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Build Your Own Windows Mobile RSS Reader : Page 3

Phones using the Windows Mobile 6 Standard do not have touch screens, which poses certain challenges when developing applications to run on them. Learn how to develop with these challenges in mind as you build a useful RSS Reader application for these devices.


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Wiring Up all the Event Handlers
With the helper functions defined, it's time to wire up all the event handlers for the various controls.

First, code the Form1_Load event handler, as shown in Listing 4.

When the form is first loaded, you have to create a root node for the TreeView control and load all existing feeds. All subscribed feeds are saved in a plain text file, with the following format:



Feed URL|Feed URL|Feed URL|

An example would be:

http://news.google.com/?output=rss|http://rss.cnn.com/rss/cnn_topstories.rss|

In the Subscribe MenuItem control's Click event handler, prompt the user to input a feed's URL and then subscribe to the feed. If the subscription is successful, save the feed URL to file:

'---Subscribe MenuItem control--- Private Sub mnuSubscribe_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles mnuSubscribe.Click Dim url As String = InputBox( _ "Please enter the feed URL", "Feed URL", "http://") If url <> String.Empty Then '---if feed is subscribed successfully--- If SubscribeFeed(url) Then '---save in feed list--- Dim textwriter As TextWriter textwriter = File.AppendText(FEEDS_LIST) textwriter.Write(url & "|") textwriter.Close() Else MsgBox("Feed not subscribed. " & _ "Please check that you have entered " & _ "the correct URL and that you have " & _ "Internet access.") End If End If End Sub

Whenever a node in the TreeView control is selected, perform a check to see if it is a posting node and enable/disable the MenuItem controls appropriately (see also Figure 6).

'---fired when each node is selected--- Private Sub TreeView1_AfterSelect( _ ByVal sender As System.Object, _ ByVal e As System.Windows.Forms.TreeViewEventArgs) _ Handles TreeView1.AfterSelect If e.Node.ImageIndex <> ICO_POST And _ e.Node.Parent IsNot Nothing Then mnuUnsubscribe.Enabled = True mnuRefreshFeed.Enabled = True Else mnuUnsubscribe.Enabled = False mnuRefreshFeed.Enabled = False End If End Sub


Figure 6. Everything's Working: Enabling/disabling the MenuItem controls.
 
Figure 7. Viewing the Post: Using the WebBrowser control in Form2.

When the user selects a post using the Select button on the navigation pad, Form2 containing the WebBrowser control will be loaded and its content set accordingly (see Figure 7). This is handled by the KeyDown event handler:

'---when the user presses the Select key on the keypad--- Private Sub TreeView1_KeyDown( _ ByVal sender As Object, _ ByVal e As System.Windows.Forms.KeyEventArgs) _ Handles TreeView1.KeyDown Dim node As TreeNode = TreeView1.SelectedNode If (e.KeyCode = System.Windows.Forms.Keys.Enter) Then '---Enter key was pressed--- If node.ImageIndex = ICO_POST Then Dim frm2 As New Form2 With frm2 .Text = node.Text .WebBrowser1.DocumentText = node.Tag .Show() End With End If End If End Sub

To unsubscribe a feed, you will need to remove the feed's URL from the text file and then remove the feed node from the TreeView control. This is handled by the Unsubscribe MenuItem control, shown in Listing 5.

When the user needs to refresh a feed, first make a backup copy of the feed XML document and proceed to subscribe to the same feed again. If the subscription is successful, remove the node containing the old feed. If the subscription is not successful (such as device is disconnected from the Internet, etc), restore the backup feed XML document. This is handled by the Refresh Feed MenuItem control, shown in Listing 6.

Finally, in the Back MenuItem control in Form2, code its Click event handler as follows:

Private Sub mnuBack_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles mnuBack.Click Me.Hide() End Sub

That’s it! Press F5 to test the application. If you are testing the application using an emulator, you need to cradle the emulator to ActiveSync first so that you have Internet connectivity on the emulator. To cradle the emulator to ActiveSync, select Tools | Device Emulator Manager… in Visual Studio 2005 and right-click on the emulator you want to test and select Cradle.

Windows Mobile 6 Basics
Now you know how to:

  • Use the TreeView control to display information hierarchically
  • Manipulate files and read and write to text files
  • Download files using the WebRequest and WebResponse classes
  • Manipulate XML files using XPath expressions



Wei-Meng Lee is a Microsoft MVP and founder of Developer Learning Solutions, a technology company specializing in hands-on training on the latest Microsoft technologies. He is an established developer and trainer specializing in .NET and wireless technologies. Wei-Meng speaks regularly at international conferences and has authored and coauthored numerous books on .NET, XML, and wireless technologies. He writes extensively on topics ranging from .NET to Mac OS X. He is also the author of the .NET Compact Framework Pocket Guide, ASP.NET 2.0: A Developer's Notebook (both from O'Reilly Media, Inc.), and Programming Sudoku (Apress). Here is Wei-Meng's blog.
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