自学教程：C++ useFallbackContent函数代码示例

bool HTMLPlugInImageElement::willRecalcStyle(StyleChange){    // FIXME: Why is this necessary?  Manual re-attach is almost always wrong.    if (!useFallbackContent() && needsWidgetUpdate() && renderer() && !isImageType())        reattach();    return true;}

RenderObject* HTMLPlugInImageElement::createRenderer(RenderArena* arena, RenderStyle* style){    // Once a PlugIn Element creates its renderer, it needs to be told when the Document goes    // inactive or reactivates so it can clear the renderer before going into the page cache.    if (!m_needsDocumentActivationCallbacks) {        m_needsDocumentActivationCallbacks = true;        document()->registerForPageCacheSuspensionCallbacks(this);    }    if (displayState() == DisplayingSnapshot) {        RenderSnapshottedPlugIn* renderSnapshottedPlugIn = new (arena) RenderSnapshottedPlugIn(this);        renderSnapshottedPlugIn->updateSnapshot(m_snapshotImage);        return renderSnapshottedPlugIn;    }    // Fallback content breaks the DOM->Renderer class relationship of this    // class and all superclasses because createObject won't necessarily    // return a RenderEmbeddedObject, RenderPart or even RenderWidget.    if (useFallbackContent())        return RenderObject::createObject(this, style);    if (isImageType()) {        RenderImage* image = new (arena) RenderImage(this);        image->setImageResource(RenderImageResource::create());        return image;    }    return new (arena) RenderEmbeddedObject(this);}

void HTMLPlugInImageElement::recalcStyle(StyleChange ch){    // FIXME: Why is this necessary?  Manual re-attach is almost always wrong.    if (!useFallbackContent() && needsWidgetUpdate() && renderer() && !isImageType())        reattach();    HTMLPlugInElement::recalcStyle(ch);}

void HTMLObjectElement::renderFallbackContent(){    if (useFallbackContent())        return;        if (!inDocument())        return;    setNeedsStyleRecalc(ReconstructRenderTree);    // Before we give up and use fallback content, check to see if this is a MIME type issue.    auto* loader = imageLoader();    if (loader && loader->image() && loader->image()->status() != CachedResource::LoadError) {        m_serviceType = loader->image()->response().mimeType();        if (!isImageType()) {            // If we don't think we have an image type anymore, then clear the image from the loader.            loader->setImage(nullptr);            return;        }    }    m_useFallbackContent = true;    // This was added to keep Acid 2 non-flaky. A style recalc is required to make fallback resources load.    // Without forcing, this may happen after all the other resources have been loaded and the document is already    // considered complete. FIXME: Would be better to address this with incrementLoadEventDelayCount instead    // or disentangle loading from style entirely.    document().updateStyleIfNeeded();}

bool HTMLPlugInElement::supportsFocus() const{    if (HTMLFrameOwnerElement::supportsFocus())        return true;    if (useFallbackContent() || !renderer() || !renderer()->isEmbeddedObject())        return false;    return !toRenderEmbeddedObject(renderer())->isPluginUnavailable();}

bool HTMLPlugInElement::supportsFocus() const{    if (HTMLFrameOwnerElement::supportsFocus())        return true;    if (useFallbackContent() || !is<RenderEmbeddedObject>(renderer()))        return false;    return !downcast<RenderEmbeddedObject>(*renderer()).isPluginUnavailable();}

void HTMLPlugInImageElement::finishParsingChildren(){    HTMLPlugInElement::finishParsingChildren();    if (useFallbackContent())        return;        setNeedsWidgetUpdate(true);    if (inDocument())        setNeedsStyleRecalc();    }

void HTMLPlugInImageElement::detach(){    // FIXME: Because of the insanity that is HTMLPlugInImageElement::recalcStyle,    // we can end up detaching during an attach() call, before we even have a    // renderer.  In that case, don't mark the widget for update.    if (attached() && renderer() && !useFallbackContent())        // Update the widget the next time we attach (detaching destroys the plugin).        setNeedsWidgetUpdate(true);    HTMLPlugInElement::detach();}

void HTMLPlugInImageElement::updateWidgetIfNecessary(){    document()->updateStyleIfNeeded();    if (!needsWidgetUpdate() || useFallbackContent() || isImageType())        return;    if (!renderEmbeddedObject() || renderEmbeddedObject()->pluginCrashedOrWasMissing())        return;    updateWidget(CreateOnlyNonNetscapePlugins);}

RenderObject* HTMLPlugInImageElement::createRenderer(RenderArena* arena, RenderStyle* style){    // Fallback content breaks the DOM->Renderer class relationship of this    // class and all superclasses because createObject won't necessarily    // return a RenderEmbeddedObject, RenderPart or even RenderWidget.    if (useFallbackContent())        return RenderObject::createObject(this, style);    if (isImageType()) {        RenderImage* image = new (arena) RenderImage(this);        image->setImageResource(RenderImageResource::create());        return image;    }    return new (arena) RenderEmbeddedObject(this);}

void HTMLPlugInImageElement::attach(){    bool isImage = isImageType();        if (!isImage)        queuePostAttachCallback(&HTMLPlugInImageElement::updateWidgetCallback, this);    HTMLPlugInElement::attach();    if (isImage && renderer() && !useFallbackContent()) {        if (!m_imageLoader)            m_imageLoader = adoptPtr(new HTMLImageLoader(this));        m_imageLoader->updateFromElement();    }}