ElementVisibilityObserver_h