QT之webkit 分析
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WebKit由三个模块组成：JavaScriptCore、WebCore 和 WebKit。WebKit作为了整个项目的名称。
其目录结构：（未校准）
WebCore
¨Page与外框相关的内容(Frame,Page,History,Focus,Window)
¨Loader加载资源及Cache
¨HTML-DOM HTML内容及解析
¨DOM- DOM CORE内容
¨XML- XML内容及解析
¨Render-排版功能
¨CSS-DOM CSS内容
¨Binding-DOM与JavascriptCore绑定的功能
¨Editing-所有与编辑相关的功能
JavascriptCore-javas
¨API-基本javas
¨Binding与其它功能绑定的功能,如:DOM,C,JNI
¨DerviedSource自动产生的代码
¨ForwordHeads头文件,无实际意义
¨PCRE-Perl-Compatible Regular Expressions
¨KJS-Javas
¨WTF-KDE的C++模板库
Unicode unicode 库
Tools tools库
CURL-url 客户端传输库
PlatForm- 与平台相关的功能,如图形图像,字体,Unicode, IO,输入法等.
在QT自带的例子中，有WebKit相关的例子。我选中previewer作为分析的项目。
QT之webkit 分析（二）
我是通过输入URL，进行跟踪分析的。下面是断点保存的调用堆栈，暂存资料。
QtWebKitd4.dll!WebCore::MainResourceLoader::loadNow(WebCore::ResourceRequest & r={...}) 行458 C++
QtWebKitd4.dll!WebCore::MainResourceLoader::load(const WebCore::ResourceRequest & r={...}, const WebCore::SubstituteData & substituteData={...}) 行494 + 0x12 字节 C++
QtWebKitd4.dll!WebCore::DocumentLoader::startLoadingMainResource(unsigned long identifier=0x00000004) 行807 + 0x32 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::continueLoadAfterWillSubmitForm(WebCore::PolicyAction __formal=PolicyUse) 行3274 + 0x16 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::continueLoadAfterNavigationPolicy(const WebCore::ResourceRequest & __formal={...}, WTF::PassRefPtr<WebCore::FormState> formState={...}, bool shouldContinue=true) 行3968 C++
QtWebKitd4.dll!WebCore::FrameLoader::callContinueLoadAfterNavigationPolicy(void * argument=0x01d424e8, const WebCore::ResourceRequest & request={...}, WTF::PassRefPtr<WebCore::FormState> formState={...}, bool shouldContinue=true) 行3906 C++
QtWebKitd4.dll!WebCore::PolicyCheck::call(bool shouldContinue=true) 行4963 + 0x3b 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::continueAfterNavigationPolicy(WebCore::PolicyAction policy=PolicyUse) 行3899 C++
QtWebKitd4.dll!WebCore::FrameLoaderClientQt::slotCallPolicyFunction(int act
QtWebKitd4.dll!WebCore::FrameLoaderClientQt::dispatchDecidePolicyForNavigationAction(void (WebCore::PolicyAction)* function=0x10018f0c, const WebCore::NavigationAction & act
QtWebKitd4.dll!WebCore::FrameLoader::checkNavigationPolicy(const WebCore::ResourceRequest & request={...}, WebCore::DocumentLoader * loader=0x00f63ff8, WTF::PassRefPtr<WebCore::FormState> formState={...}, void (void *, const WebCore::ResourceRequest &, WTF::PassRefPtr<WebCore::FormState>, bool)* function=0x1004e661, void * argument=0x01d424e8) 行3868 C++
QtWebKitd4.dll!WebCore::FrameLoader::loadWithDocumentLoader(WebCore::DocumentLoader * loader=0x00f63ff8, WebCore::FrameLoadType type=FrameLoadTypeRedirectWithLockedHistory, WTF::PassRefPtr<WebCore::FormState> prpFormState={...}) 行2291 C++
QtWebKitd4.dll!WebCore::FrameLoader::loadWithNavigationAction(const WebCore::ResourceRequest & request={...}, const WebCore::NavigationAction & act
QtWebKitd4.dll!WebCore::FrameLoader::loadURL(const WebCore::KURL & newURL={...}, const WebCore::String & referrer={...}, const WebCore::String & frameName={...}, WebCore::FrameLoadType newLoadType=FrameLoadTypeRedirectWithLockedHistory, WebCore::Event * event=0x00000000, WTF::PassRefPtr<WebCore::FormState> prpFormState={...}) 行2174 C++
QtWebKitd4.dll!WebCore::FrameLoaderClientQt::createFrame(const WebCore::KURL & url={...}, const WebCore::String & name={...}, WebCore::HTMLFrameOwnerElement * ownerElement=0x00f681a0, const WebCore::String & referrer={...}, bool allowsScrolling=false, int marginWidth=0xffffffff, int marginHeight=0xffffffff) 行981 + 0x70 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::loadSubframe(WebCore::HTMLFrameOwnerElement * ownerElement=0x00f681a0, const WebCore::KURL & url={...}, const WebCore::String & name={...}, const WebCore::String & referrer={...}) 行472 + 0x74 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::requestFrame(WebCore::HTMLFrameOwnerElement * ownerElement=0x00f681a0, const WebCore::String & urlString={...}, const WebCore::AtomicString & frameName={...}) 行442 + 0x29 字节 C++
QtWebKitd4.dll!WebCore::HTMLFrameElementBase::openURL() 行105 C++
QtWebKitd4.dll!WebCore::HTMLFrameElementBase::setNameAndOpenURL() 行161 C++
QtWebKitd4.dll!WebCore::HTMLFrameElementBase::setNameAndOpenURLCallback(WebCore::Node * n=0x00f681a0) 行166 C++
QtWebKitd4.dll!WebCore::ContainerNode::dispatchPostAttachCallbacks() 行572 + 0x7 字节 C++
QtWebKitd4.dll!WebCore::ContainerNode::attach() 行587 C++
QtWebKitd4.dll!WebCore::Element::attach() 行648 C++
QtWebKitd4.dll!WebCore::HTMLFrameElementBase::attach() 行194 C++
QtWebKitd4.dll!WebCore::HTMLFrameElement::attach() 行67 C++
QtWebKitd4.dll!WebCore::HTMLParser::insertNode(WebCore::Node * n=0x00f681a0, bool flat=false) 行351 C++
QtWebKitd4.dll!WebCore::HTMLParser::parseToken(WebCore::Token * t=0x00f65fd0) 行256 + 0x19 字节 C++
> QtWebKitd4.dll!WebCore::HTMLTokenizer::processToken() 行1902 + 0x20 字节 C++
QtWebKitd4.dll!WebCore::HTMLTokenizer::parseTag(WebCore::SegmentedString & src={...}, WebCore::HTMLTokenizer::State state={...}) 行1484 + 0x12 字节 C++
QtWebKitd4.dll!WebCore::HTMLTokenizer::write(const WebCore::SegmentedString & str={...}, bool appendData=true) 行1730 + 0x23 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::write(const char * str=0x01d3f5c0, int len=0x000001df, bool flush=false) 行1039 + 0x23 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::addData(const char * bytes=0x01d3f5c0, int length=0x000001df) 行1891 C++
QtWebKitd4.dll!WebCore::FrameLoaderClientQt::committedLoad(WebCore::DocumentLoader * loader=0x00f881e0, const char * da
QtWebKitd4.dll!WebCore::FrameLoader::committedLoad(WebCore::DocumentLoader * loader=0x00f881e0, const char * da
QtWebKitd4.dll!WebCore::DocumentLoader::commitLoad(const char * da
QtWebKitd4.dll!WebCore::DocumentLoader::receivedData(const char * da
QtWebKitd4.dll!WebCore::FrameLoader::receivedData(const char * da
QtWebKitd4.dll!WebCore::MainResourceLoader::addData(const char * da
QtWebKitd4.dll!WebCore::ResourceLoader::didReceiveData(const char * da
QtWebKitd4.dll!WebCore::MainResourceLoader::didReceiveData(const char * da
QtWebKitd4.dll!WebCore::ResourceLoader::didReceiveData(WebCore::ResourceHandle * __formal=0x00fb9aa0, const char * da
QtWebKitd4.dll!WebCore::QNetworkReplyHandler::forwardData() 行341 C++
QtWebKitd4.dll!WebCore::QNetworkReplyHandler::qt_metacall(QMetaObject::Call _c=InvokeMetaMethod, int _id=0x00000002, void * * _a=0x00fba378) 行74 C++
QtCored4.dll!QMetaCallEvent::placeMetaCall(QObject * object=0x00f810d0) 行478 C++
QtCored4.dll!QObject::event(QEvent * e=0x01d3ee18) 行1102 + 0x14 字节 C++
QtGuid4.dll!QApplicationPrivate::notify_helper(QObject * receiver=0x00f810d0, QEvent * e=0x01d3ee18) 行4065 + 0x11 字节 C++
QtGuid4.dll!QApplication::notify(QObject * receiver=0x00f810d0, QEvent * e=0x01d3ee18) 行3605 + 0x10 字节 C++
QtCored4.dll!QCoreApplication::notifyInternal(QObject * receiver=0x00f810d0, QEvent * event=0x01d3ee18) 行610 + 0x15 字节 C++
QtCored4.dll!QCoreApplication::sendEvent(QObject * receiver=0x00f810d0, QEvent * event=0x01d3ee18) 行213 + 0x39 字节 C++
QtCored4.dll!QCoreApplicationPrivate::sendPostedEvents(QObject * receiver=0x00000000, int event_type=0x00000000, QThreadData * da
QtCored4.dll!QEventDispatcherWin32::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags={...}) 行679 + 0x10 字节 C++
QtGuid4.dll!QGuiEventDispatcherWin32::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags={...}) 行1182 + 0x15 字节 C++
QtCored4.dll!QEventLoop::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags={...}) 行150 C++
QtCored4.dll!QEventLoop::exec(QFlags<enum QEventLoop::ProcessEventsFlag> flags={...}) 行201 + 0x2d 字节 C++
QtCored4.dll!QCoreApplication::exec() 行888 + 0x15 字节 C++
QtGuid4.dll!QApplication::exec() 行3526 C++
previewer.exe!main(int argc=0x00000001, char * * argv=0x00e78e20) 行51 + 0x6 字节 C++
previewer.exe!WinMain(HINSTANCE__ * instance=0x00400000, HINSTANCE__ * prevInstance=0x00000000, char * __formal=0x001520d9, int cmdShow=0x00000001) 行137 + 0x12 字节 C++
previewer.exe!__tmainCRTStartup() 行574 + 0x35 字节 C
previewer.exe!WinMainCRTStartup() 行399 C
kernel32.dll!7c82f23b()
[下面的框架可能不正确和/或缺失，没有为 kernel32.dll 加载符号]
QT之webkit 分析（三）
分三个阶段对QWebView进行分析：初始化（获取数据）、HTML解析、页面显示。从QT自带的文档中可以知道：
QWebView -> QWebPage => QWebFrame（一个QWebPage含多个QWebFrame）
在界面中选择了Open URL，输入URL之后，调用的是：void MainWindow::openUrl()
void MainWindow::openUrl()
{
bool ok;
QString url = QInputDialog::getText(this, tr("Enter a URL"),
tr("URL:"), QLineEdit::Normal, "http://", &ok);
if (ok && !url.isEmpty()) {
centralWidget->webView->setUrl(url);
}
}
调用的是QWebView::setUrl()
void QWebView::setUrl(const QUrl &url)
{
page()->mainFrame()->setUrl(url);
}
其中page()是获取QWebPage指针，QWebPage::mainFrame()获取的是QWebFrame指针。
所以调用的是：QWebFrame::setUrl()
void QWebFrame::setUrl(const QUrl &url)
{
d->frame->loader()->begin(ensureAbsoluteUrl(url));
d->frame->loader()->end();
load(ensureAbsoluteUrl(url));
}
ensureAbsoluteUrl()函数作用是，确保URL是绝对URL（完整URL）。所谓相对URL是指没有输入http://或者https://等前缀的web地址。先看第一句的调用。其中隐含了从QUrl到KURL的变换。
void FrameLoader::begin(const KURL& url, bool dispatch, SecurityOrigin* origin)
{
// We need to take a reference to the security origin because |clear|
// might destroy the document that owns it.
RefPtr<SecurityOrigin> forcedSecurityOrigin = origin;
bool resetScripting = !(m_isDisplayingInitialEmptyDocument && m_frame->document() && m_frame->document()->securityOrigin()->isSecureTransitionTo(url));
clear(resetScripting, resetScripting); // 清除上一次的数据，为本次装载准备
if (resetScripting)
m_frame->script()->updatePlatformScriptObjects(); // 在Windows平台下，这是空函数
if (dispatch)
dispatchWindowObjectAvailable();
m_needsClear = true;
m_isComplete = false;
m_didCallImplicitClose = false;
m_isLoadingMainResource = true;
m_isDisplayingInitialEmptyDocument = m_creatingInitialEmptyDocument;
KURL ref(url);
ref.setUser(String());
ref.setPass(String());
ref.setRef(String());
m_outgoingReferrer = ref.string();
m_URL = url;
RefPtr<Document> document;
if (!m_isDisplayingInitialEmptyDocument && m_client->shouldUsePluginDocument(m_responseMIMEType))
document = PluginDocument::create(m_frame);
else
document = DOMImplementation::createDocument(m_responseMIMEType, m_frame, m_frame->inViewSourceMode()); // 创建DOM文件，m_responseMIMEType不同实体不同。
// 如果是"text/html"创建HTMLDocument实体；"application/xhtml+xml"创建Document实体
// 如果是"application/x-ftp-directory"则是FTPDirectoryDocument实体
// text/vnd.wap.wml 对应 WMLDocument 实体（无线）
// "application/pdf" /"text/plain" 对应 PluginDocument实体
// 如果是MediaPlayer::supportsType(type)，创建的是MediaDocument实体
// "image/svg+xml" 对应 SVGDocument实体
m_frame->setDocument(document);
document->setURL(m_URL);
if (m_decoder)
document->setDecoder(m_decoder.get());
if (forcedSecurityOrigin)
document->setSecurityOrigin(forcedSecurityOrigin.get());
m_frame->domWindow()->setURL(document->url());
m_frame->domWindow()->setSecurityOrigin(document->securityOrigin());
updatePolicyBaseURL(); // 更新排布策略的基础URL
Settings* settings = document->settings();
document->docLoader()->setAutoLoadImages(settings && settings->loadsImagesAutomatically());
if (m_documentLoader) {
String dnsPrefetchControl = m_documentLoader->response().httpHeaderField("X-DNS-Prefetch-Control");
if (!dnsPrefetchControl.isEmpty())
document->parseDNSPrefetchControlHeader(dnsPrefetchControl);
}
#if FRAME_LOADS_USER_STYLESHEET
KURL userStyleSheet = settings ? settings->userStyleSheetLocation() : KURL();
if (!userStyleSheet.isEmpty())
m_frame->setUserStyleSheetLocation(userStyleSheet);
#endif
restoreDocumentState();
document->implicitOpen();
if (m_frame->view())
m_frame->view()->setContentsSize(IntSize());
#if USE(LOW_BANDWIDTH_DISPLAY)
// Low bandwidth display is a first pass display without external resources
// used to give an instant visual feedback. We currently on
// HTML documents in the top frame.
if (document->isHTMLDocument() && !m_frame->tree()->parent() && m_useLowBandwidthDisplay) {
m_pendingSourceInLowBandwidthDisplay = String();
m_finishedParsingDuringLowBandwidthDisplay = false;
m_needToSwitchOutLowBandwidthDisplay = false;
document->setLowBandwidthDisplay(true);
}
#endif
}
看其中document->implicitOpen()的代码：
void Document::implicitOpen()
{
cancelParsing();
clear();
m_tokenizer = createTokenizer();
setParsing(true);
}
Tokenizer *HTMLDocument::createTokenizer()
{
bool reportErrors = false;
if (frame())
if (Page* page = frame()->page())
reportErrors = page->inspectorController()->windowVisible();
return new HTMLTokenizer(this, reportErrors);
}
新创建的HTMLTokenizer对象，就是HTML的解析器。
回到QWebFrame::setUrl()的第二句：d->frame->loader()->end();
只是把上次未完的解析停止：
void FrameLoader::endIfNotLoadingMainResource()
{
if (m_isLoadingMainResource || !m_frame->page())
return;
// http://bugs.webkit.org/show_bug.cgi?id=10854
// The frame's last ref may be removed and it can be deleted by checkCompleted(),
// so we'll add a protective refcount
RefPtr<Frame> protector(m_frame);
// make sure nothing's left in there
if (m_frame->document()) {
write(0, 0, true);
m_frame->document()->finishParsing();
} else
// WebKit partially uses WebCore when loading non-HTML docs. In these cases doc==nil, but
// WebCore is enough involved that we need to checkCompleted() in order for m_bComplete to
// become true. An example is when a subframe is a pure text doc, and that subframe is the
// last on
checkCompleted();
}
再来看QWebFrame::setUrl()的第三句：load(ensureAbsoluteUrl(url));
void QWebFrame::load(const QUrl &url)
{
load(QNetworkRequest(ensureAbsoluteUrl(url)));
}
新建一个QNetworkRequest对象，然后调用
void load(const QNetworkRequest &request,
QNetworkAccessManager::Operation operation = QNetworkAccessManager::GetOperation,
const QByteArray &body = QByteArray());
看其代码：
void QWebFrame::load(const QNetworkRequest &req,
QNetworkAccessManager::Operation operation,
const QByteArray &body)
{
if (d->parentFrame())
d->page->d->insideOpenCall = true;
QUrl url = ensureAbsoluteUrl(req.url());
WebCore::ResourceRequest request(url);
switch (operation) {
case QNetworkAccessManager::HeadOperation:
request.setHTTPMethod("HEAD");
break;
case QNetworkAccessManager::GetOperation:
request.setHTTPMethod("GET");
break;
case QNetworkAccessManager::PutOperation:
request.setHTTPMethod("PUT");
break;
case QNetworkAccessManager::PostOperation:
request.setHTTPMethod("POST");
break;
case QNetworkAccessManager::UnknownOperation:
// eh?
break;
}
QList<QByteArray> httpHeaders = req.rawHeaderList();
for (int i = 0; i < httpHeaders.size(); ++i) {
const QByteArray &headerName = httpHeaders.at(i);
request.addHTTPHeaderField(QString::fromLatin1(headerName), QString::fromLatin1(req.rawHeader(headerName)));
}
if (!body.isEmpty())
request.setHTTPBody(WebCore::FormData::create(body.constData(), body.size()));
d->frame->loader()->load(request);
if (d->parentFrame())
d->page->d->insideOpenCall = false;
}
看关键的FrameLoader::load()
void FrameLoader::load(const ResourceRequest& request)
{
load(request, SubstituteData());
}
void FrameLoader::load(const ResourceRequest& request, const SubstituteData& substituteData)
{
if (m_inStopAllLoaders)
return;
// FIXME: is this the right place to reset loadType? Perhaps this should be done after loading is finished or aborted.
m_loadType = FrameLoadTypeStandard;
load(m_client->createDocumentLoader(request, substituteData).get());
}
上面m_client对应的是FrameLoaderClientQt实体，m_client->createDocumentLoader()创建的是DocumentLoader对象。进一步看FrameLoader::load(DocumentLoader *)的代码：
void FrameLoader::load(DocumentLoader* newDocumentLoader)
{
ResourceRequest& r = newDocumentLoader->request();
addExtraFieldsToMainResourceRequest(r);
FrameLoadType type;
if (shouldTreatURLAsSameAsCurrent(newDocumentLoader->originalRequest().url())) {
r.setCachePolicy(ReloadIgnoringCacheData);
type = FrameLoadTypeSame;
} else
type = FrameLoadTypeStandard;
if (m_documentLoader)
newDocumentLoader->setOverrideEncoding(m_documentLoader->overrideEncoding());
// When we loading alternate content for an unreachable URL that we're
// visiting in the history list, we treat it as a reload so the history list
// is appropriately maintained.
//
// FIXME: This seems like a dangerous overloading of the meaning of "FrameLoadTypeReload" ...
// shouldn't a more explicit type of reload be defined, that means roughly
// "load without affecting history" ?
if (shouldReloadToHandleUnreachableURL(newDocumentLoader)) {
ASSERT(type == FrameLoadTypeStandard);
type = FrameLoadTypeReload;
}
loadWithDocumentLoader(newDocumentLoader, type, 0);
}
QT之webkit 分析（四）
void FrameLoader::loadWithDocumentLoader(DocumentLoader* loader, FrameLoadType type, PassRefPtr<FormState> prpFormState)
{
ASSERT(m_client->hasWebView());
// Unfortunately the view must be non-nil, this is ultimately due
// to parser requiring a FrameView. We should fix this dependency.
ASSERT(m_frame->view());
m_policyLoadType = type;
RefPtr<FormState> formState = prpFormState;
bool isFormSubmission = formState;
const KURL& newURL = loader->request().url();
if (shouldScrollToAnchor(isFormSubmission, m_policyLoadType, newURL)) {
RefPtr<DocumentLoader> oldDocumentLoader = m_documentLoader;
NavigationAction act
oldDocumentLoader->setTriggeringAction(act
stopPolicyCheck();
checkNavigationPolicy(loader->request(), oldDocumentLoader.get(), formState,
callContinueFragmentScrollAfterNavigationPolicy, this);
} else {
if (Frame* parent = m_frame->tree()->parent())
loader->setOverrideEncoding(parent->loader()->documentLoader()->overrideEncoding());
stopPolicyCheck();
setPolicyDocumentLoader(loader);
checkNavigationPolicy(loader->request(), loader, formState,
callContinueLoadAfterNavigationPolicy, this);
}
}
上面调用checkNavigationPolicy()是关键，看其实现：
void FrameLoader::checkNavigationPolicy(const ResourceRequest& request, DocumentLoader* loader,
PassRefPtr<FormState> formState, NavigationPolicyDecisionFunction function, void* argument)
{
NavigationAction act
if (act
act
loader->setTriggeringAction(act
}
// Don't ask more than on
// This avoids confusion on the part of the client.
if (equalIgnoringHeaderFields(request, loader->lastCheckedRequest()) || (!request.isNull() && request.url().isEmpty())) {
function(argument, request, 0, true);
loader->setLastCheckedRequest(request);
return;
}
// We are always willing to show alternate content for unreachable URLs;
// treat it like a reload so it maintains the right state for b/f list.
if (loader->substituteData().isValid() && !loader->substituteData().failingURL().isEmpty()) {
if (isBackForwardLoadType(m_policyLoadType))
m_policyLoadType = FrameLoadTypeReload;
function(argument, request, 0, true);
return;
}
loader->setLastCheckedRequest(request);
m_policyCheck.set(request, formState.get(), function, argument);
m_delegateIsDecidingNavigationPolicy = true;
m_client->dispatchDecidePolicyForNavigationAction(&FrameLoader::continueAfterNavigationPolicy,
act
m_delegateIsDecidingNavigationPolicy = false;
}
其中m_client是FrameLoaderClientQt实体指针，
void FrameLoaderClientQt::dispatchDecidePolicyForNavigationAction(FramePolicyFunction function, const WebCore::NavigationAction& act
{
Q_ASSERT(!m_policyFunction);
Q_ASSERT(m_webFrame);
m_policyFunction = function;
#if QT_VERSION < 0x040400
QWebNetworkRequest r(request);
#else
QNetworkRequest r(request.toNetworkRequest());
#endif
QWebPage*page = m_webFrame->page();
if (!page->d->acceptNavigationRequest(m_webFrame, r, QWebPage::NavigationType(act
if (act
m_frame->loader()->resetMultipleFormSubmissionProtection();
if (act
ResourceRequest emptyRequest;
m_frame->loader()->activeDocumentLoader()->setLastCheckedRequest(emptyRequest);
}
slotCallPolicyFunction(PolicyIgnore);
return;
}
slotCallPolicyFunction(PolicyUse);
}
void FrameLoaderClientQt::slotCallPolicyFunction(int act
{
if (!m_frame || !m_policyFunction)
return;
FramePolicyFunction function = m_policyFunction;
m_policyFunction = 0;
(m_frame->loader()->*function)(WebCore::PolicyAction(act
}
用函数指针回调，FrameLoader::continueAfterNavigationPolicy(PolicyAction policy)，参数为PolicyUse
void FrameLoader::continueAfterNavigationPolicy(PolicyAction policy)
{
PolicyCheck check = m_policyCheck;
m_policyCheck.clear();
bool shouldContinue = policy == PolicyUse;
switch (policy) {
case PolicyIgnore:
check.clearRequest();
break;
case PolicyDownload:
m_client->startDownload(check.request());
check.clearRequest();
break;
case PolicyUse: {
ResourceRequest request(check.request());
if (!m_client->canHandleRequest(request)) {
handleUnimplementablePolicy(m_client->cannotShowURLError(check.request()));
check.clearRequest();
shouldContinue = false;
}
break;
}
}
check.call(shouldContinue);
}
上面调用的是PolicyCheck::call()，参数为true
void PolicyCheck::call(bool shouldContinue)
{
if (m_navigationFunction)
m_navigationFunction(m_argument, m_request, m_formState.get(), shouldContinue);
if (m_newWindowFunction)
m_newWindowFunction(m_argument, m_request, m_formState.get(), m_frameName, shouldContinue);
ASSERT(!m_contentFunction);
}
m_navigationFunction又是一个函数指针，指向的是FrameLoader::callContinueLoadAfterNavigationPolicy()
void FrameLoader::callContinueLoadAfterNavigationPolicy(void* argument,
const ResourceRequest& request, PassRefPtr<FormState> formState, bool shouldContinue)
{
FrameLoader* loader = static_cast<FrameLoader*>(argument);
loader->continueLoadAfterNavigationPolicy(request, formState, shouldContinue);
}
void FrameLoader::continueLoadAfterNavigationPolicy(const ResourceRequest&, PassRefPtr<FormState> formState, bool shouldContinue)
{
// If we loaded an alternate page to replace an unreachableURL, we'll get in here with a
// nil policyDataSource because loading the alternate page will have passed
// through this method already, nested; otherwise, policyDataSource should still be set.
ASSERT(m_policyDocumentLoader || !m_provisionalDocumentLoader->unreachableURL().isEmpty());
bool isTargetItem = m_provisionalHistoryItem ? m_provisionalHistoryItem->isTargetItem() : false;
// Two reasons we can't continue:
// 1) Navigation policy delegate said we can't so request is nil. A primary case of this
// is the user responding Cancel to the form repost nag sheet.
// 2) User responded Cancel to an alert popped up by the before unload event handler.
// The "before unload" event handler runs on
bool canContinue = shouldContinue && (!isLoadingMainFrame() || m_frame->shouldClose());
if (!canContinue) {
// If we were waiting for a quick redirect, but the policy delegate decided to ignore it, then we
// need to report that the client redirect was cancelled.
if (m_quickRedirectComing)
clientRedirectCancelledOrFinished(false);
setPolicyDocumentLoader(0);
// If the navigation request came from the back/forward menu, and we punt on it, we have the
// problem that we have optimistically moved the b/f cursor already, so move it back. For sanity,
// we on
if ((isTargetItem || isLoadingMainFrame()) && isBackForwardLoadType(m_policyLoadType))
if (Page* page = m_frame->page()) {
Frame* mainFrame = page->mainFrame();
if (HistoryItem* resetItem = mainFrame->loader()->m_currentHistoryItem.get()) {
page->backForwardList()->goToItem(resetItem);
Settings* settings = m_frame->settings();
page->setGlobalHistoryItem((!settings || settings->privateBrowsingEnabled()) ? 0 : resetItem);
}
}
return;
}
FrameLoadType type = m_policyLoadType;
stopAllLoaders();
// <rdar://problem/6250856> - In certain circumstances on pages with multiple frames, stopAllLoaders()
// might detach the current FrameLoader, in which case we should bail on this newly defunct load.
if (!m_frame->page())
return;
setProvisionalDocumentLoader(m_policyDocumentLoader.get());
m_loadType = type;
setState(FrameStateProvisional);
setPolicyDocumentLoader(0);
if (isBackForwardLoadType(type) && loadProvisionalItemFromCachedPage())
return;
if (formState)
m_client->dispatchWillSubmitForm(&FrameLoader::continueLoadAfterWillSubmitForm, formState);
else
continueLoadAfterWillSubmitForm();
}
void FrameLoader::continueLoadAfterWillSubmitForm(PolicyAction)
{
if (!m_provisionalDocumentLoader)
return;
// DocumentLoader calls back to our prepareForLoadStart
m_provisionalDocumentLoader->prepareForLoadStart();
// The load might be cancelled inside of prepareForLoadStart(), nulling out the m_provisionalDocumentLoader,
// so we need to null check it again.
if (!m_provisionalDocumentLoader)
return;
// 先看活动的DocumentLoader能否装载
DocumentLoader* activeDocLoader = activeDocumentLoader();
if (activeDocLoader && activeDocLoader->isLoadingMainResource())
return;
// 看Cache中能否装载
m_provisionalDocumentLoader->setLoadingFromCachedPage(false);
unsigned long identifier = 0;
if (Page* page = m_frame->page()) {
identifier = page->progress()->createUniqueIdentifier();
dispatchAssignIdentifierToInitialRequest(identifier, m_provisionalDocumentLoader.get(), m_provisionalDocumentLoader->originalRequest());
}
if (!m_provisionalDocumentLoader->startLoadingMainResource(identifier))
m_provisionalDocumentLoader->updateLoading();
}
上面的装载过程，如果是第一次并且只有m_provisionalDocumentLoader的话，只会执行最后一中装载。
bool DocumentLoader::startLoadingMainResource(unsigned long identifier)
{
ASSERT(!m_mainResourceLoader);
m_mainResourceLoader = MainResourceLoader::create(m_frame);
m_mainResourceLoader->setIdentifier(identifier);
// FIXME: Is there any way the extra fields could have not been added by now?
// If not, it would be great to remove this line of co
frameLoader()->addExtraFieldsToMainResourceRequest(m_request);
if (!m_mainResourceLoader->load(m_request, m_substituteData)) {
// FIXME: If this should really be caught, we should just ASSERT this doesn't happen;
// should it be caught by other parts of WebKit or other parts of the app?
LOG_ERROR("could not create WebResourceHandle for URL %s -- should be caught by policy handler level", m_request.url().string().ascii().da
m_mainResourceLoader = 0;
return false;
}
return true;
}
创建MainResourceLoader对象，并调用load()
bool MainResourceLoader::load(const ResourceRequest& r, const SubstituteData& substituteData)
{
ASSERT(!m_handle);
m_substituteData = substituteData;
#if ENABLE(OFFLINE_WEB_APPLICATIONS)
// Check if this request should be loaded from the application cache
if (!m_substituteData.isValid() && frameLoader()->frame()->settings() && frameLoader()->frame()->settings()->offlineWebApplicationCacheEnabled()) {
ASSERT(!m_applicationCache);
m_applicationCache = ApplicationCacheGroup::cacheForMainRequest(r, m_documentLoader.get());
if (m_applicationCache) {
// Get the resource from the application cache. By definition, cacheForMainRequest() returns a cache that contains the resource.
ApplicationCacheResource* resource = m_applicationCache->resourceForRequest(r);
m_substituteData = SubstituteData(resource->da
resource->response().mimeType(),
resource->response().textEncodingName(), KURL());
}
}
#endif
ResourceRequest request(r);
bool defer = defersLoading();
if (defer) {
bool shouldLoadEmpty = shouldLoadAsEmptyDocument(r.url());
if (shouldLoadEmpty)
defer = false;
}
if (!defer) {
if (loadNow(request)) {
// Started as an empty document, but was redirected to something non-empty.
ASSERT(defersLoading());
defer = true;
}
}
if (defer)
m_initialRequest = request;
return true;
}
继续深入看MainResourceLoader::loadNow()
bool MainResourceLoader::loadNow(ResourceRequest& r)
{
bool shouldLoadEmptyBeforeRedirect = shouldLoadAsEmptyDocument(r.url());
ASSERT(!m_handle);
ASSERT(shouldLoadEmptyBeforeRedirect || !defersLoading());
// Send this synthetic delegate callback since clients expect it, and
// we no longer send the callback from within NSURLConnection for
// initial requests.
willSendRequest(r, ResourceResponse());
// <rdar://problem/4801066>
// willSendRequest() is liable to make the call to frameLoader() return NULL, so we need to check that here
if (!frameLoader())
return false;
const KURL& url = r.url();
bool shouldLoadEmpty = shouldLoadAsEmptyDocument(url) && !m_substituteData.isValid();
if (shouldLoadEmptyBeforeRedirect && !shouldLoadEmpty && defersLoading())
return true;
if (m_substituteData.isValid())
handleDataLoadSoon(r);
else if (shouldLoadEmpty || frameLoader()->representationExistsForURLScheme(url.protocol()))
handleEmptyLoad(url, !shouldLoadEmpty);
else
m_handle = ResourceHandle::create(r, this, m_frame.get(), false, true, true);
return false;
}
主要两个调用：willSendRequest()和ResourceHandle::create()，前面一个估计是发送请求前的相关设定；后一个就是请求发送了。先看前一个：
void MainResourceLoader::willSendRequest(ResourceRequest& newRequest, const ResourceResponse& redirectResponse)
{
// Note that there are no asserts here as there are for the other callbacks. This is due to the
// fact that this "callback" is sent when starting every load, and the state of callback
// deferrals plays less of a part in this function in preventing the bad beha
// callbacks is meant to prevent.
ASSERT(!newRequest.isNull());
// The additional processing can do anything including possibly removing the last
// reference to this object; on
RefPtr<MainResourceLoader> protect(this);
// Update cookie policy base URL as URL changes, except for subframes, which use the
// URL of the main frame which doesn't change when we redirect.
if (frameLoader()->isLoadingMainFrame())
newRequest.setMainDocumentURL(newRequest.url());
// If we're fielding a redirect in response to a POST, force a load from origin, since
// this is a common site technique to return to a page viewing some da
// just modified.
// Also, POST requests always load from origin, but this does not affect subresources.
if (newRequest.cachePolicy() == UseProtocolCachePolicy && isPostOrRedirectAfterPost(newRequest, redirectResponse))
newRequest.setCachePolicy(ReloadIgnoringCacheData);
ResourceLoader::willSendRequest(newRequest, redirectResponse);
// Don't set this on the first request. It is set when the main load was started.
m_documentLoader->setRequest(newRequest);
// FIXME: Ideally we'd stop the I/O until we hear back from the navigation policy delegate
// listener. But there's no way to do that in practice. So instead we cancel later if the
// listener tells us to. In practice that means the navigation policy needs to be decided
// synchronously for these redirect cases.
ref(); // balanced by deref in continueAfterNavigationPolicy
frameLoader()->checkNavigationPolicy(newRequest, callContinueAfterNavigationPolicy, this);
}
主要是调用ResourceLoader::willSendRequest()函数：
void ResourceLoader::willSendRequest(ResourceRequest& request, const ResourceResponse& redirectResponse)
{
// Protect this in this delegate method since the additional processing can do
// anything including possibly derefing this; on
RefPtr<ResourceLoader> protector(this);
ASSERT(!m_reachedTerminalState);
if (m_sendResourceLoadCallbacks) {
if (!m_identifier) {
m_identifier = m_frame->page()->progress()->createUniqueIdentifier();
frameLoader()->assignIdentifierToInitialRequest(m_identifier, request);
}
frameLoader()->willSendRequest(this, request, redirectResponse);
}
m_request = request;
}
进一步调用FrameLoader::willSendRequest()
void FrameLoader::willSendRequest(ResourceLoader* loader, ResourceRequest& clientRequest, const ResourceResponse& redirectResponse)
{
applyUserAgent(clientRequest);
dispatchWillSendRequest(loader->documentLoader(), loader->identifier(), clientRequest, redirectResponse);
}
更多的调用：
void FrameLoader::dispatchWillSendRequest(DocumentLoader* loader, unsigned long identifier, ResourceRequest& request, const ResourceResponse& redirectResponse)
{
StringImpl* oldRequestURL = request.url().string().impl();
m_documentLoader->didTellClientAboutLoad(request.url());
m_client->dispatchWillSendRequest(loader, identifier, request, redirectResponse);
// If the URL changed, then we want to put that new URL in the "did tell client" set too.
if (oldRequestURL != request.url().string().impl())
m_documentLoader->didTellClientAboutLoad(request.url());
if (Page* page = m_frame->page())
page->inspectorController()->willSendRequest(loader, identifier, request, redirectResponse);
}
囧~~还有下一步吗？？
m_client->dispatchWillSendRequest()实际调用的是FrameLoaderClientQt::dispatchWillSendRequest()，目前是一个空函数（仅在dump的时候打印信息）。
void InspectorController::willSendRequest(DocumentLoader*, unsigned long identifier, ResourceRequest& request, const ResourceResponse& redirectResponse)
{
if (!enabled())
return;
InspectorResource* resource = m_resources.get(identifier).get();
if (!resource)
return;
resource->startTime = currentTime();
if (!redirectResponse.isNull()) {
updateResourceRequest(resource, request);
updateResourceResponse(resource, redirectResponse);
}
if (resource != m_mainResource && windowVisible()) {
if (!resource->scriptObject)
addScriptResource(resource);
else
updateScriptResourceRequest(resource);
updateScriptResource(resource, resource->startTime, resource->responseReceivedTime, resource->endTime);
if (!redirectResponse.isNull())
updateScriptResourceResponse(resource);
}
}
在这里设定了开始时间，猜测是供请求超时判断用的，请求超时的定时器在何处设定有待进一步分析。
看都是一些Resource的更新，感觉意义不大，不再进一步追踪。回到MainResourceLoader::loadNow()，看下一步ResourceHandle::create()
PassRefPtr<ResourceHandle> ResourceHandle::create(const ResourceRequest& request, ResourceHandleClient* client,
Frame* frame, bool defersLoading, bool shouldContentSniff, bool mightDownloadFromHandle)
{
RefPtr<ResourceHandle> newHandle(adoptRef(new ResourceHandle(request, client, defersLoading, shouldContentSniff, mightDownloadFromHandle)));
if (!request.url().isValid()) {
newHandle->scheduleFailure(InvalidURLFailure);
return newHandle.release();
}
// 检查端口号(port)是否合法
if (!portAllowed(request)) {
newHandle->scheduleFailure(BlockedFailure);
return newHandle.release();
}
if (newHandle->start(frame))
return newHandle.release();
return 0;
}
看关键的ResourceHandle::start调用：
bool ResourceHandle::start(Frame* frame)
{
if (!frame)
return false;
Page *page = frame->page();
// If we are no longer attached to a Page, this must be an attempted load from an
// on
if (!page)
return false;
getInternal()->m_frame = static_cast<FrameLoaderClientQt*>(frame->loader()->client())->webFrame();
#if QT_VERSION < 0x040400
return QWebNetworkManager::self()->add(this, getInternal()->m_frame->page()->d->networkInterface);
#else
ResourceHandleInternal *d = getInternal();
d->m_job = new QNetworkReplyHandler(this, QNetworkReplyHandler::LoadMode(d->m_defersLoading));
return true;
#endif
}
新创建了一个QNetworkReplyHandler对象，QNetworkReplyHandler在构造的时候会调用QNetworkReplyHandler::start()
void QNetworkReplyHandler::start()
{
m_shouldStart = false;
ResourceHandleInternal* d = m_resourceHandle->getInternal();
QNetworkAccessManager* manager = d->m_frame->page()->networkAccessManager();
const QUrl url = m_request.url();
const QString scheme = url.scheme();
// Post requests on files and da
// fast/forms/form-post-urlencoded.html and for fast/forms/button-state-restore.html
// we still need to retrieve the file/da
if (m_method == QNetworkAccessManager::PostOperation
&& (!url.toLocalFile().isEmpty() || url.scheme() == QLatin1String("da
m_method = QNetworkAccessManager::GetOperation;
m_startTime = QDateTime::currentDateTime().toTime_t();
switch (m_method) {
case QNetworkAccessManager::GetOperation:
m_reply = manager->get(m_request);
break;
case QNetworkAccessManager::PostOperation: {
FormDataIODevice* postDevice = new FormDataIODevice(d->m_request.httpBody());
m_reply = manager->post(m_request, postDevice);
postDevice->setParent(m_reply);
break;
}
case QNetworkAccessManager::HeadOperation:
m_reply = manager->head(m_request);
break;
case QNetworkAccessManager::PutOperation: {
FormDataIODevice* putDevice = new FormDataIODevice(d->m_request.httpBody());
m_reply = manager->put(m_request, putDevice);
putDevice->setParent(m_reply);
break;
}
case QNetworkAccessManager::UnknownOperation: {
m_reply = 0;
ResourceHandleClient* client = m_resourceHandle->client();
if (client) {
ResourceError error(url.host(), 400 /*bad request*/,
url.toString(),
QCoreApplication::translate("QWebPage", "Bad HTTP request"));
client->didFail(m_resourceHandle, error);
}
return;
}
}
m_reply->setParent(this);
connect(m_reply, SIGNAL(finished()),
this, SLOT(finish()), Qt::QueuedConnection);
// For http(s) we know that the headers are complete upon metaDataChanged() emission, so we
// can send the response as early as possible
if (scheme == QLatin1String("http") || scheme == QLatin1String("https"))
connect(m_reply, SIGNAL(metaDataChanged()),
this, SLOT(sendResponseIfNeeded()), Qt::QueuedConnection);
connect(m_reply, SIGNAL(readyRead()),
this, SLOT(forwardData()), Qt::QueuedConnection);
}
看到了熟悉的QNetworkAccessManager、QNetworkReply。跟踪至此，初始化和URL请求发送基本完成。
QT之webkit 分析（五）
前面分析WebView初始化的时候，在QNetworkReplyHandler::start()里有设定读取数据的处理函数：
connect(m_reply, SIGNAL(finished()),
this, SLOT(finish()), Qt::QueuedConnection);
// For http(s) we know that the headers are complete upon metaDataChanged() emission, so we
// can send the response as early as possible
if (scheme == QLatin1String("http") || scheme == QLatin1String("https"))
connect(m_reply, SIGNAL(metaDataChanged()),
this, SLOT(sendResponseIfNeeded()), Qt::QueuedConnection);
connect(m_reply, SIGNAL(readyRead()),
this, SLOT(forwardData()), Qt::QueuedConnection);
先看QNetworkReplyHandler::forwardData()
void QNetworkReplyHandler::forwardData()
{
m_shouldForwardData = (m_loadMode == LoadDeferred);
if (m_loadMode == LoadDeferred)
return;
sendResponseIfNeeded();
// don't emit the "Document has moved here" type of HTML
if (m_redirected)
return;
if (!m_resourceHandle)
return;
QByteArray da
ResourceHandleClient* client = m_resourceHandle->client();
if (!client)
return;
if (!da
client->didReceiveData(m_resourceHandle, da
}
实际就是两个调用：read()和didReceiveData()。其中QNetworkReply::read()前面分析过不再重复；
ResourceHandleClient* client->didReceiveData()实际调用的是MainResourceLoader::didReceiveData()
void MainResourceLoader::didReceiveData(const char* da
{
ASSERT(da
ASSERT(length != 0);
// There is a bug in CFNetwork where callbacks can be dispatched even when loads are deferred.
// See <rdar://problem/6304600> for more details.
#if !PLATFORM(CF)
ASSERT(!defersLoading());
#endif
// The additional processing can do anything including possibly removing the last
// reference to this object; on
RefPtr<MainResourceLoader> protect(this);
ResourceLoader::didReceiveData(da
}
进一步看其调用：
void ResourceLoader::didReceiveData(const char* da
{
// Protect this in this delegate method since the additional processing can do
// anything including possibly derefing this; on
RefPtr<ResourceLoader> protector(this);
addData(da
// FIXME: If we get a resource with more than 2B bytes, this co
// However, with today's computers and networking speeds, this won't happen in practice.
// Could be an issue with a giant local file.
if (m_sendResourceLoadCallbacks && m_frame)
frameLoader()->didReceiveData(this, da
}
在ResourceLoader类中addData()是虚函数，client->didReceiveData()中client指针实际的实体为MainResourceLoader对象，所以addData()先调用MainResourceLoader::addData()
void MainResourceLoader::addData(const char* da
{
ResourceLoader::addData(da
frameLoader()->receivedData(da
}
这里只有两个调用，前一个是将接收到的数据保存到一个buffer中，供后续语法扫描使用（猜测的），暂不深入分析。看frameLoader->receivedData()
void FrameLoader::receivedData(const char* da
{
activeDocumentLoader()->receivedData(da
void DocumentLoader::receivedData(const char* da
{
m_gotFirstByte = true;
if (doesProgressiveLoad(m_response.mimeType()))
commitLoad(da
}
其中doesProgressiveLoad()会测试MIME的类型，重点是commitLoad()
void DocumentLoader::commitLoad(const char* da
{
// Both unloading the old page and parsing the new page may execute JavaS
// by starting a new load, so retain temporarily.
RefPtr<DocumentLoader> protect(this);
commitIfReady();
if (FrameLoader* frameLoader = DocumentLoader::frameLoader())
frameLoader->committedLoad(this, da
前面一个调用：commitIfReady()是清理前一次页面扫描的中间数据；committedLoad()才是正题。
void FrameLoader::committedLoad(DocumentLoader* loader, const char* da
{
if (ArchiveFactory::isArchiveMimeType(loader->response().mimeType()))
return;
m_client->committedLoad(loader, da
}
其中m_client指向的是FrameLoaderClientQT对象实体。
void FrameLoaderClientQt::committedLoad(WebCore::DocumentLoader* loader, const char* da
{
if (!m_pluginView) {
if (!m_frame)
return;
FrameLoader *fl = loader->frameLoader();
if (m_firstData) {
fl->setEncoding(m_response.textEncodingName(), false);
m_firstData = false;
}
fl->addData(da
// We re-check here as the plugin can have been created
if (m_pluginView) {
if (!m_hasSentResponseToPlugin) {
m_pluginView->didReceiveResponse(loader->response());
// didReceiveResponse sets up a new stream to the plug-in. on a full-page plug-in, a failure in
// setting up this stream can cause the main document load to be cancelled, setting m_pluginView
// to null
if (!m_pluginView)
return;
m_hasSentResponseToPlugin = true;
}
m_pluginView->didReceiveData(da
}
}
其中fl->setEncoding()是根据服务器返回的HTML数据流设定编码格式（例如：中文gb2312），另外处理了其他一些事情，例如Redirect等。fl->addData()是关键：
void FrameLoader::addData(const char* bytes, int length)
{
ASSERT(m_workingURL.isEmpty());
ASSERT(m_frame->document());
ASSERT(m_frame->document()->parsing());
write(bytes, length);
}
上面的FrameLoader::write()调用，启动了HTML/JS分析扫描。后一篇深入HTML扫描分析。
QT之webkit 分析（六）
在继续分析FrameLoader::write()之前，先回到《QT分析之WebKit（二）》。那里曾经保存了一个完整的调用堆栈，
……
QtWebKitd4.dll!WebCore::HTMLTokenizer::write(const WebCore::SegmentedString & str={...}, bool appendData=true) 行1730 + 0x23 字节 C++
QtWebKitd4.dll!WebCore::FrameLoader::write(const char *
可知调用的次序为：FrameLoader::write()调用了HTMLTokenizer::write()。
下面是FrameLoader::write()的定义：
void write(const char* str, int len = -1, bool flush = false);
这里包含了两个缺省值调用定义，在前一篇，调用的形式是：write(bytes, length);
实际传递的的是：write(bytes, length, false);
接着看write()的实现：
void FrameLoader::write(const char* str, int len, bool flush)
{
if (len == 0 && !flush)
return;
if (len == -1)
len = strlen(str);
Tokenizer* tokenizer = m_frame->document()->tokenizer();
if (tokenizer && tokenizer->wantsRawData()) {
if (len > 0)
tokenizer->writeRawData(str, len);
return;
}
if (!m_decoder) {
Settings* settings = m_frame->settings();
m_decoder = TextResourceDecoder::create(m_responseMIMEType, settings ? settings->defaultTextEncodingName() : String());
if (m_encoding.isEmpty()) {
Frame* parentFrame = m_frame->tree()->parent();
if (parentFrame && parentFrame->document()->securityOrigin()->canAccess(m_frame->document()->securityOrigin()))
m_decoder->setEncoding(parentFrame->document()->inputEncoding(), TextResourceDecoder::DefaultEncoding);
} else {
m_decoder->setEncoding(m_encoding,
m_encodingWasChosenByUser ? TextResourceDecoder::UserChosenEncoding : TextResourceDecoder::EncodingFromHTTPHeader);
}
m_frame->document()->setDecoder(m_decoder.get());
}
String decoded = m_decoder->decode(str, len);
if (flush)
decoded += m_decoder->flush();
if (decoded.isEmpty())
return;
#if USE(LOW_BANDWIDTH_DISPLAY)
if (m_frame->document()->inLowBandwidthDisplay())
m_pendingSourceInLowBandwidthDisplay.append(decoded);
#endif
if (!m_receivedData) {
m_receivedData = true;
if (m_decoder->encoding().usesVisualOrdering())
m_frame->document()->setVisuallyOrdered();
m_frame->document()->recalcStyle(Node::Force);
}
if (tokenizer) {
ASSERT(!tokenizer->wantsRawData());
tokenizer->write(decoded, true);
}
}
怎么和HTMLTokenizer关联的呢？就是在《QT分析之WebKit（三）》初始化Document对象的时候关联上的。
DOMImplementation::createDocument()
上面程序做了一些边缘的工作，例如设定编码（因为可以在HTTP协议、HTML的TITLE部分或者浏览器特别指定编码），主要是新建一个decoder另外一个是调用tokenizer->write()
QT之webkit 分析（七）
String TextResourceDecoder::decode(const char* da
{
if (!m_checkedForBOM)
checkForBOM(da
bool movedDataToBuffer = false;
if (m_contentType == CSS && !m_checkedForCSSCharset)
if (!checkForCSSCharset(da
return "";
if ((m_contentType == HTML || m_contentType == XML) && !m_checkedForHeadCharset) // HTML and XML
if (!checkForHeadCharset(da
return "";
// Do the auto-detect if our default encoding is on
// FIXME: It seems wrong to change our encoding downstream after we have already done some decoding.
if (m_source != UserChosenEncoding && m_source != AutoDetectedEncoding && encoding().isJapanese())
detectJapaneseEncoding(da
ASSERT(encoding().isValid());
if (m_buffer.isEmpty())
return m_decoder.decode(da
if (!movedDataToBuffer) {
size_t oldSize = m_buffer.size();
m_buffer.grow(oldSize + len);
memcpy(m_buffer.da
}
String result = m_decoder.decode(m_buffer.da
m_buffer.clear();
return result;
}
再回到tokenizer->write(decoded, true);看其具体实现：
bool HTMLTokenizer::write(const SegmentedString& str, bool appendData)
{
if (!m_buffer)
return false;
if (m_parserStopped)
return false;
SegmentedString source(str);
if (m_executingScript)
source.setExcludeLineNumbers();
if ((m_executingScript && appendData) || !m_pendingScripts.isEmpty()) {
// don't parse; we will do this later
if (m_currentPrependingSrc)
m_currentPrependingSrc->append(source);
else {
m_pendingSrc.append(source);
#if PRELOAD_SCANNER_ENABLED
if (m_preloadScanner && m_preloadScanner->inProgress() && appendData)
m_preloadScanner->write(source);
#endif
}
return false;
}
#if PRELOAD_SCANNER_ENABLED
if (m_preloadScanner && m_preloadScanner->inProgress() && appendData)
m_preloadScanner->end();
#endif
if (!m_src.isEmpty())
m_src.append(source);
else
setSrc(source);
// On
if (m_timer.isActive())
return false;
bool wasInWrite = m_inWrite;
m_inWrite = true;
#ifdef INSTRUMENT_LAYOUT_SCHEDULING
if (!m_doc->ownerElement())
printf("Beginning write at time %d ", m_doc->elapsedTime());
#endif
int processedCount = 0;
double startTime = currentTime();
Frame* frame = m_doc->frame();
State state = m_state;
while (!m_src.isEmpty() && (!frame || !frame->loader()->isScheduledLocationChangePending())) {
if (!continueProcessing(processedCount, startTime, state))
break;
// do we need to enlarge the buffer?
checkBuffer();
UChar cc = *m_src;
bool wasSkipLF = state.skipLF();
if (wasSkipLF)
state.setSkipLF(false);
if (wasSkipLF && (cc == ' '))
m_src.advance();
else if (state.needsSpecialWriteHandling()) {
// it's imp
if (state.hasEntityState())
state = parseEntity(m_src, m_dest, state, m_cBufferPos, false, state.hasTagState());
else if (state.inPlainText())
state = parseText(m_src, state);
else if (state.inAnySpecial())
state = parseSpecial(m_src, state);
else if (state.inComment())
state = parseComment(m_src, state);
else if (state.inDoctype())
state = parseDoctype(m_src, state);
else if (state.inServer())
state = parseServer(m_src, state);
else if (state.inProcessingInstruction())
state = parseProcessingInstruction(m_src, state);
else if (state.hasTagState())
state = parseTag(m_src, state);
else if (state.startTag()) {
state.setStartTag(false);
switch(cc) {
case '/':
break;
case '!': {
// or
searchCount = 1; // Look for ' m_doctypeSearchCount = 1;
break;
}
case '?': {
// xml processing instruction
state.setInProcessingInstruction(true);
tquote = NoQuote;
state = parseProcessingInstruction(m_src, state);
continue;
break;
}
case '%':
if (!m_brokenServer) {
// <% server stuff, handle as comment %>
state.setInServer(true);
tquote = NoQuote;
state = parseServer(m_src, state);
continue;
}
// else fall through
default: {
if( ((cc >= 'a') && (cc <= 'z')) || ((cc >= 'A') && (cc <= 'Z'))) {
// Start of a Start-Tag
} else {
// Invalid tag
// Add as is
*m_dest = '<';
m_dest++;
continue;
}
}
}; // end case
processToken();
m_cBufferPos = 0;
state.setTagState(TagName);
state = parseTag(m_src, state);
}
} else if (cc == '&' && !m_src.escaped()) {
m_src.advancePastNonNewline();
state = parseEntity(m_src, m_dest, state, m_cBufferPos, true, state.hasTagState());
} else if (cc == '<' && !m_src.escaped()) {
m_currentTagStartLineNumber = m_lineNumber;
m_src.advancePastNonNewline();
state.setStartTag(true);
state.setDiscardLF(false);
} else if (cc == ' ' || cc == ' ') {
if (state.discardLF())
// Ignore this LF
state.setDiscardLF(false); // We have discarded 1 LF
else {
// Process this LF
*m_dest++ = ' ';
if (cc == ' ' && !m_src.excludeLineNumbers())
m_lineNumber++;
}
/* Check for MS-DOS CRLF sequence */
if (cc == ' ')
state.setSkipLF(true);
m_src.advance(m_lineNumber);
} else {
state.setDiscardLF(false);
*m_dest++ = cc;
m_src.advancePastNonNewline();
}
}
#ifdef INSTRUMENT_LAYOUT_SCHEDULING
if (!m_doc->ownerElement())
printf("Ending write at time %d ", m_doc->elapsedTime());
#endif
m_inWrite = wasInWrite;
m_state = state;
if (m_noMoreData && !m_inWrite && !state.loadingExtScript() && !m_executingScript && !m_timer.isActive()) {
end(); // this actually causes us to be deleted
return true;
}
return false;
}
在调用的时候，因为调用参数decoded是String类型的，所以先隐含转化成SegmentedString。SegmentedString可以附带行号，也可以不带行号（可以设定）。上面程序中的while循环主体，就是一个分析程序主体。
QT之webkit 分析（八）
邓侃的博客
http://blog.sina.com.cn/s/blog_46d0a3930100d5pt.html【20】WebKit的结构与解构
从指定一个HTML文本文件，到绘制出一幅布局复杂，字体多样，内含图片音频视 频等等多媒体内容的网页，这是一个复杂的过程。在这个过程中Webkit所做的一切，都是围绕DOM Tree和Rendering Tree这两个核心。上一章我们谈到这两棵树各自的功用，这一章，我们借一个简单的HTML文件，展示一下DOM Tree和Rendering Tree的具体构成，同时解剖一下Webkit是如何构造这两棵树的。Figure 1. From HTML to webpage, and the underlying DOM
tree and rendering tree.
Courtesy http://farm4.static.flickr.com/3351/3556972420_23a30366c2_o.jpg
1. DOM Tree 与 Rendering Tree 的结构
Figure 1中左上是一个简单的HTML文本文件，右上是Webkit rendering engine绘制出来的页面。页面的内容包括一个标题，“AI”，一行正文，“Ape's Intelligence”，以及一幅照片。整个页面分成前后两个层面，标题和正文绘制在前一个层面，照片处于后一个层面。L君和我亦步亦趋地跟踪了，从解析这个HTML文本文件，到生成DOM Tree和Rendering Tree的整个流程，目的是为了了解DOM Tree和Rendering Tree的具体成份，以及构造的各个步骤。
先说Figure 1中左下角的DOM Tree。基本上HTML文本文件中每个tag，在webkit/webcore/html中都有一个class与之对应。譬如<HTML> tag 对应HTMLHtmlElement，<HEAD> tag 对应HTMLHeadElement，<STYLE> tag 对应HTMLStyleElement 等等。比较特别的是DOM Tree的根节点，HTMLDocument，在HTML文本文件中没有哪个tag与之对应。关于HTMLDocument的作用，我们稍后介绍。整个 DOM Tree的结构，与HTML文本文件中各个tags的嵌套关系也一一对应。一言以蔽之，DOM Tree就是把HTML文本文件翻译成object树状结构。
需要强调的是，DOM Tree是一个通用数据结构，任何XML文本文件都可以翻译成DOM Tree，而不仅仅限于HTML文本文件。webkit/webcore/html 中林林总总html classes，基本上都是webkit/webcore/dom 中的某个class的子类，也就是说，/html 是 /dom的一个特例。这样的设计，为将来把Webkit拓展到HTML格式以外的页面的布局和渲染，埋下了伏笔。所以严格地讲，Figure 1中左下的DOM Tree，实际上是一个HTML DOM Tree。
再看Rendering Tree，显著的特点在于，
a. 整个Rendering Tree树状结构，与HTML DOM Tree树状结构一一对应。也就是说，几乎每个HTML DOM Tree中的节点，在Rendering Tree中都有对应的节点。节点与节点之间的父子或兄弟关系也一一对应。
例外的是，在HTML DOM Tree有HTMLStyleElement叶子节点，而在Rendering Tree中，没有相应的叶子节点。原因是，Rendering Tree各个节点，都涉及页面中某块区域的布局和渲染。而HTMLStyleElement，并不直接涉及某块区域的布局和渲染，HTML DOM Tree中HTMLStyleElement叶子节点包含的内容，已经融入Rendering Tree中RenderImage叶子节点的属性中去了。另外，因为Rendering Tree中不存在与HTMLStyleElement相应的叶子节点，所以，与HTMLHeadElement对应的节点也没有必要存在。
b. webkit/webcore/rendering中各个class与HTML tags并没有一一对应的关系。
Rendering Tree是一个通用的规划页面布局和渲染的机制，这个通用机制可以服务于HTML页面，但是并不仅仅限于为HTML页面服务，我们可以用 Rendering Tree来规划其它格式的页面的布局和渲染。以DOM Tree和Rendering Tree为核心的Webkit渲染机，是一个功能强大，扩展性良好的通用渲染机。它不仅可以用来绘制HTML页面，也可以用来渲染其它格式的页面，譬如可 以用它来制作email阅读和管理器，制作数据库管理工具，甚至制作游戏界面。
稍微让人有点吃惊的是，对于 HTMLHtmlElement，HTMLBodyElement，HTMLHeadingElement和HTMLParagraphElement， 在Rendering Tree中通通以RenderBlock呼应。如果说HTMLHeadingElement和HTMLParagraphElement的区别不大，仅仅 是字体和对齐方式有些微小的差别，所以Rendering Tree可以用RenderBlock来统一应对。那么问题是，HTMLHtmlElement和HTMLBodyElement是两种容器，总是出现在 DOM Tree的中部，而从来不会作为叶子节点出现，对应于这样的容器节点，为什么Rendering Tree不另设一种class，与RenderBlock有所区别呢？不过话又说回来，这不是个大问题，最多是个美感的问题。
Figure 2. The construction sequence of the root of the DOM tree.
Courtesy http://farm4.static.flickr.com/3010/3554310018_e34d271344_o.jpg
2. DOM Tree 与 Rendering Tree 的根节点
前一节中我们提到HTMLDocument是一个比较特殊的class，它是整个HTML DOM Tree的根节点，但是不对应任何HTML tag。JavaS
HTML文本文件，通常是以<HTML>开头，以</HTML>结尾。但是<HTML> tag并不对应DOM Tree的根节点，而是根以下的第一个子节点，即HTMLHtmlElement节点。
初看Figure 2 觉得有点意外，当用户在浏览器里打开一个空白页面的时候，立刻生成了DOM Tree的根节点HTMLDocument，与Rendering Tree的根节点RenderView。而这个时候，用户并没有给定URL，也就是说，对于浏览器来讲，这时候具体的HTML文本文件并不存在。根节点与 具体HTML内容相脱节，或许暗示了Webkit的两个设计思路，
a. DOM Tree的根节点HTMLDocument，与Rendering Tree的根节点RenderView，可以重复利用。
当用户在同一个浏览器页面中，先后打开两个不同的URLs，也就是两个不同的HTML文本文时，HTMLDocument和RenderView两个根节点并没有发生改变，改变的是HTMLHtmlElement以下的子树，以及对应的Rendering Tree的子树。
为什么这样设计？原因是HTMLDocument和RenderView服从于浏览器页面的设置，譬如页面的大小和在整个屏幕中的位置等等。这些设置与页面中要显示什 么的内容无关。同时HTMLDocument绑定HTMLTokenizer和HTMLParser，这两个构件也与某一个具体的HTML内容无关。
b. 同一个DOM Tree的根节点可以悬挂多个HTML子树，同一个Rendering Tree的根节点可以悬挂多个RenderBlock子树。
在 我们目前所见到的浏览器中，每一个页面通常只显示一个HTML文件。虽然一个HTML文件可以分割成多个frames，每个frame承载一个独立的 HTML文件，但是从DOM Tree结构来讲，HTMLDocument根节点以下，只有一个子节点，这个子节点是HTMLHtmlElement，它领衔某个HTML文本文件对应 的子树。Rendering Tree也一样，目前我们见到的网页中，一个RenderView根节点以下，也只有一个RenderBlock子节点。
但是Webkit的设计，却允许同一个根以下，悬挂多个HTML子树。虽然我们目前没有看到一个页面中，并存多个HTML文件，并存多个布局和渲染风格的 情景，但是Webkit为将来的拓展留下了空间。前文中所设想的个性化，多皮肤，多视角的浏览器页面绘制，用Webkit实现起来难度不大。
Figure 3. The construction sequence of the DOM Tree and the Rendering Tree.
Courtesy http://farm4.static.flickr.com/3627/3554182242_b0bec88534_b.jpg
3. DOM Tree 与 Rendering Tree 的构筑
HTMLDocument 根节点包含的最重要的构件是HTMLTokenizer，而HTMLTokenizer又包含HTMLParser这个构件。HTMLTokenizer 从前到后读取HTML文本文件中每一个字符，并从中提取出各个HTML tags以及它们的内容。而HTMLParser不仅负责HTML DOM Tree的构筑，而且也同时负责Rendering Tree的构筑。
在Figure 3中，从第8步到第11步，HTMLParser根据一个HTML Tag生成一个HTML DOM Tree节点。从第12步到第17步，生成相应的Rendering Tree的节点，并把它和HTML DOM Tree的节点勾连在一起。这张图的细节过多，读解不容易。Figure 4把第8步到第17步演示了一下。
Figure 4. An illustration of the construction of a DOM tree node and its corresponding Rendering tree node.
Courtesy http://farm4.static.flickr.com/3306/3554259140_3deb9736ea_o.jpg
值得注意的是，每当HTMLParser生成一个DOM Tree的节点的时候，相应地，也同时生成一个Rendering Tree节点。然后把它们两个新节点勾连在一起。换而言之，Rendering Tree与DOM Tree同步生长。
Webkit 值得赞赏的地方非常多，但是HTMLParser让DOM Tree和Rendering Tree同步生长的做法，却值得商榷。如果同步生长，那么Rendering Tree必然平铺直叙地刻板地忠实于DOM Tree。假设先生成DOM Tree，再生成Rendering Tree，把两者割裂开，就有机会让Webkit发挥更加奇妙的布局和渲染。平铺直叙固然符合大多数人在大多数时间里的阅读习惯，但是离经叛道的设计，也会有市场。一个例子就是上一章末尾处那张多视点的地图。如果让DOM Tree与Rendering Tree同步生长，这样的布局和渲染是难以想像的。
QT之webkit 分析（九）
WebKit的显示，继续转邓侃博士的blog。【21】WebKit，为了布局，忙并美丽着
如果没有1440年以后活字印刷术的大规模普及，或许就不会有文艺复兴运动，更不会有后来的启蒙运动。如果没有这两个运动的开展，或许就不会有世界范围的工业化。
在活字印刷术出现以前，每出版一本书，都必须先刻制一套模版，称为雕版，每套雕版上的每一个字，都是手工雕刻的。不仅制作雕版费时费力，而且有了错误不容易 更改。活字印刷术的进步在于，可以预先批量生产各种样式和大小的字体，称为活字。需要出版某一本书籍时，先制作该书的页面模版，模版做好以后，只需要把这 些活字摆放在模版上即可。如果出现错误，只需要调换某些活字，既省时又省力。如果某本书的模版不需要长期保存，还可以把模版中摆放的活字拆解下来，在印刷 其它图书时用，节约成本。
活字印刷术没有解决的问题，1. 图像的印刷。起初不能印刷笔触丰富，层次复杂的图像，一直到1796年，石板印刷术(lithography)出现以后，才能印刷表现手段丰富的图像。 2. 灵活的布局排版。纸张大小不同，布局排版也不同，布局变了，需要重新摆放活字，而且有时候还需要改变字体和大小。
灵活的布局排版对于纸质书籍来说，或许并不太重要，但是对于电脑浏览器来说，却必须实现完全的自动化。否则，每当用户改变浏览器窗口的大小的时候，页面内容就不能正确显示。对于手机浏览器来说，布局排版的自动化尤其重要，因为不同手机的屏幕不一致，而且屏幕分辨率也不同。
但是即便是浏览器，也没有摆脱传统的排版方式。所谓传统的排版方式，基本是横平竖直的，单一的鸟瞰视角。
Figure 1. Incunabulum, the end of 15'th century
Courtesy http://www.citrinitas.com/history_of_viscom/images/printing/venice-1505.jpg
Figure 2. City of Words, by Vito Acconi, 1999
Courtesy http://upload.wikimedia.org/wikipedia/en/6/63/%27City_of_Words%27%2C_lithograph_by_Vito_Acconci%2C_1999.jpg
Figure 1 中显示的是1490年代的书籍，不难看出，现代书报中广泛使用的双列，边注，页码，首字母大写等等，都是继承了500多年以前的做法。而CSS规范，囊括了所有这些页面设计的要素。
在当今信息爆炸的形势下，如何安排页面的布局排版，在有限的页面面积内，承载更多内容，突出读者关注的内容，增强页面设计的视觉美感，成为不可回避的问题。 例如，手机购物的UI设计，既要包含商品简介，又要包含用户意见反馈，还要包含实物照片，以及各个不同商场的标价等等。完美的页面设计，不仅要求简练而清 晰，而且也不能遗漏相关内容，实在是一件困难的事情。可以说，手机购物之所以不普及，与手机购物的UI设计笨拙而丑陋是相关的。
要巧妙地 设计手机应用的UI设计，终极而言，需要突破传统的单一鸟瞰视角的方式，Figure 2 就是这方面的尝试。Webkit能不能做到这一点？原理上是可以做到的，但是必须修改源代码。但是在改造以前，我们还是先踏踏实实研究一下，Webkit 的布局排版的内部机制是什么。只有充分了解对方之长，才有可能改进对方之短。
读解Webkit排版布局与绘制的具体实现以前，首先需要明确的是，Webkit把排版布局(layout)，与绘制(paint)，分开处理。
Layout负责确定Render Tree中，每个叶子和中间节点的位置。每个节点在屏幕上的显示，都呈长方形格局。所谓位置，指的是这个长方形左上角起始坐标(X,Y)，以及长方形的宽度和高度。每个中间节点的长方形，里面嵌套着若干小长方形，对应这个中间节点的后代节点等等。
在Layout过程结束以后，Webkit启动 Paint过程，负责把Render Tree中各个叶子节点，在相应的位置绘制出来。Webkit 把具体绘制的工作，交给第三方图形工具库(Graphics Library)去完成。常用的第三方图形工具库包括QT，GTK+，Wx，Skia，Cairo等等。
打个比方，图形工具库相当于活字，以及绘制图像的石板(lithography)，它们负责paint。而从严格意义上来说，Webkit的主要工作是layout，也就是排版布局，相当于版面模版。
关于图形库，台湾的开源高手，黄敬群(Jim Huang / jserv)，写过一篇介绍Google Skia 图形库的文章(http://blog.linux.org.tw/~jserv/archives/002095.html)。文中谈到，
Google 为了搭建Android平台，于2005年8月并购了Android公司。同年11月份，Google还收购了Skia公司。2007年11 月，Google发布Android，并公开部分源代码。当人们热衷于探究Android Dalvik VM的奥秘的时候，忽略了Skia的意义。
2008年9月，Google发布了以改良的Webkit为核心的Chrome PC浏览器。当人们热衷于探究V8 JavaS
Skia是一个2D图形工具库，该产品的特色在于，能够在手机等等移动设备中，以较低的内存和CPU消耗，呈现高品质的2D图形。
Skia 的创办人，Mike Reed，是图形技术方面的顶尖人物。Mike早年任职于Apple，参与QuickDraw GX项目，处理字型和图像显示。后来他跳槽到OpenWave，开发手机浏览器。在OpenWave工作期间，与Benoit Schillings合作，在50-300KB的内存空间内，提供图层之间alpha blended方式的预览，以及全功能向量矩阵转换等等，真可谓螺丝壳里做道场。后来Benoit Schillings离开OpenWave，去Trolltech任职CTO。Trolltech的主打产品是大名鼎鼎的QT。再后来Trolltech 被Nokia并购，Benoit随之加入Nokia。Benoit Schillings离开OpenWave不久，Mike Reed也离开了OpenWave，去创建Skia公司。
Courtesy http://www.flickr.com/photos/87209438%40N00/3609632247/sizes/l/
Courtesy http://www.flickr.com/photos/87209438%40N00/3609632249/sizes/l/
Figure 3 和 Figure 4， 分别显示了Webkit执行排版布局(layout)，以及绘制(paint)的两个过程。仔细查看这两张sequence diagrams，会发现以下特点，
1. Layout 和 Paint 这两个过程完全分开。开始执行Paint过程以前，必然预先执行过Layout，否则图形库就不知道在哪里写字以及显示图像。但是这并不意味 着，Layout执行结束后，随即就立刻执行Paint。实际上，Layout执行结束后，触发一个事件，这个事件启动Paint过程。但是Paint过 程也可以被其它事件触发，譬如屏幕内容的切换，以及把隐藏的浏览器窗口复原等等。
2. Layout 涵盖了所有CSS规定的布局要素。包括页面边缘与内容之间的空白，文字对插入图像的避让(floating)，单列与多列，上下层覆盖(z-index)等等。
3. 图像，视频播放器插件，Applet等等，在 Layout 被称作 Replaced Render Object。这些 Replaced 元素的宽度和高度可以由CSS规定。如果CSS没有规定，就解析这些元素的数据流，譬如一个JPG照片的metadata里，规定了这幅照片原件的宽度和 高度。如果元素自己也没有规定宽度高度，就使用Webkit提供的缺省值。
4. 文字的宽度根据页面的排版来确定。譬如一页中包含多列文字，则每列文字宽度相等。每列文字的宽度，乘以列数，加上列与列之间的夹缝，加上页面边缘空白等 等，应当等于页面总的宽度。假设页面总的宽度已知，边缘空白，和列与列之间的夹缝的宽度也已知，就可以反推文字的宽度。
5. Render Tree中每个节点在屏幕上的显示，都呈长方形格局。前面第3点和第4点，描述了宽度的确定。而高度的确定，取决于这个中间节点的所有后代节点的高度的总 和。对于 Replaced 元素来说，它的高度相对比较容易确定，而文字段落的高度，需要根据字数，字型，以及字体大小计算得出。
6. 在 Layout 过程中，反复出现以 Repaint 为开头的子过程，例如 repaintAfterLayoutIfNeed
7. 相对于 Layout 过程，Paint 过程的逻辑要简单得多。Paint的过程，大致按照深度优先的顺序，遍历整棵RenderTree。也就是说，从最左边的叶子节点开始，从左向右逐个绘制 RenderTree所有可以显示的叶子节点。所谓“可以显示的叶子节点”，是因为CSS中可以规定，不显示某些叶子。
反复研究以上Layout和Paint的过程，我们有以下看法。
1. Layout 是一个计算量很繁重的过程。之所以繁重，主要体现在估算完每个RenderTree节点的宽度尤其是高度以后，需要相应调整这个节点的前辈节点以及左邻右舍兄弟节点的位置。对于文字段落而言，它的高度有赖于字数，字体和大小，所以估算不容易准确。
有没有可能把Layout 过程，与第一遍 Paint 过程合二为一？ 只要遍历一次RenderTree的所有叶子节点，绘制图像并码字。Paint过程结束后，各个叶子节点对应的长方形的起始位置的(X,Y)坐标，以及宽 度和高度都自然迎刃而解。然后再由叶子节点开始，逐步确定RenderTree中，各个中间节点的起始位置和宽度高度。这样做的好处是，可以大大降低 Layout 过程的成本。
2. Layout 过程假设每个RenderTree 的节点都对应一个长方形屏幕区域。受限于这个规定，类似于Figure 2的效果，就显示不出来。有没有可能取消这个限制？SVG不仅提供了强大的绘图能力，而且也提供了强大的排版布局能力。能不能把CSS当着SVG格式的一 个子集来看待？
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