84     m_callback.set(request, formState.get(), function, argument);//callContinueLoadAfterNavigationPolicy

一旦客戶端指示FrameLoader將本次加載視為一個導航，FrameLoader就推動DocumentLoader進入”provisional”狀態，在該狀態，DocumentLoader會發起一個網絡請求，并等待以確定網絡請求將發起一個下載還是一個新的document。
——>page->d->acceptNavigationRequest（Source/WebKit/qt/WebCoreSupport/FrameLoaderClientQt.cpp:1271）
        callPolicyFunction(Source/WebKit/qt/WebCoreSupport/FrameLoaderClientQt.cpp:1283)
        回調函數？如上
         //——>(m_frame->loader()->policyChecker()->*function)(action) (245)
——>callback.call(Source/WebCore/loader/PolicyChecker.cpp:160)
——>m_navigationFunction(Source/WebCore/loader/PolicyCallback.cpp:103)
——>loader->continueLoadAfterNavigationPolicy(Source/WebCore/loader/FrameLoader.cpp:2977)

2974 void FrameLoader::callContinueLoadAfterNavigationPolicy(void* argument,
2975     const ResourceRequest& request, PassRefPtr<FormState> formState, bool shouldContinue)
2976 {
2977     FrameLoader* loader = static_cast<FrameLoader*>(argument);
2978     loader->continueLoadAfterNavigationPolicy(request, formState, shouldContinue);
2979 }

——>continueLoadAfterWillSubmitForm(Source/WebCore/loader/FrameLoader.cpp:3105)
——>m_provisionalDocumentLoader->startLoadingMainResource(Source/WebCore/loader/FrameLoader.cpp:2572)

接下去，DocumentLoader會創建一個MainResourceLoader對象，這個對象主要用來通過ResourceHandle接口同平臺網絡庫進行交互。將MainResourceLoader和DocumentLoader分開來主要有兩個目的：（1）MainResourceLoader讓DocumentLoader從處理ResourceHandle回調的細節中抽身出來（2）降低MainResourceLoader的生命周期和DocumentLoader的生命周期（同Document綁定）的耦合度。

——>m_mainResourceLoader->load(Source/WebCore/loader/DocumentLoader.cpp:798)
——>loadNow(Source/WebCore/loader/MainResourceLoader.cpp:612)
——>ResourceHandle::create(Source/WebCore/loader/MainResourceLoader.cpp:585)
——>newHandle->start(Source/WebCore/platform/network/ResourceHandle.cpp:71)
——>new QNetworkReplyHandler(Source/WebCore/platform/network/qt/ResourceHandleQt.cpp:100)
       ——>r.toNetworkRequest(Source/WebCore/platform/network/qt/QNetworkReplyHandler.cpp:399) 設置http頭
——>m_queue.push 提交事件？ (Source/WebCore/platform/network/qt/QNetworkReplyHandler.cpp:401)
——>flush();(Source/WebCore/platform/network/qt/QNetworkReplyHandler.cpp:164)         
——>(m_replyHandler->*(m_enqueuedCalls.takeFirst()))(); （Source/WebCore/platform/network/qt/QNetworkReplyHandler.cpp:195）
——>QNetworkReplyHandler::start() (Source/WebCore/platform/network/qt/QNetworkReplyHandler.cpp:659)
        建立連接(ESTABLISHED)、發送請求 665： QNetworkReply* reply = sendNetworkRequest(d->m_context->networkAccessManager(), d->m_firstRequest); 
         ——>601 WebCore::QNetworkReplyHandler::sendNetworkRequest 
               ——>626 manager->get(m_request)
      665     QNetworkReply* reply = sendNetworkRequest(d->m_context->networkAccessManager(), d->m_firstRequest);
      669     m_replyWrapper = new QNetworkReplyWrapper(&m_queue, reply, m_resourceHandle->shouldContentSniff() && d->m_context-
    >mimeSniffingEnabled(), this);
               ——>208 QNetworkReplyWrapper::QNetworkReplyWrapper(QNetworkReplyHandlerCallQueue* queue, QNetworkReply* reply, bool
    sniffMIMETypes, QObject* parent)
                  209     : QObject(parent)
                  210     , m_reply(reply)
                  211     , m_queue(queue)
                  212     , m_responseContainsData(false)
                  213     , m_sniffMIMETypes(sniffMIMETypes)
                  214 {
                   215     Q_ASSERT(m_reply);
                  216 
                  217     // setFinished() must be the first that we connect, so isFinished() is updated when running other slots.
                   //QT通過此種方式異步回調??(yes, 信號、槽機制，http://www.ibm.com/developerworks/cn/linux/guitoolkit/qt/signal-slot/ )
                  218     connect(m_reply, SIGNAL(finished()), this, SLOT(setFinished()));
                  219     connect(m_reply, SIGNAL(finished()), this, SLOT(receiveMetaData()));
                  220     connect(m_reply, SIGNAL(readyRead()), this, SLOT(receiveMetaData()));
                  221 }

一旦加載系統接收到足夠的信息可以確定資源確實代表了document，FrameLoader就將DocumentLoader推向”committed”狀態，在該狀態中，frame將顯示document。