Use Shadow DOM to display fallback content for images

Source/core/html/HTMLImageElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 39 matching lines @@
 
     int width(bool ignorePendingStylesheets = false);
     int height(bool ignorePendingStylesheets = false);
 
     int naturalWidth() const;
     int naturalHeight() const;
     const String& currentSrc() const;
 
     bool isServerMap() const;
 
-    const AtomicString& altText() const;
+    virtual String altText() const OVERRIDE FINAL;
 
     CompositeOperator compositeOperator() const { return m_compositeOperator; }
 
     ImageResource* cachedImage() const { return imageLoader().image(); }
     void setImageResource(ImageResource* i) { imageLoader().setImage(i); };
 
     void setLoadingImageDocument() { imageLoader().setLoadingImageDocument(); }
 
     void setHeight(int);
 
@@ skipping 11 matching lines @@
 
     virtual bool canContainRangeEndPoint() const OVERRIDE { return false; }
 
     void addClient(ImageLoaderClient* client) { imageLoader().addClient(client); }
     void removeClient(ImageLoaderClient* client) { imageLoader().removeClient(client); }
 
     virtual const AtomicString imageSourceURL() const OVERRIDE;
 
     virtual HTMLFormElement* formOwner() const OVERRIDE;
     void formRemovedFromTree(const Node& formRoot);
+    virtual void ensureFallbackContent() OVERRIDE FINAL;
+    virtual void ensurePrimaryContent() OVERRIDE FINAL;
 
     // CanvasImageSourceImplementations
     virtual PassRefPtr<Image> getSourceImageForCanvas(SourceImageMode, SourceImageStatus*) const;
     virtual bool wouldTaintOrigin(SecurityOrigin*) const OVERRIDE;
     virtual FloatSize sourceSize() const OVERRIDE;
     virtual FloatSize defaultDestinationSize() const OVERRIDE;
     virtual const KURL& sourceURL() const OVERRIDE;
 
     // public so that HTMLPictureElement can call this as well.
     void selectSourceURL(ImageLoader::UpdateFromElementBehavior);
+    void reattachFallbackContent();
+    void setUseFallbackContent() { m_useFallbackContent = true; ensureUserAgentShadowRoot(); }
+    void setIsFallbackImage() { m_isFallbackImage = true; }
+
 protected:
     explicit HTMLImageElement(Document&, HTMLFormElement* = 0, bool createdByParser = false);
 
     virtual void didMoveToNewDocument(Document& oldDocument) OVERRIDE;
+    virtual bool useFallbackContent() const { return m_useFallbackContent; }
 
+    virtual void didAddUserAgentShadowRoot(ShadowRoot&) OVERRIDE;
+    virtual PassRefPtr<RenderStyle> customStyleForRenderer() OVERRIDE;
 private:
     virtual bool areAuthorShadowsAllowed() const OVERRIDE { return false; }
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
     virtual bool isPresentationAttribute(const QualifiedName&) const OVERRIDE;
     virtual void collectStyleForPresentationAttribute(const QualifiedName&, const AtomicString&, MutableStylePropertySet*) OVERRIDE;
 
     virtual void attach(const AttachContext& = AttachContext()) OVERRIDE;
     virtual RenderObject* createRenderer(RenderStyle*) OVERRIDE;
 
@@ skipping 11 matching lines @@
     virtual bool shouldRegisterAsExtraNamedItem() const OVERRIDE { return true; }
     virtual bool isInteractiveContent() const OVERRIDE;
     virtual Image* imageContents() OVERRIDE;
 
     void resetFormOwner();
     ImageCandidate findBestFitImageFromPictureParent();
     void setBestFitURLAndDPRFromImageCandidate(const ImageCandidate&);
     HTMLImageLoader& imageLoader() const { return *m_imageLoader; }
     void notifyViewportChanged();
     void createMediaQueryListIfDoesNotExist();
+    Text* altTextNode() { return m_altTextNode; }
 
     OwnPtrWillBeMember<HTMLImageLoader> m_imageLoader;
     RefPtrWillBeMember<ViewportChangeListener> m_listener;
 #if ENABLE(OILPAN)
     Member<HTMLFormElement> m_form;
 #else
     WeakPtr<HTMLFormElement> m_form;
 #endif
+    Text* m_altTextNode;

[esprehn, 2014/09/09 19:52:41]

There's no reason for this, just do
userAgentShadowRoot()->getElementById("alt-text") whenever you need to get it,
then use setTextContent to update it.

     CompositeOperator m_compositeOperator;
     AtomicString m_bestFitImageURL;
     float m_imageDevicePixelRatio;
     unsigned m_formWasSetByParser : 1;
     unsigned m_elementCreatedByParser : 1;
     // Intrinsic sizing is viewport dependant if the 'w' descriptor was used for the picked resource.
     unsigned m_intrinsicSizingViewportDependant : 1;
     // Effective size is viewport dependant if the sizes attribute's effective size used v* length units.
     unsigned m_effectiveSizeViewportDependant : 1;
+    unsigned m_useFallbackContent : 1;
+    unsigned m_isFallbackImage : 1;
 };
 
 } // namespace blink
 
 #endif // HTMLImageElement_h