Start autoplay muted videos with autoplay attribute when they are visible.

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

+// 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.
+
+#include "core/dom/ElementVisibilityObserver.h"
+
+#include "core/dom/Element.h"
+#include "core/dom/IntersectionObserverCallback.h"
+#include "core/frame/LocalFrame.h"
+
+namespace blink {
+
+class ElementVisibilityObserver::ElementVisibilityCallback : public IntersectionObserverCallback {

[esprehn, 2016/06/14 16:01:18]

move into IntersectionObserver constructor for C++ function callbacks

+public:
+    ElementVisibilityCallback(ElementVisibilityObserver* observer)
+        : m_observer(observer)
+    {}
+
+    void handleEvent(const HeapVector<Member<IntersectionObserverEntry>>& entries, IntersectionObserver&) override
+    {
+        DCHECK_EQ(entries.size(), 1u);
+
+        bool isVisible = entries[0]->intersectionRatio() > 0.f;
+        (*m_observer->m_callback.get())(isVisible);
+    }
+
+    ExecutionContext* getExecutionContext() const override
+    {
+        return &m_observer->m_element->document();
+    }
+
+    DEFINE_INLINE_TRACE()
+    {
+        IntersectionObserverCallback::trace(visitor);
+        visitor->trace(m_observer);
+    }
+
+private:
+    Member<ElementVisibilityObserver> m_observer;
+};
+
+ElementVisibilityObserver::ElementVisibilityObserver(Element* element, std::unique_ptr<VisibilityCallback> callback)
+    : m_element(element)
+    , m_callback(std::move(callback))
+    , m_intersectionObserverCallback(new ElementVisibilityCallback(this))
+{
+}
+
+ElementVisibilityObserver::~ElementVisibilityObserver() = default;
+
+void ElementVisibilityObserver::start()
+{
+    DCHECK(!m_intersectionObserver);
+
+    Frame* mainFrame = m_element->document().frame()->tree().top();
+    if (!mainFrame || !mainFrame->isLocalFrame())
+        return;
+
+    Node* root = toLocalFrame(mainFrame)->document();
+    m_intersectionObserver = new IntersectionObserver(*m_intersectionObserverCallback, *root, Vector<Length>(), Vector<float>({0.f}));

[esprehn, 2016/06/14 16:01:18]

Please add IntersectionObserver::create() or constructor for C++ that takes a
WTF::Function<void(Vector<IntersectionRecord>)>

+    m_intersectionObserver->observe(m_element);
+}
+
+void ElementVisibilityObserver::stop()
+{
+    DCHECK(m_intersectionObserver);
+
+    m_intersectionObserver->unobserve(m_element);
+    m_intersectionObserver = nullptr;
+}
+
+DEFINE_TRACE(ElementVisibilityObserver)
+{
+    visitor->trace(m_element);
+    visitor->trace(m_intersectionObserver);
+    visitor->trace(m_intersectionObserverCallback);
+}
+
+} // namespace blink