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前言

web-ui和js交互，为了避免阻塞web-ui的消息循环，不能直接PostTask或直接bind来干活，需要用和UI相关的BrowserThread::PostTask.

通过实验可知，即使用BrowserThread::PostTask，发起的任务很频繁时，web-ui也卡的很。

实验

BrowserThread::PostDelayedTask的用法例子

// Class acting as a controller of the chrome://download-internals WebUI.class DownloadInternalsUIMessageHandler : public content::WebUIMessageHandler,                                          public download::Logger::Observer { public:  DownloadInternalsUIMessageHandler();  ~DownloadInternalsUIMessageHandler() override;  // content::WebUIMessageHandler implementation.  void RegisterMessages() override;

在WebUIMessageHandler子类中，找一个只有初始化时才执行的成员函数，PostDelayedTask即可。PostDelayedTask非常适合在消息循环中绑定自己类的成员函数来干活。

void DownloadInternalsUIMessageHandler::HandleGetServiceStatus(    const base::ListValue* args) {  AllowJavascript();  const base::Value* callback_id;  CHECK(args->Get(0, &callback_id));  ResolveJavascriptCallback(*callback_id,                            download_service_->GetLogger()->GetServiceStatus());	// 这里是服务下载有效, 启动一次本web-ui, 这里只进入一次，很适合自动启动任务	post_ui_task_for_download();}void DownloadInternalsUIMessageHandler::post_ui_task_for_download(){	// http://192.168.2.222/a.txt // 可以用http下载的测试文件	m_str_url_by_js_first = "http://192.168.2.222/a.txt";	BrowserThread::PostDelayedTask(		BrowserThread::UI,		FROM_HERE,		base::BindOnce(&DownloadInternalsUIMessageHandler::HandleStartDownload_url_again, base::Unretained(this)),		base::TimeDelta::FromSeconds(2)); // 延时是以秒为单位的, 都是约数设置10秒延时，有时7秒发起任务，有时11秒发起任务}

void DownloadInternalsUIMessageHandler::HandleStartDownload_url_again(){	if (!m_str_url_by_js_first.empty()) {		GURL url_now = GURL(m_str_url_by_js_first);		HandleStartDownload_url(url_now);	}}void DownloadInternalsUIMessageHandler::HandleStartDownload_url(GURL& url){	if (!url.is_valid()) {		LOG(WARNING) << "Can't parse download URL, try to enter a valid URL.";		return;	}	download::DownloadParams params;	params.guid = base::GenerateGUID();	params.client = download::DownloadClient::DEBUGGING;	params.request_params.method = "GET";	params.request_params.url = url;	net::NetworkTrafficAnnotationTag traffic_annotation =		net::DefineNetworkTrafficAnnotation("download_internals_webui_source", R"(          semantics {            sender: "Download Internals Page"            description:              "Starts a download with background download service in WebUI."            trigger:              "User clicks on the download button in "              "chrome://download-internals."            data: "None"            destination: WEBSITE          }          policy {            cookies_allowed: YES            cookies_store: "user"            setting: "This feature cannot be disabled by settings."            policy_exception_justification: "Not implemented."          })");	params.traffic_annotation =		net::MutableNetworkTrafficAnnotationTag(traffic_annotation);	DCHECK(download_service_);	download_service_->StartDownload(params);}

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