前言

web-ui和js互動，為了避免阻塞web-ui的訊息迴圈，不能直接PostTask或直接bind來幹活，需要用和UI相關的BrowserThread::PostTask.

通過實驗可知，即使用BrowserThread::PostTask，發起的任務很頻繁時，web-ui也卡的很。
查了chromium工程中的已有實現，看看BrowserThread都怎麼用。發現人家都是用BrowserThread::PostDelayedTask，我也用BrowserThread::PostDelayedTask來發起任務，在任務指定的本類屬回撥中，再用BrowserThread::PostDelayedTask發起下一個任務。這時，web-ui很流暢，一點都不卡。

實驗

BrowserThread::PostDelayedTask的用法例子
web-ui中程式碼很少，只有初始化的程式碼。
幹活都時繫結一個WebUIMessageHandler子類來幹活。

// 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子類中，找一個只有初始化時才執行的成員函式，PostDelayedTask即可。PostDelayedTask非常適合在訊息迴圈中繫結自己類的成員函式來幹活。

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());


	// 這裡是服務下載有效, 啟動一次本web-ui, 這裡只進入一次，很適合自動啟動任務
	post_ui_task_for_download();
}

void DownloadInternalsUIMessageHandler::post_ui_task_for_download()
{
	// http://192.168.2.222/a.txt // 可以用http下載的測試檔案
	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)); // 延時是以秒為單位的, 都是約數設定10秒延時，有時7秒發起任務，有時11秒發起任務
}

 

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);
}