chromium - post task for web-ui

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。
查了chromium工程中的已有实现，看看BrowserThread都怎么用。发现人家都是用BrowserThread::PostDelayedTask，我也用BrowserThread::PostDelayedTask来发起任务，在任务指定的本类属回调中，再用BrowserThread::PostDelayedTask发起下一个任务。这时，web-ui很流畅，一点都不卡。

实验

BrowserThread::PostDelayedTask的用法例子
web-ui中代码很少，只有初始化的代码。
干活都时绑定一个WebUIMessageHandler子类来干活。

// 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);
}