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With FCL 2.0, Microsoft introduced the WebBrowser class (namespace System.Windows.Forms), which enables applications to incorporate Web browsing capabilities. The control provides methods for navigating Web pages and maintains its own history of Web sites visited. It also generates events as the user interacts with the content displayed in the control so that your application can respond to events such as the user clicking the links displayed in the content. This tutorial is intended for students and programmers who are already familiar with the fundamentals of Visual Basic GUIs and event handling.
[Note: This tutorial is an excerpt (Section 23.9) of Chapter 23, Networking, from our forthcoming textbook Visual Basic 2005 How to Program, 3/e. This tutorial may refer to other chapters or sections of the book that are not included here.
Permission Information: Deitel, Harvey M. and Paul J., Visual Basic 2005 How to Program, ©2005.
Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.]
Figure 23.8 demonstrates the WebBrowser class's capabilities. Class FrmBrowser provides the basic functionality of a Web browser, allowing the user to navigate to a URL, move backward and forward through the history of visited sites, and reload the current Web page.
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1 ' Fig. 23.8: FrmBrowser.vb |
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2 ' WebBrowser class example.
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3 |
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4 Public Class FrmBrowser |
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5 ' navigate back one page |
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6 Private Sub btnBack_Click(ByVal sender As System.Object, _ |
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7 ByVal e As System.EventArgs) Handles btnBack.Click |
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8 |
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9 webBrowser.GoBack() |
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10 End Sub ' btnBack_Click |
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11 |
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12 ' navigate forward one page |
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13 Private Sub btnForward_Click(ByVal sender As System.Object, _ |
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14 ByVal e As System.EventArgs) Handles btnForward.Click |
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15 |
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16 webBrowser.GoForward() |
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17 End Sub ' btnForward_Click |
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18 |
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19 ' stop loading the current page |
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20 Private Sub btnStop_Click(ByVal sender As System.Object, _ |
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21 ByVal e As System.EventArgs) Handles btnStop.Click |
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22 |
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23 webBrowser.Stop() |
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24 End Sub ' btnStop_Click |
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25 |
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26 ' reload the current page |
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27 Private Sub btnReload_Click(ByVal sender As System.Object, _ |
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28 ByVal e As System.EventArgs) Handles btnReload.Click |
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29 |
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30 webBrowser.Refresh() |
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31 End Sub ' btnReload_Click |
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32 |
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33 ' navigate to the user's home page |
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34 Private Sub btnHome_Click(ByVal sender As System.Object, _ |
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35 ByVal e As System.EventArgs) Handles btnHome.Click |
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36 |
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37 webBrowser.GoHome() |
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38 End Sub ' btnHome_Click |
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39 |
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40 ' if the user pressed enter, navigate to the specified URL |
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41 Private Sub txtNavigation_KeyDown(ByVal sender As System.Object, _ |
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42 ByVal e As System.Windows.Forms.KeyEventArgs) _ |
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43 Handles txtNavigation.KeyDown |
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44 |
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45 If e.KeyCode = Keys.Enter Then |
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46 webBrowser.Navigate(txtNavigation.Text) |
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47 End If |
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48 End Sub ' txtNavigation_KeyDown |
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49 |
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50 ' enable btnStop while the current page is loading |
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51 Private Sub webBrowser_Navigating(ByVal sender As System.Object, _ |
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52 ByVal e As System.Windows.Forms.WebBrowserNavigatingEventArgs) _ |
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53 Handles webBrowser.Navigating |
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54 |
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55 btnStop.Enabled = True |
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56 End Sub ' webBrowser_Navigating |
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57 |
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58 ' update the status text |
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59 Private Sub webBrowser_StatusTextChanged( _ |
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60 ByVal sender As System.Object, ByVal e As System.EventArgs) _ |
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61 Handles webBrowser.StatusTextChanged |
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62 |
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63 txtStatus.Text = webBrowser.StatusText |
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64 End Sub ' webBrowser_StatusTextChanged |
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65 |
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66 ' update the ProgressBar for how much of the page has been loaded |
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67 Private Sub webBrowser_ProgressChanged( _ |
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68 ByVal sender As System.Object, ByVal e As _ |
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69 System.Windows.Forms.WebBrowserProgressChangedEventArgs) _ |
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70 Handles webBrowser.ProgressChanged |
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71 ' Check if e.MaximumProgress is 0 or |
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72 ' if e.MaximumProgress is less than e.CurrentProgress |
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73 If e.MaximumProgress <> 0 And _ |
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74 e.MaximumProgress >= e.CurrentProgress Then |
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75 |
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76 prgPage.Value = Convert.ToInt32( _ |
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77 100 * e.CurrentProgress / e.MaximumProgress) |
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78 End If |
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79 End Sub ' webBrowser_ProgressChanged |
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80 |
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81 ' update the web browser's controls appropriately |
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82 Private Sub webBrowser_DocumentCompleted( _ |
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83 ByVal sender As System.Object, ByVal e As _ |
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84 System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) _ |
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85 Handles webBrowser.DocumentCompleted |
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86 ' set the text in txtNavigation to the current page's URL |
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87 txtNavigation.Text = webBrowser.Url.ToString() |
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88 |
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89 ' enable or disable btnBack and btnForward |
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90 btnBack.Enabled = webBrowser.CanGoBack |
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91 btnForward.Enabled = webBrowser.CanGoForward |
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92 |
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93 ' disable btnStop |
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94 btnStop.Enabled = False |
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95 |
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96 ' clear the prgPage
97 prgPage.Value = 0 |
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98 End Sub ' webBrowser_DocumentCompleted |
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99 |
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100 ' update the title of the Browser |
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101 Private Sub webBrowser_DocumentTitleChanged( _ |
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102 ByVal sender As System.Object, ByVal e As System.EventArgs) _ |
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103 Handles webBrowser.DocumentTitleChanged |
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104 |
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105 Me.Text = webBrowser.DocumentTitle & " - Browser" |
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106 End Sub ' webBrowser_DocumentTitleChanged |
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107 End Class ' FrmBrowser |
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Lines 6–38 define five Click event handlers, one for each of the five navigation Buttons that appear at the top of the Form. Each event handler calls a corresponding WebBrowser method. WebBrowser method GoBack (line 9) navigates back to the previous page in the navigation history. Method GoForward (line 16) navigates forward to the next page in the navigation history. Method Stop (line 23) stops loading the current page. Method Refresh (line 30) reloads the current page. Method GoHome (line 37) navigates to the user's home page, as defined under Internet Explorer's settings (under Tools > Internet Options… in the Home page section).
The TextBox to the right of the navigation buttons allows the user to enter the URL of a Web site to browse. When the user types each keystroke in the TextBox, the event handler in lines 41–48 executes. If the key pressed was Enter, line 46 calls WebBrowser method Navigate to retrieve the document at the specified URL.
A WebBrowser object generates a Navigating event when it starts loading a new page. When this occurs, the event handler in lines 51–56 executes, and line 55 enables btnStop so that the user can cancel the loading of the Web page.
Typically, a user can see the status of a loading Web page at the bottom of the browser window. For this reason, we include a TextBox control (named txtStatus) and a ProgressBar control (named prgPage) at the bottom of our Form. A WebBrowser object generates a StatusTextChanged event when its StatusText property changes. The event handler for this event (lines 59–64) assigns the new contents of the WebBrowser's StatusText property to txtStatus's Text property (line 63) so that the user can monitor the WebBrowser's status messages. A WebBrowser object generates a ProgressChanged event when its page-loading progress is updated. The ProgressChanged event handler (lines 67–79) updates prgPage's Value (lines 76–77) to reflect how much of the current document has been loaded.
When the WebBrowser finishes loading a document, it generates a DocumentCompleted event. This executes the event handler in lines 82–98. Line 87 updates the contents of txtNavigation so that it shows the URL of the currently loaded page (WebBrowser property Url). This is particularly important if the user browses to another Web page by clicking a link in the existing page. Lines 90–91 use properties CanGoBack and CanGoForward to determine whether the back and forward buttons should be enabled or disabled. Since the document is now loaded, line 94 disables btnStop. Line 97 sets prgPage's Value to 0 to indicate that no content is currently being loaded.
Lines 101–106 define an event handler for the DocumentTitleChanged event, which occurs when a new document is loaded in the WebBrowser. Line 105 sets FrmBrowser's Text property (which is displayed in the Form's title bar) to the WebBrowser's current DocumentTitle.
