谷歌浏览器的源码分析(26)

消息的流通过程，是一个不同类相互交流的过程，如果不了解这个过程，根本就不知道这些类是怎么样相互协作的。由于上一次说到ViewHostMsg_RequestResource消息已经发送出来，它的处理过徎其实就是一般资源的消息处理过程，下面就来看看这个消息的处理过程，如下：
1. base::MessagePumpWin::Run函数消息
2. base::MessagePumpWin::RunWithDispatcher
3. base::MessagePumpWin::DoRunLoop
4. base::MessagePumpWin::WaitForWork
5. base::MessagePumpWin::SignalWatcher
6. IPC::Channel::OnObjectSignaled 通道的消息处理。
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

消息通过上面12个函数的流转，到达到处理这个消息的函数ResourceDispatcherHost::BeginRequest，它在里面就会把消息变成一个网络下载请求，函数代码如下：
#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) {

是否已经传送完成，或者关闭。
#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

保证所有插件已经加载。
#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

构造事件处理器。
#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

构造下载请求。
#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

设置上传数据。
#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

安装一个CrossSiteEventHandler事件处理器。
#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

创建一个缓冲区处理。
#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

开始调用内部处理请求函数。
#108 BeginRequestInternal(request, request_data.mixed_content);
#109 }

通过上面的分析，已经知道消息转换为一个请求任务URLRequest，这个任务就需要交给后面的工作进程来处理了，它是通过函数BeginRequestInternal来把任务进一步发送出去。