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);
}
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()));
}