public async Task ScrapeAsync (IProgress <IProgressReportable> progress)
{
Uri host;
var runningTasks = new List <Task> ();
lock (options)
{
host = new Uri (options.Uri);
var start = new LinkItem (options.Uri, null); // Provide a starting scrape task
finishedLinkItems.Add (start);
Interlocked.Increment (ref totalLinkItems);
runningTasks.Add (DoScrapeAsync (start, options.HRefXPathExpression, options.ImageXPathExpression, host, progress));
}
// Use a do-while loop so that the loop executes at least once
do
{
runningTasks.Remove (await Task.WhenAny (runningTasks));
lock (options)
{
while (queuedLinkItems.Any () && (runningTasks.Count < options.MaxConcurrentOperations))
{
LinkItem link;
if (!queuedLinkItems.TryDequeue (out link)) continue;
finishedLinkItems.Add (link); // Add the dequeued link to the finished item bag as soon as possible
runningTasks.Add (DoScrapeAsync (link, options.HRefXPathExpression, options.ImageXPathExpression, host, progress));
}
}
}
while (runningTasks.Any ());
}
protected async Task <Page> ScrapePageAsync (LinkItem link, string hRefXPath, string imageXPath, Uri host, IProgress <IProgressReportable> progress)
{
var document = new HtmlDocument ();
try
{
using (var client = new WebClient ()) document.LoadHtml (await client.DownloadStringTaskAsync (new Uri (link.HRef)));
}
catch (WebException)
{
return null;
}
var linkItems = ScrapeLinkItems (document.DocumentNode.SelectNodes (hRefXPath), host);
var imageItems = ScrapeImageItems (document.DocumentNode.SelectNodes (imageXPath), host);
QueueLinksForScraping (linkItems);
progress?.Report (new PageProgressReport (finishedLinkItems.Count, Interlocked.Read (ref totalLinkItems)));
return new Page (linkItems, imageItems);
}
protected async Task DoScrapeAsync (LinkItem link, string hRefXPath, string imageXPath, Uri host, IProgress <IProgressReportable> progress)
{
var page = await ScrapePageAsync (link, hRefXPath, imageXPath, host, progress);
if (page == null) return;
await SaveImagesAsync (await ScrapeImagesAsync (page, host, progress), progress);
}