ResourceLoadPriority FrameFetchContext::modifyPriorityForExperiments(ResourceLoadPriority priority, Resource::Type type, const FetchRequest& request, ResourcePriority::VisibilityStatus visibility)
{
    // An image fetch is used to distinguish between "early" and "late" scripts in a document
    if (type == Resource::Image)
        m_imageFetched = true;

    // If Settings is null, we can't verify any experiments are in force.
    if (!frame()->settings())
        return priority;

    // If enabled, drop the priority of all resources in a subframe.
    if (frame()->settings()->lowPriorityIframes() && !frame()->isMainFrame())
        return ResourceLoadPriorityVeryLow;

    // Async/Defer scripts.
    if (type == Resource::Script && FetchRequest::LazyLoad == request.defer())
        return frame()->settings()->fetchIncreaseAsyncScriptPriority() ? ResourceLoadPriorityMedium : ResourceLoadPriorityLow;

    // Runtime experiment that change how we prioritize resources.
    // The toggles do not depend on each other and can be flipped individually
    // though the cumulative result will depend on the interaction between them.
    // Background doc: https://docs.google.com/document/d/1bCDuq9H1ih9iNjgzyAL0gpwNFiEP4TZS-YLRp_RuMlc/edit?usp=sharing

    // Increases the priorities for CSS, Scripts, Fonts and Images all by one level
    // and parser-blocking scripts and visible images by 2.
    // This is used in conjunction with logic on the Chrome side to raise the threshold
    // of "layout-blocking" resources and provide a boost to resources that are needed
    // as soon as possible for something currently on the screen.
    int modifiedPriority = static_cast<int>(priority);
    if (frame()->settings()->fetchIncreasePriorities()) {
        if (type == Resource::CSSStyleSheet || type == Resource::Script || type == Resource::Font || type == Resource::Image)
            modifiedPriority++;
    }

    // Always give visible resources a bump, and an additional bump if generally increasing priorities.
    if (visibility == ResourcePriority::Visible) {
        modifiedPriority++;
        if (frame()->settings()->fetchIncreasePriorities())
            modifiedPriority++;
    }

    if (frame()->settings()->fetchIncreaseFontPriority() && type == Resource::Font)
        modifiedPriority++;

    if (type == Resource::Script) {
        // Reduce the priority of late-body scripts.
        if (frame()->settings()->fetchDeferLateScripts() && request.forPreload() && m_imageFetched)
            modifiedPriority--;
        // Parser-blocking scripts.
        if (frame()->settings()->fetchIncreasePriorities() && !request.forPreload())
            modifiedPriority++;
    }

    // Clamp priority
    modifiedPriority = std::min(static_cast<int>(ResourceLoadPriorityHighest), std::max(static_cast<int>(ResourceLoadPriorityLowest), modifiedPriority));
    return static_cast<ResourceLoadPriority>(modifiedPriority);
}
Ejemplo n.º 2
0
ResourcePtr<ImageResource> ImageResource::fetch(FetchRequest& request, ResourceFetcher* fetcher)
{
    if (request.resourceRequest().requestContext() == WebURLRequest::RequestContextUnspecified)
        request.mutableResourceRequest().setRequestContext(WebURLRequest::RequestContextImage);
    if (fetcher->context().pageDismissalEventBeingDispatched()) {
        KURL requestURL = request.resourceRequest().url();
        if (requestURL.isValid() && fetcher->context().canRequest(Resource::Image, request.resourceRequest(), requestURL, request.options(), request.forPreload(), request.originRestriction()))
            fetcher->context().sendImagePing(requestURL);
        return 0;
    }

    if (fetcher->clientDefersImage(request.resourceRequest().url()))
        request.setDefer(FetchRequest::DeferredByClient);

    return toImageResource(fetcher->requestResource(request, ImageResourceFactory()));
}