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Visual Basic 2005
How to Program, 3/e

ISBN:
0-13-186900-0
© 2005
pages: ~1500

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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.]

23.9 WebBrowser Class

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.

Fig. 23.8 | WebBrowser class example.

1   ' Fig. 23.8: FrmBrowser.vb
2   ' WebBrowser class example.
3   
4   Public Class FrmBrowser
5       ' navigate back one page
6       Private Sub btnBack_Click(ByVal sender As System.Object, _
7         ByVal e As System.EventArgs) Handles btnBack.Click
8   
9         webBrowser.GoBack()
10      End Sub ' btnBack_Click
11      
12       ' navigate forward one page
13       Private Sub btnForward_Click(ByVal sender As System.Object, _
14         ByVal e As System.EventArgs) Handles btnForward.Click
15   
16         webBrowser.GoForward()
17       End Sub ' btnForward_Click
18   
19       ' stop loading the current page
20      Private Sub btnStop_Click(ByVal sender As System.Object, _
21         ByVal e As System.EventArgs) Handles btnStop.Click
22   
23         webBrowser.Stop()
24       End Sub ' btnStop_Click
25   
26       ' reload the current page
27       Private Sub btnReload_Click(ByVal sender As System.Object, _
28         ByVal e As System.EventArgs) Handles btnReload.Click
29   
30         webBrowser.Refresh()
31      End Sub ' btnReload_Click
32   
33          ' navigate to the user's home page
34          Private Sub btnHome_Click(ByVal sender As System.Object, _
35         ByVal e As System.EventArgs) Handles btnHome.Click
36   
37         webBrowser.GoHome()
38      End Sub ' btnHome_Click
39      
40       ' if the user pressed enter, navigate to the specified URL
41       Private Sub txtNavigation_KeyDown(ByVal sender As System.Object, _
42         ByVal e As System.Windows.Forms.KeyEventArgs) _
43         Handles txtNavigation.KeyDown
44   
45         If e.KeyCode = Keys.Enter Then
46            webBrowser.Navigate(txtNavigation.Text)
47          End If
48       End Sub ' txtNavigation_KeyDown
49      
50       ' enable btnStop while the current page is loading
51       Private Sub webBrowser_Navigating(ByVal sender As System.Object, _
52         ByVal e As System.Windows.Forms.WebBrowserNavigatingEventArgs) _
53         Handles webBrowser.Navigating
54   
55         btnStop.Enabled = True
56      End Sub ' webBrowser_Navigating
57   
58       ' update the status text
59       Private Sub webBrowser_StatusTextChanged( _
60         ByVal sender As System.Object, ByVal e As System.EventArgs) _
61         Handles webBrowser.StatusTextChanged
62   
63         txtStatus.Text = webBrowser.StatusText
64       End Sub ' webBrowser_StatusTextChanged
65   
66       ' update the ProgressBar for how much of the page has been loaded
67       Private Sub webBrowser_ProgressChanged( _
68         ByVal sender As System.Object, ByVal e As _
69         System.Windows.Forms.WebBrowserProgressChangedEventArgs) _
70         Handles webBrowser.ProgressChanged
71          ' Check if e.MaximumProgress is 0 or
72          ' if e.MaximumProgress is less than e.CurrentProgress
73         If e.MaximumProgress <> 0 And _
74            e.MaximumProgress >= e.CurrentProgress Then
75   
76            prgPage.Value = Convert.ToInt32( _
77               100 * e.CurrentProgress / e.MaximumProgress)
78          End If
79       End Sub ' webBrowser_ProgressChanged
80   
81       ' update the web browser's controls appropriately
82      Private Sub webBrowser_DocumentCompleted( _
83         ByVal sender As System.Object, ByVal e As _
84         System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) _
85         Handles webBrowser.DocumentCompleted
86          ' set the text in txtNavigation to the current page's URL
87         txtNavigation.Text = webBrowser.Url.ToString()
88   
89          ' enable or disable btnBack and btnForward
90         btnBack.Enabled = webBrowser.CanGoBack
91         btnForward.Enabled = webBrowser.CanGoForward
92   
93          ' disable btnStop
94         btnStop.Enabled = False
95   
96          ' clear the prgPage
97         prgPage.Value = 0
98      End Sub ' webBrowser_DocumentCompleted
99   
100       ' update the title of the Browser
101      Private Sub webBrowser_DocumentTitleChanged( _
102         ByVal sender As System.Object, ByVal e As System.EventArgs) _
103         Handles webBrowser.DocumentTitleChanged
104   
105         Me.Text = webBrowser.DocumentTitle & " - Browser"
106      End Sub ' webBrowser_DocumentTitleChanged
107    End Class ' FrmBrowser

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.