Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageElement.cpp

Index: Source/core/html/HTMLImageElement.cpp
diff --git a/Source/core/html/HTMLImageElement.cpp b/Source/core/html/HTMLImageElement.cpp
index 11dbf5900f5bfb702b0ce14706a5d21970ab80a1..92c21f2dfee2a94af57fc5615c02fe47657c1bbf 100644
--- a/Source/core/html/HTMLImageElement.cpp
+++ b/Source/core/html/HTMLImageElement.cpp
@@ -37,11 +37,13 @@
 #include "core/html/HTMLAnchorElement.h"
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLFormElement.h"
+#include "core/html/HTMLImageFallbackHelper.h"
 #include "core/html/HTMLSourceElement.h"
 #include "core/html/canvas/CanvasRenderingContext.h"
 #include "core/html/parser/HTMLParserIdioms.h"
 #include "core/html/parser/HTMLSrcsetParser.h"
 #include "core/inspector/ConsoleMessage.h"
+#include "core/rendering/RenderBlockFlow.h"
 #include "core/rendering/RenderImage.h"
 #include "platform/MIMETypeRegistry.h"
 #include "platform/RuntimeEnabledFeatures.h"
@@ -79,14 +81,18 @@ private:
 HTMLImageElement::HTMLImageElement(Document& document, HTMLFormElement* form, bool createdByParser)
     : HTMLElement(imgTag, document)
     , m_imageLoader(HTMLImageLoader::create(this))
+    , m_altTextNode(0)
     , m_compositeOperator(CompositeSourceOver)
     , m_imageDevicePixelRatio(1.0f)
     , m_formWasSetByParser(false)
     , m_elementCreatedByParser(createdByParser)
     , m_intrinsicSizingViewportDependant(false)
     , m_effectiveSizeViewportDependant(false)
+    , m_useFallbackContent(false)
+    , m_isFallbackImage(false)
 {
     ScriptWrappable::init(this);
+    setHasCustomStyleCallbacks();
     if (form && form->inDocument()) {
 #if ENABLE(OILPAN)
         m_form = form;
@@ -237,9 +243,12 @@ void HTMLImageElement::setBestFitURLAndDPRFromImageCandidate(const ImageCandidat
 
 void HTMLImageElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
-    if (name == altAttr) {
-        if (renderer() && renderer()->isImage())
-            toRenderImage(renderer())->updateAltText();
+    if (name == altAttr || name == titleAttr) {
+        if (Text* text = altTextNode()) {
+            String alt = altText();
+            if (text->data() != alt)
+                text->setData(alt);
+        }
     } else if (name == srcAttr || name == srcsetAttr || name == sizesAttr) {
         selectSourceURL(ImageLoader::UpdateIgnorePreviousError);
     } else if (name == usemapAttr) {
@@ -255,7 +264,7 @@ void HTMLImageElement::parseAttribute(const QualifiedName& name, const AtomicStr
     }
 }
 
-const AtomicString& HTMLImageElement::altText() const
+String HTMLImageElement::altText() const
 {
     // lets figure out the alt text.. magic stuff
     // http://www.w3.org/TR/1998/REC-html40-19980424/appendix/notes.html#altgen
@@ -318,6 +327,9 @@ RenderObject* HTMLImageElement::createRenderer(RenderStyle* style)
     if (style->hasContent())
         return RenderObject::createObject(this, style);
 
+    if (m_useFallbackContent)
+        return new RenderBlockFlow(this);
+
     RenderImage* image = new RenderImage(this);
     image->setImageResource(RenderImageResource::create());
     image->setImageDevicePixelRatio(m_imageDevicePixelRatio);
@@ -342,13 +354,9 @@ void HTMLImageElement::attach(const AttachContext& context)
         if (renderImageResource->hasImage())
             return;
 
-        // If we have no image at all because we have no src attribute, set
-        // image height and width for the alt text instead.
         if (!imageLoader().image() && !renderImageResource->cachedImage())
-            renderImage->setImageSizeForAltText();
-        else
-            renderImageResource->setImageResource(imageLoader().image());
-
+            return;
+        renderImageResource->setImageResource(imageLoader().image());
     }
 }
 
@@ -648,6 +656,11 @@ void HTMLImageElement::selectSourceURL(ImageLoader::UpdateFromElementBehavior be
         document().mediaQueryMatcher().addViewportListener(m_listener);
     }
     imageLoader().updateFromElement(behavior);
+
+    if (imageLoader().image() || (imageLoader().hasPendingActivity() && !imageSourceURL().isEmpty()))
+        ensurePrimaryContent();
+    else
+        ensureFallbackContent();
 }
 
 const KURL& HTMLImageElement::sourceURL() const
@@ -655,4 +668,43 @@ const KURL& HTMLImageElement::sourceURL() const
     return cachedImage()->response().url();
 }
 
+void HTMLImageElement::didAddUserAgentShadowRoot(ShadowRoot&)
+{
+    m_altTextNode = createAltTextShadowTree(*this);
+}
+
+void HTMLImageElement::ensureFallbackContent()
+{
+    if (m_useFallbackContent || m_isFallbackImage)
+        return;
+    setUseFallbackContent();
+    reattachFallbackContent();
+}
+
+void HTMLImageElement::ensurePrimaryContent()
+{
+    if (!m_useFallbackContent)
+        return;
+    m_useFallbackContent = false;
+    reattachFallbackContent();
+}
+
+void HTMLImageElement::reattachFallbackContent()
+{
+    // This can happen inside of attach() in the middle of a recalcStyle so we need to
+    // reattach synchronously here.
+    if (document().inStyleRecalc())
+        reattach();
+    else
+        lazyReattachIfAttached();
+}
+
+PassRefPtr<RenderStyle> HTMLImageElement::customStyleForRenderer()
+{
+    RefPtr<RenderStyle> newStyle = originalStyleForRenderer();
+
+    if (!m_useFallbackContent)
+        return newStyle;
+    return customStyleForAltText(*this, newStyle);
+}
 }