How to make asynchronous calls using HtmlAgilityPack?

问题

I'm trying to get the table with id table-matches available here. The problem is that table is loaded using ajax so I don't get the full html code when I download the page:

string url = "http://www.oddsportal.com/matches/soccer/20180701/";

using (HttpClient client = new HttpClient())
{
    using (HttpResponseMessage response = client.GetAsync(url).Result)
    {
        using (HttpContent content = response.Content)
        {
            string result = content.ReadAsStringAsync().Result;
        }
    }
}

the html returned does not contains any table, so I tried to see if there is a problem of the library, infact I setted on Chrome (specifically on the Dev console F12) javascript off and same result on the browser.

Fox fix this problem I though to use a WebBrowser, in particular:

webBrowser.Navigate("oddsportal.com/matches/soccer/20140221/"); 
HtmlElementCollection elements = webBrowser.Document.GetElementsByTagName("table");

but I want ask if I can load also the full html doing asynchronus calls, someone has encountered a similar problem?

Could you please share a solution? Thanks.

回答1:

The main issue with this page is that content inside table-matches is loaded via ajax. And neither HttpClient nor HtmlAgilityPack unable to wait for ajax to be executed. Therefore, you need different approach.

Approach #1 - Use any headless browser like PuppeteerSharp

using PuppeteerSharp;
using System;
using System.Threading.Tasks;

namespace PuppeteerSharpDemo
{
    class Program
    {
        private static String url = "http://www.oddsportal.com/matches/soccer/20180701/";

        static void Main(string[] args)
        {
            var htmlAsTask = LoadAndWaitForSelector(url, "#table-matches .table-main");
            htmlAsTask.Wait();
            Console.WriteLine(htmlAsTask.Result);

            Console.ReadKey();
        }

        public static async Task<string> LoadAndWaitForSelector(String url, String selector)
        {
            var browser = await Puppeteer.LaunchAsync(new LaunchOptions
            {
                Headless = true,
                ExecutablePath = @"c:\Program Files (x86)\Google\Chrome\Application\chrome.exe"
            });
            using (Page page = await browser.NewPageAsync())
            {
                await page.GoToAsync(url);
                await page.WaitForSelectorAsync(selector);
                return await page.GetContentAsync();
            }
        }
    }
}

In purpose of cleanness, I've posted output here here. And once you get html content you are able to parse it with HtmlAgilityPack.

Approach #2 - Use pure Selenium WebDriver. Can be launched in headless mode.

using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;
using OpenQA.Selenium.Support.UI;
using System;

namespace SeleniumDemo
{
    class Program
    {
        private static IWebDriver webDriver;
        private static TimeSpan defaultWait = TimeSpan.FromSeconds(10);
        private static String targetUrl = "http://www.oddsportal.com/matches/soccer/20180701/";
        private static String driversDir = @"../../Drivers/";

        static void Main(string[] args)
        {
            webDriver = new ChromeDriver(driversDir);
            webDriver.Navigate().GoToUrl(targetUrl);
            IWebElement table = webDriver.FindElement(By.Id("table-matches"));
            var innerHtml = table.GetAttribute("innerHTML");
        }

        #region (!) I didn't even use this, but it can be useful (!)
        public static IWebElement FindElement(By by)
        {
            try
            {
                WaitForAjax();
                var wait = new WebDriverWait(webDriver, defaultWait);
                return wait.Until(driver => driver.FindElement(by));
            }
            catch
            {
                return null;
            }
        }

        public static void WaitForAjax()
        {
            var wait = new WebDriverWait(webDriver, defaultWait);
            wait.Until(d => (bool)(d as IJavaScriptExecutor).ExecuteScript("return jQuery.active == 0"));
        }
        #endregion
    }
}

Approach #3 - Simulate ajax requests

If you analyse the page loading using Fiddler or browser's profiler (F12) you can see that all data is coming with these two requests:

So you can try to execute them directly using HttpClient. But in this case you may need to track authorization headers and maybe something else with each HTTP request.

来源：https://stackoverflow.com/questions/51457307/how-to-make-asynchronous-calls-using-htmlagilitypack

标签

html-agility-pack