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Posted 12 Aug 2002

Small Web Agents using VB - Part II

, 12 Aug 2002
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Small Web Agents using VB - Part II


In the previous article, we saw a simple VB application which will pull out the HTML page of a particular URL. In this article, we will build a small web crawler which will crawl through all the links in the given URL.

  1. Setting up the Visual Basic Environment with required Components and Libraries:
    • Open Visual Basic and create a new project (user Standard EXE).
    • Select Project -> References from the main menu and add the following Microsoft Libraries:
      • Microsoft HTML Object Library
    • Add Microsoft Windows Common Controls to the toolbox as follows. Select Project -> Components from the main menu. The Components window will open. With the controls tab selected, scroll down and click the check box preceding the components:
      • Microsoft Windows Common Control 6.x
  2. Set up the UI for the Crawler
    • Add a label, two button controls, a listbox, and a treeview control as below:

      Click to enlarge image

  3. Add the code for the Crawler:
    • On click of the start button, populate the list box with all the links under the given URL:
      Private Sub cmdStart_Click()
      	'1 will populate lstlinks with all the parent links 
               'in the requested URL
      	getLinks txtURL.Text, 1
      End Sub
    • The getlinks function based on the second parameter populates either the listbox or the treeview. Here since the parameter is 1, it populates the listbox with all the links under the URL:
    • Private Sub getLinks(strURL As String, iParentChild As Integer, _
      	Optional iParentNo As Integer)
          Dim objLink As HTMLLinkElement
          Dim objMSHTML As New MSHTML.HTMLDocument
          Dim objDoc As New MSHTML.HTMLDocument
          Dim objNode As Node
          Set objDoc = objMSHTML.createDocumentFromUrl(txtURL.Text, vbNullString)
          MousePointer = vbHourglass
          While objDoc.readyState <> "complete"
          'get all Links
          For Each objLink In objDoc.links
              If iParentChild = 1 Then
                  lstLinks.AddItem objLink
              ElseIf iParentChild = 2 Then
                  'lstInnerLinks.AddItem objLink
                  Set objNode = trvLinks.Nodes.Add(iParentNo, tvwChild)
                  objNode.Text = objLink
                  'objNode.Image = "leaf"
              End If
          MousePointer = vbNormal
      End Sub
    • If the user wishes to go further down with some of the links, then she/he can select the links and press Get Inner Links Button:
      Private Sub cmdGet_Click()
          Dim iCount As Integer
          'Dim objNode As New Node
          If lstLinks.SelCount = 0 Then
              MsgBox "Please Select a Link"
              Exit Sub
              'objNode.Text = lstLinks.Text
              'For iCount = 0 To lstLinks.ListCount - 1
              iCount = 0
              While iCount <= lstLinks.ListCount - 1
                  If lstLinks.Selected(iCount) Then
                      trvLinks.Nodes.Add , , , lstLinks.List(iCount)
                      getLinks lstLinks.List(iCount), 2, trvLinks.Nodes.Count
                      lstLinks.RemoveItem (iCount)
                      iCount = iCount + 1
                  End If
          End If
      End Sub
    • All the inner links will get populated inside the Treeview. Now if the user further wishes to drilldown, he can double click on those URLs in the treeview:
      Private Sub trvLinks_DblClick()
          getLinks trvLinks.SelectedItem.Text, 2, trvLinks.SelectedItem.Index
      End Sub
    • Then finally the screen would look something like this:

      Click to enlarge image


  • 12th August, 2002: Initial post


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Gopi Subramanian
Web Developer
India India
No Biography provided

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Comments and Discussions

QuestionIs there a part I for this article. Pin
legoman5527-Mar-06 21:07
memberlegoman5527-Mar-06 21:07 

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