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消息的流通過程，是一個不同類相互交流的過程，如果不了解這個過程，根本就不知道這些類是怎么樣相互協作的。由于上一次說到 ViewHostMsg_RequestResource 消息已經發送出來，它的處理過徎其實就是一般資源的消息處理過程，下面就來看看這個消息的處理過程，如下：

1. ?????? base::MessagePumpWin::Run 函數消息

2. ?????? base::MessagePumpWin::RunWithDispatcher

3. ?????? base::MessagePumpWin::DoRunLoop

4. ?????? base::MessagePumpWin::WaitForWork

5. ?????? base::MessagePumpWin::SignalWatcher

6. ?????? IPC::Channel::OnObjectSignaled 通道的消息處理。

7. ?????? IPC::Channel::ProcessIncomingMessages

8. ?????? IPC::SyncChannel::SyncContext::OnMessageReceived

9. ?????? IPC::ChannelProxy::Context::TryFilters

10. ??? ResourceMessageFilter::OnMessageReceived

11. ??? IPC::MessageWithTuple<Tuple2<int,ViewHostMsg_Resource_Request> >::Dispatch<ResourceMessageFilter,int,ViewHostMsg_Resource_Request const &>

12. ??? ResourceMessageFilter::OnRequestResource

13. ??? ResourceDispatcherHost::BeginRequest

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消息通過上面 12 個函數的流轉，到達到處理這個消息的函數 ResourceDispatcherHost::BeginRequest ，它在里面就會把消息變成一個網絡下載請求，函數代碼如下：

#001 ? void ResourceDispatcherHost::BeginRequest(

#002 ????? Receiver* receiver,

#003 ????? HANDLE render_process_handle,

#004 ????? int render_process_host_id,

#005 ????? int render_view_id,

#006 ????? int request_id,

#007 ????? const ViewHostMsg_Resource_Request& request_data,

#008 ????? URLRequestContext* request_context,

#009 ????? IPC::Message* sync_result) {

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是否已經傳送完成，或者關閉。

#010 ??? if (is_shutdown_ ||

#011 ??????? !ShouldServiceRequest(render_process_host_id, request_data)) {

#012 ??? ?? // Tell the renderer that this request was disallowed.

#013 ????? receiver->Send(new ViewMsg_Resource_RequestComplete(

#014 ????????? render_view_id,

#015 ????????? request_id,

#016 ????????? URLRequestStatus(URLRequestStatus::FAILED, net::ERR_ABORTED)));

#017 ????? return;

#018 ??? }

#019 ?

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保證所有插件已經加載。

#020 ??? // Ensure the Chrome plugins are loaded, as they may intercept network

#021 ??? // requests. ? Does nothing if they are already loaded.

#022 ??? // TODO(mpcomplete): This takes 200 ms! ? Investigate parallelizing this by

#023 ??? // starting the load earlier in a BG thread.

#024 ??? plugin_service_->LoadChromePlugins(this);

#025 ?

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構造事件處理器。

#026 ??? // Construct the event handler.

#027 ??? scoped_refptr<EventHandler> handler;

#028 ??? if (sync_result) {

#029 ???? ? handler = new SyncEventHandler(receiver, request_data.url, sync_result);

#030 ??? } else {

#031 ????? handler = new AsyncEventHandler(receiver,

#032 ????????????????????????????????????? render_process_host_id,

#033 ????????????????????????????????????? render_view_id,

#034 ????????????????????????????????????? render_process_handle,

#035 ????????????????????????????????????? request_data.url,

#036 ????????????????????????????????????? this);

#037 ??? }

#038 ?

#039 ??? if (HandleExternalProtocol(request_id, render_process_host_id, render_view_id,

#040 ?????????????????????????????? request_data.url, request_data.resource_type,

#041 ?????????????????????????????? handler)) {

#042 ????? return;

#043 ??? }

#044 ?

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構造下載請求。

#045 ??? // Construct the request.

#046 ??? URLRequest* request = new URLRequest(request_data.url, this);

#047 ??? request->set_method(request_data.method);

#048 ??? request->set_policy_url(request_data.policy_url);

#049 ??? request->set_referrer(request_data.referrer.spec());

#050 ??? request->SetExtraRequestHeaders(request_data.headers);

#051 ??? request->set_load_flags(request_data.load_flags);

#052 ??? request->set_context(request_context);

#053 ??? request->set_origin_pid(request_data.origin_pid);

#054 ?

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設置上傳數據。

#055 ??? // Set upload data.

#056 ??? uint64 upload_size = 0;

#057 ??? if (!request_data.upload_content.empty()) {

#058 ????? scoped_refptr<net::UploadData> upload = new net::UploadData();

#059 ????? upload->set_elements(request_data.upload_content); ? // Deep copy.

#060 ????? request->set_upload(upload);

#061 ????? upload_size = upload->GetContentLength();

#062 ??? }

#063 ?

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安裝一個 CrossSiteEventHandler 事件處理器。

#064 ??? // Install a CrossSiteEventHandler if this request is coming from a

#065 ??? // RenderViewHost with a pending cross-site request. ? We only check this for

#066 ??? // MAIN_FRAME requests.

#067 ?? ? // TODO(mpcomplete): remove "render_process_host_id != -1"

#068 ??? // ?????????????????? when http://b/viewIssue?id=1080959 is fixed.

#069 ??? if (request_data.resource_type == ResourceType::MAIN_FRAME &&

#070 ??????? render_process_host_id != -1 &&

#071 ??? ???? Singleton<CrossSiteRequestManager>::get()->

#072 ??????????? HasPendingCrossSiteRequest(render_process_host_id, render_view_id)) {

#073 ????? // Wrap the event handler to be sure the current page's onunload handler

#074 ????? // has a chance to run before we render the new page.

#075 ????? handler = new CrossSiteEventHandler(handler,

#076 ????????????????????????????????????????? render_process_host_id,

#077 ????????????????????????????????????????? render_view_id,

#078 ??????????????????????????????????????? ?? this);

#079 ??? }

#080 ?

#081 ??? if (safe_browsing_->enabled() &&

#082 ??????? safe_browsing_->CanCheckUrl(request_data.url)) {

#083 ????? handler = new SafeBrowsingEventHandler(handler,

#084 ???????????????????????????????????????????? render_process_host_id,

#085 ???????????????????????????????????????????? render_view_id,

#086 ???????????????????????????????????????????? request_data.url,

#087 ???????????????????????????????????????????? request_data.resource_type,

#088 ??????????????????????????????????????? ????? safe_browsing_,

#089 ???????????????????????????????????????????? this);

#090 ??? }

#091 ?

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創建一個緩沖區處理。

#092 ??? // Insert a buffered event handler before the actual one.

#093 ??? handler = new BufferedEventHandler(handler, this, request);

#094 ?

#095 ??? // Make extra info and read footer (contains request ID).

#096 ??? ExtraRequestInfo* extra_info =

#097 ??????? new ExtraRequestInfo(handler,

#098 ???????????????????????????? request_id,

#099 ???????????????????????????? render_process_host_id,

#100 ??????????????????????? ????? render_view_id,

#101 ???????????????????????????? request_data.mixed_content,

#102 ???????????????????????????? request_data.resource_type,

#103 ???????????????????????????? upload_size);

#104 ??? extra_info->allow_download =

#105 ??????? ResourceType::IsFrame(request_data.resource_type);

#106 ??? request->set_user_data(extra_info); ? // takes pointer ownership

#107 ?

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開始調用內部處理請求函數。

#108 ??? BeginRequestInternal(request, request_data.mixed_content);

#109 ? }

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通過上面的分析，已經知道消息轉換為一個請求任務 URLRequest ，這個任務就需要交給后面的工作進程來處理了，它是通過函數 BeginRequestInternal 來把任務進一步發送出去。

谷歌瀏覽器的源碼分析(26)