一种解决方案是使用SemaphoreSlim维持有限的资源池WebBrowser反对scrape并行网站。为所有人共享公共消息循环也是有意义的WebBrowser实例。
以下是它的实现方式，基于我的控制台网络scraper code你链接了。新的部分是WebBrowserPool类（警告：仅进行了轻微测试）：
using Microsoft.Win32;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace AsyncWebBrowserScraper
{
class Program
{
// by Noseratio - https://stackoverflow.com/a/23819021/1768303
// test: web-scrape a list of URLs
static async Task ScrapeSitesAsync(string[] urls, CancellationToken token)
{
using (var pool = new WebBrowserPool(maxParallel: 2, token: token))
{
// cancel each site in 30s or when the main token is signalled
var timeout = (int)TimeSpan.FromSeconds(30).TotalMilliseconds;
var results = urls.ToDictionary(
url => url, url => pool.ScrapeSiteAsync(url, timeout));
await Task.WhenAll(results.Values);
foreach (var url in results.Keys)
{
Console.WriteLine("URL:\n" + url);
string html = results[url].Result;
Console.WriteLine("HTML:\n" + html);
}
}
}
// entry point
static void Main(string[] args)
{
try
{
WebBrowserExt.SetFeatureBrowserEmulation(); // enable HTML5
var cts = new CancellationTokenSource((int)TimeSpan.FromMinutes(3).TotalMilliseconds);
var task = ScrapeSitesAsync(
new[] { "http://example.com", "http://example.org", "http://example.net", "http://www.bing.com", "http://www.google.com" },
cts.Token);
task.Wait();
Console.WriteLine("Press Enter to exit...");
Console.ReadLine();
}
catch (Exception ex)
{
while (ex is AggregateException && ex.InnerException != null)
ex = ex.InnerException;
Console.WriteLine(ex.Message);
Environment.Exit(-1);
}
}
}
/// <summary>
/// WebBrowserPool the pool of WebBrowser objects sharing the same message loop
/// </summary>
public class WebBrowserPool : IDisposable
{
MessageLoopApartment _apartment; // a WinFroms STA thread with message loop
readonly SemaphoreSlim _semaphore; // regulate available browsers
readonly Queue<WebBrowser> _browsers; // the pool of available browsers
readonly HashSet<Task> _pendingTasks; // keep track of pending tasks for proper cancellation
readonly CancellationTokenSource _cts; // global cancellation (for Dispose)
public WebBrowserPool(int maxParallel, CancellationToken token)
{
if (maxParallel < 1)
throw new ArgumentException("maxParallel");
_cts = CancellationTokenSource.CreateLinkedTokenSource(token);
_apartment = new MessageLoopApartment();
_semaphore = new SemaphoreSlim(maxParallel);
_browsers = new Queue<WebBrowser>();
_pendingTasks = new HashSet<Task>();
// init the pool of WebBrowser objects
_apartment.Invoke(() =>
{
while (--maxParallel >= 0)
_browsers.Enqueue(new WebBrowser());
});
}
// Navigate to a site and get a snapshot of its DOM HTML
public async Task<string> ScrapeSiteAsync(string url, int timeout, CancellationToken token = default(CancellationToken))
{
var navigationCts = CancellationTokenSource.CreateLinkedTokenSource(token, _cts.Token);
var combinedToken = navigationCts.Token;
// we have a limited number of WebBrowser objects available, so await the semaphore
await _semaphore.WaitAsync(combinedToken);
try
{
if (timeout != Timeout.Infinite)
navigationCts.CancelAfter(timeout);
// run the main logic on the STA thread
return await _apartment.Run(async () =>
{
// acquire the 1st available WebBrowser from the pool
var webBrowser = _browsers.Dequeue();
try
{
var task = webBrowser.NavigateAsync(url, combinedToken);
_pendingTasks.Add(task); // register the pending task
try
{
return await task;
}
finally
{
// unregister the completed task
_pendingTasks.Remove(task);
}
}
finally
{
// return the WebBrowser to the pool
_browsers.Enqueue(webBrowser);
}
}, combinedToken);
}
finally
{
_semaphore.Release();
}
}
// Dispose of WebBrowserPool
public void Dispose()
{
if (_apartment == null)
throw new ObjectDisposedException(this.GetType().Name);
// cancel and wait for all pending tasks
_cts.Cancel();
var task = _apartment.Run(() => Task.WhenAll(_pendingTasks.ToArray()));
try
{
task.Wait();
}
catch
{
if (!task.IsCanceled)
throw;
}
// dispose of WebBrowser objects
_apartment.Run(() =>
{
while (_browsers.Any())
_browsers.Dequeue().Dispose();
});
_apartment.Dispose();
_apartment = null;
}
}
/// <summary>
/// WebBrowserExt - WebBrowser extensions
/// by Noseratio - https://stackoverflow.com/a/22262976/1768303
/// </summary>
public static class WebBrowserExt
{
const int POLL_DELAY = 500;
// navigate and download
public static async Task<string> NavigateAsync(this WebBrowser webBrowser, string url, CancellationToken token)
{
// navigate and await DocumentCompleted
var tcs = new TaskCompletionSource<bool>();
WebBrowserDocumentCompletedEventHandler handler = (s, arg) =>
tcs.TrySetResult(true);
using (token.Register(
() => { webBrowser.Stop(); tcs.TrySetCanceled(); },
useSynchronizationContext: true))
{
webBrowser.DocumentCompleted += handler;
try
{
webBrowser.Navigate(url);
await tcs.Task; // wait for DocumentCompleted
}
finally
{
webBrowser.DocumentCompleted -= handler;
}
}
// get the root element
var documentElement = webBrowser.Document.GetElementsByTagName("html")[0];
// poll the current HTML for changes asynchronosly
var html = documentElement.OuterHtml;
while (true)
{
// wait asynchronously, this will throw if cancellation requested
await Task.Delay(POLL_DELAY, token);
// continue polling if the WebBrowser is still busy
if (webBrowser.IsBusy)
continue;
var htmlNow = documentElement.OuterHtml;
if (html == htmlNow)
break; // no changes detected, end the poll loop
html = htmlNow;
}
// consider the page fully rendered
token.ThrowIfCancellationRequested();
return html;
}
// enable HTML5 (assuming we're running IE10+)
// more info: https://stackoverflow.com/a/18333982/1768303
public static void SetFeatureBrowserEmulation()
{
if (System.ComponentModel.LicenseManager.UsageMode != System.ComponentModel.LicenseUsageMode.Runtime)
return;
var appName = System.IO.Path.GetFileName(System.Diagnostics.Process.GetCurrentProcess().MainModule.FileName);
Registry.SetValue(@"HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION",
appName, 10000, RegistryValueKind.DWord);
}
}
/// <summary>
/// MessageLoopApartment
/// STA thread with message pump for serial execution of tasks
/// by Noseratio - https://stackoverflow.com/a/22262976/1768303
/// </summary>
public class MessageLoopApartment : IDisposable
{
Thread _thread; // the STA thread
TaskScheduler _taskScheduler; // the STA thread's task scheduler
public TaskScheduler TaskScheduler { get { return _taskScheduler; } }
/// <summary>MessageLoopApartment constructor</summary>
public MessageLoopApartment()
{
var tcs = new TaskCompletionSource<TaskScheduler>();
// start an STA thread and gets a task scheduler
_thread = new Thread(startArg =>
{
EventHandler idleHandler = null;
idleHandler = (s, e) =>
{
// handle Application.Idle just once
Application.Idle -= idleHandler;
// return the task scheduler
tcs.SetResult(TaskScheduler.FromCurrentSynchronizationContext());
};
// handle Application.Idle just once
// to make sure we're inside the message loop
// and SynchronizationContext has been correctly installed
Application.Idle += idleHandler;
Application.Run();
});
_thread.SetApartmentState(ApartmentState.STA);
_thread.IsBackground = true;
_thread.Start();
_taskScheduler = tcs.Task.Result;
}
/// <summary>shutdown the STA thread</summary>
public void Dispose()
{
if (_taskScheduler != null)
{
var taskScheduler = _taskScheduler;
_taskScheduler = null;
// execute Application.ExitThread() on the STA thread
Task.Factory.StartNew(
() => Application.ExitThread(),
CancellationToken.None,
TaskCreationOptions.None,
taskScheduler).Wait();
_thread.Join();
_thread = null;
}
}
/// <summary>Task.Factory.StartNew wrappers</summary>
public void Invoke(Action action)
{
Task.Factory.StartNew(action,
CancellationToken.None, TaskCreationOptions.None, _taskScheduler).Wait();
}
public TResult Invoke<TResult>(Func<TResult> action)
{
return Task.Factory.StartNew(action,
CancellationToken.None, TaskCreationOptions.None, _taskScheduler).Result;
}
public Task Run(Action action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
}
public Task<TResult> Run<TResult>(Func<TResult> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
}
public Task Run(Func<Task> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
}
public Task<TResult> Run<TResult>(Func<Task<TResult>> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
}
}
}
