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;
 
+// Base class for ElementVisibilityObserver and ElementViewportRatioObserver.
+class VisibilityObserverBase
+    : public GarbageCollectedFinalized<VisibilityObserverBase> {
+  WTF_MAKE_NONCOPYABLE(VisibilityObserverBase);
+
+ public:
+  VisibilityObserverBase(Element*);

[miu, 2016/11/11 08:18:33]

Add 'explicit' keyword.

[xjz, 2016/11/11 17:57:48]

Done.

+  virtual ~VisibilityObserverBase();
+
+  virtual void start() = 0;
+  void stop();
+
+  DECLARE_VIRTUAL_TRACE();
+
+ protected:
+  Member<Element> m_element;
+  Member<IntersectionObserver> m_intersectionObserver;
+};
+
 // 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()|.
 // 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
-    : public GarbageCollectedFinalized<ElementVisibilityObserver> {
+    : public VisibilityObserverBase {
   WTF_MAKE_NONCOPYABLE(ElementVisibilityObserver);
 
  public:
   using VisibilityCallback = Function<void(bool), WTF::SameThreadAffinity>;
 
   ElementVisibilityObserver(Element*, std::unique_ptr<VisibilityCallback>);
-  virtual ~ElementVisibilityObserver();
+  ~ElementVisibilityObserver() override;

[miu, 2016/11/11 08:18:33]

Since these subclasses are final, should all the 'override' keywords be 'final'
instead?

[xjz, 2016/11/11 17:57:48]

Done.

 
-  void start();
-  void stop();
+  void start() override;
 
   void deliverObservationsForTesting();
 
-  DECLARE_VIRTUAL_TRACE();
-
  private:
   class ElementVisibilityCallback;
 
   void onVisibilityChanged(
       const HeapVector<Member<IntersectionObserverEntry>>&);
+  std::unique_ptr<VisibilityCallback> m_callback;
+};
 
-  Member<Element> m_element;
-  Member<IntersectionObserver> m_intersectionObserver;
-  std::unique_ptr<VisibilityCallback> m_callback;
+// ElementViewportRatioObserver monitors the element/viewport ratio. Creating an
+// ElementViewportRatioObserver is a no-op with regards to CPU cycle. The
+// observing has be started by calling |start()| and can be stopped with
+// |stop()|. When creating an instance, the caller will have to pass a callback
+// taking a float as an argument. The float is the new element/viewport ratio.
+class ElementViewportRatioObserver final : public VisibilityObserverBase {
+  WTF_MAKE_NONCOPYABLE(ElementViewportRatioObserver);
+
+ public:
+  using RatioChangeCallback = Function<void(float), WTF::SameThreadAffinity>;
+
+  ElementViewportRatioObserver(Element*, std::unique_ptr<RatioChangeCallback>);
+  ~ElementViewportRatioObserver() override;
+
+  void start() override;
+
+ private:
+  void onVisibilityChanged(
+      const HeapVector<Member<IntersectionObserverEntry>>&);
+  std::unique_ptr<RatioChangeCallback> m_callback;
 };
 
 }  // namespace blink
 
 #endif  // ElementVisibilityObserver_h