c＃WebBrowser DocumentText只能工作一次但不能循环吗？

Question

I am interested in checking the content of a website, the content changes frequently and when I view the website on any browser, it refreshes itself every 30 seconds. I want to know when the content has changed.

I am using winforms and I want to just click a button to start a loop, every 30 seconds. I don't want to hit the website too frequently, in fact the web pages own refresh is more than enough for my needs.

My code works when I click a button (btnCheckWebsite), if I wait a minute and then click btnCheckWebsite again, my message box pops up because the web page has changed. This is great however I want to do this in a while loop. When I un-comment my while loop, the DocumentText never changes. I have debugged it and for some reason it's the same text every time, even when the web page has changed in the real world, it stays the same in my code.

So my question is why can't I use a loop and what can I do instead to run this repeatedly without any input from me?

As a bonus, I would like to remove the .Refresh() I added this because it won't work without it however as I understand it, this refreshes the whole page. When I use a browser I see the page updating even when I don't refresh the whole page.

Just for background info, I did start by having a WebBrowser control on my form, the page refreshes automatically. I used the same code and have the same problem, interestingly, the WebBrowser control on my windows form refreshes by itself no problem, until I click btnCheckWebsite and then it stops refreshing! Also I know about webrequest but I don't know how to use it for my purposes.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace Check_Website
{
    public partial class Form1 : Form
    {
        public WebBrowser _memoryWebBrowser = new WebBrowser();
        String _previousSource = "emptySource";

        public Form1()
        {
            InitializeComponent();

           _memoryWebBrowser.Navigate(new Uri("http://www.randomurl.com/"));

        }

        private void btnCheckWebsite_Click(object sender, EventArgs e)
        {
            //I want to un-comment this while loop and let my code run itself but it stops working
            //when I introduce my while loop.

            //while (1 < 2 )
            //{
                //Thread.Sleep(30000);

                checkWebsite();

            //}
        }

        private void checkWebsite()
        {
            //Why do I need this refresh? I would rather not have to hit the web page with a refresh.
            //When I view the webpage it refreshed with new data however when I use a WebBrowser
            //the refresh just doesn't happen unless I call Refresh.
            _memoryWebBrowser.Refresh();

            Thread.Sleep(500);

            while (((_memoryWebBrowser.ReadyState != WebBrowserReadyState.Complete) || (_memoryWebBrowser.DocumentText.Length < 3000)))
            {
                Thread.Sleep(1000);
            }


            String source = _memoryWebBrowser.DocumentText;

            if ((source != _previousSource) && (_previousSource != "emptySource"))
            {
                //Hey take a look at the interesting new stuff on this web page!!
                MessageBox.Show("Great news, there's new stuff on this web page www.randomurl.co.uk!!" );
            }

            _previousSource = source;

        }
    }
}

Answer 1

You'd need to do your processing upon DocumentCompleted event. This event is asynchronous, so if you want to do this in a loop, the execution thread must pump messages for this event to fire. In a WinFroms app, your UI thread is already pumping messages in Applicaiton.Run , and the only other endorsed way to enter nested message loop on the same thread is via a modal form (here's how it can be done , see in the comments). Another (IMO, better) way of doing such Navigate / DocumentCompleted logic without a nested message loop is by using async/await , here's how . In the classic sense, this is not exactly a loop, but conceptually and syntactically it might be exactly what you're looking for.

Answer 2

You can catch the WebBrowser.Navigated Event to get notified when the page was reloaded. So you wouldn't need a loop for that. (I meant the ready loop)

Just navigate every 30 seconds to the page in a loop and in the Navigated Event you can check whether the site has changed or not.

Answer 3

您最好连接DocumentCompleted事件以检查其DocumentText属性！

Answer 4

The WebBrowser Element is very buggy and has much overhead for your needs. Instead of that you should use WebRequest. Because you said you don't know how to use, here's an (working) example for you.

using System;
using System.Windows.Forms;
using System.Net;
using System.IO;

namespace Check_Website
{
    public partial class Form1 : Form
    {
        String _previousSource = string.Empty;
        System.Windows.Forms.Timer timer;

        private System.Windows.Forms.CheckBox cbCheckWebsite;
        private System.Windows.Forms.TextBox tbOutput;

        public Form1()
        {
            InitializeComponent();

            this.cbCheckWebsite = new System.Windows.Forms.CheckBox();
            this.tbOutput = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // cbCheckWebsite
            // 
            this.cbCheckWebsite.AutoSize = true;
            this.cbCheckWebsite.Location = new System.Drawing.Point(12, 12);
            this.cbCheckWebsite.Name = "cbCheckWebsite";
            this.cbCheckWebsite.Size = new System.Drawing.Size(80, 17);
            this.cbCheckWebsite.TabIndex = 0;
            this.cbCheckWebsite.Text = "checkBox1";
            this.cbCheckWebsite.UseVisualStyleBackColor = true;
            // 
            // tbOutput
            // 
            this.tbOutput.Location = new System.Drawing.Point(12, 35);
            this.tbOutput.Multiline = true;
            this.tbOutput.Name = "tbOutput";
            this.tbOutput.Size = new System.Drawing.Size(260, 215);
            this.tbOutput.TabIndex = 1;
            // 
            // Form1
            // 
            this.ClientSize = new System.Drawing.Size(284, 262);
            this.Controls.Add(this.tbOutput);
            this.Controls.Add(this.cbCheckWebsite);
            this.Name = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);
            this.PerformLayout();

            timer = new System.Windows.Forms.Timer();
            timer.Interval = 30000;
            timer.Tick += timer_Tick;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            timer.Start();
        }

        void timer_Tick(object sender, EventArgs e)
        {
            if (!cbCheckWebsite.Checked) return;

            WebRequest request = WebRequest.Create("http://localhost/check_website.html");
            request.Method = "GET";

            WebResponse response = request.GetResponse();

            string newContent;
            using (var sr = new StreamReader(response.GetResponseStream()))
            {
                newContent = sr.ReadToEnd();
            }

            tbOutput.Text += newContent + "\r\n";

            if (_previousSource == string.Empty)
            {
                tbOutput.Text += "Nah. It's empty";
            }
            else if (_previousSource == newContent)
            {
                tbOutput.Text += "Nah. Equals the old content";
            }
            else
            {
                tbOutput.Text += "Oh great. Something happened";
            }

            _previousSource = newContent;
        }
    }
}