Use a ResizeObserver to determine default font-size for text tracks

third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp

Index: third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
diff --git a/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp b/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
index f5bf1fcc1dbac936fff2b804114640bf8c961ae0..d0ee2e8408d812e68783d51dec55610402a582e7 100644
--- a/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
+++ b/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
@@ -29,24 +29,94 @@
 
 #include "core/html/track/TextTrackContainer.h"
 
+#include "core/dom/ResizeObserver.h"
+#include "core/dom/ResizeObserverCallback.h"
+#include "core/dom/ResizeObserverEntry.h"
 #include "core/html/HTMLVideoElement.h"
 #include "core/html/track/CueTimeline.h"
-#include "core/layout/LayoutTextTrackContainer.h"
+#include "core/layout/LayoutBlockFlow.h"
+#include "core/layout/LayoutVideo.h"
 
 namespace blink {
 
+namespace {
+
+class VideoElementResizeCallback final : public ResizeObserverCallback {
+ public:
+  VideoElementResizeCallback(TextTrackContainer& container)
+      : ResizeObserverCallback(), m_textTrackContainer(container) {}
+
+  void handleEvent(const HeapVector<Member<ResizeObserverEntry>>& entries,
+                   ResizeObserver*) override {
+    DCHECK_EQ(entries.size(), 1u);
+    DCHECK(isHTMLVideoElement(entries[0]->target()));
+    m_textTrackContainer->updateDefaultFontSize(
+        entries[0]->target()->layoutObject());
+  }
+
+  DEFINE_INLINE_VIRTUAL_TRACE() {
+    visitor->trace(m_textTrackContainer);
+    ResizeObserverCallback::trace(visitor);
+  }
+
+ private:
+  Member<TextTrackContainer> m_textTrackContainer;
+};
+}

[mlamouri (slow - plz ping), 2017/04/08 12:57:57]

style: leave empty line before end of namespace and add a end of namespace
comment (// anonymous namespace).

[fs, 2017/04/10 13:36:27]

Done.

+
 TextTrackContainer::TextTrackContainer(Document& document)
-    : HTMLDivElement(document) {}
+    : HTMLDivElement(document), m_defaultFontSize(0) {}
 
-TextTrackContainer* TextTrackContainer::create(Document& document) {
-  TextTrackContainer* element = new TextTrackContainer(document);
+DEFINE_TRACE(TextTrackContainer) {
+  visitor->trace(m_videoSizeObserver);
+  HTMLDivElement::trace(visitor);
+}
+
+TextTrackContainer* TextTrackContainer::create(HTMLMediaElement& mediaElement) {
+  TextTrackContainer* element = new TextTrackContainer(mediaElement.document());
   element->setShadowPseudoId(
       AtomicString("-webkit-media-text-track-container"));
+  if (isHTMLVideoElement(mediaElement))
+    element->observeSizeChanges(mediaElement);
   return element;
 }
 
 LayoutObject* TextTrackContainer::createLayoutObject(const ComputedStyle&) {
-  return new LayoutTextTrackContainer(this);
+  return new LayoutBlockFlow(this);
+}
+
+void TextTrackContainer::observeSizeChanges(Element& element) {
+  m_videoSizeObserver =
+      ResizeObserver::create(document(), new VideoElementResizeCallback(*this));
+  m_videoSizeObserver->observe(&element);
+}
+
+void TextTrackContainer::updateDefaultFontSize(
+    LayoutObject* mediaLayoutObject) {
+  if (!mediaLayoutObject || !mediaLayoutObject->isVideo())
+    return;
+  // FIXME: The video size is used to calculate the font size (a workaround
+  // for lack of per-spec vh/vw support) but the whole media element is used
+  // for cue rendering. This is inconsistent. See also the somewhat related
+  // spec bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28105
+  LayoutSize videoSize =
+      toLayoutVideo(*mediaLayoutObject).replacedContentRect().size();
+  LayoutUnit smallestDimension =
+      std::min(videoSize.height(), videoSize.width());
+  float fontSize = smallestDimension * 0.05f;
+
+  // Avoid excessive FP precision issue.
+  // C11 5.2.4.2.2:9 requires assignment and cast to remove extra precision, but
+  // the behavior is currently not portable. fontSize may have precision higher
+  // than m_fontSize thus straight comparison can fail despite they cast to the
+  // same float value.
+  volatile float& currentFontSize = m_defaultFontSize;
+  float oldFontSize = currentFontSize;
+  currentFontSize = fontSize;
+  if (currentFontSize == oldFontSize)
+    return;
+  setInlineStyleProperty(CSSPropertyFontSize, m_defaultFontSize,
+                         CSSPrimitiveValue::UnitType::Pixels);
 }
 
 void TextTrackContainer::updateDisplay(HTMLMediaElement& mediaElement,