Introduction
This article describes the construction of a very simple custom server control used to display the content returned from an RSS feed. The control consumes a public web service to retrieve the current data from an RSS feed, and the control displays the content along with a user defined label. The public web service returns the feed content as straight HTML which greatly simplifies the process of displaying the data.
Figure 1: RSS Feed Custom Control in Use
Getting Started
The files included with this project include a web control library project and a demonstration web site. In order to get started, open the included zip file and install the two projects onto your file system. Open IIS and create a virtual directory for the web application. Open the solution into Visual Studio 2005 and make any changes necessary to bring both projects into the solution. Once properly configured, your Solution Explorer should show these projects, references, and files:
Figure 2: Solution Explorer with Web App and Control Library
In examining the solution, note that the RSS control library contains only a single control and that control is called “RssControl”. This project also includes a web reference that points to http://www.webservicex.net/WS/WSDetails.aspx?WSID=54&CATID=12; this public site supplies the web service web method called RssToHtml; this method retrieves the RSS feed content and converts it to HTML through its GetHTML web method.
The web application contains only a single web page (default.aspx) and includes a reference to the RSS DLL. It also contains a reference to my weather forecasting DLL which I added to the project as eye wash.
The web application serves as a container used to test the custom control. The page itself contains no additional code; there are four separate instances of the RSS Control on the page, along with a single instance of the weather forecasting control. The RSS controls are all pointing to different RSS feeds made available by the New York Times.
The Code: RSS Control
The RSS custom control is constructed to retrieve the information from the web service upon initialization; the information retrieved is the content from the specified RSS feed converted to HTML by the web service. Upon initialization, the HTML is retrieved and when the control is rendered, a label control is added and its text property is set to display the HTML. The control exposes two properties of interest, one is the RSS feed’s URL and the other is the title for the RSS control. The title and RSS feed properties may be set by the user.
In examining the code, note that only the default imports are included in the project. The class itself inherits from the WebControl class.
Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Text
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
<ToolboxData("<{0}:RssCC runat=server></{0}:RssCC>")> _
Public Class RssCC
Inherits WebControl
Following the class declaration, four member variables are declared within a declarations region; one for the web service and three string variables used to contain the RSS feed, the user defined RSS Feed Title, and one for the content returned by the web service.
#Region "Declarations"
Private mNews As net.webservicex.www.RSStoHTML
Private mMessage As String
Private mRssUrl As String
Private mRssTitle As String
#End Region
Next up is the methods region which contains the initialization event handler for the RSS control:
#Region "Methods"
Private Sub RssCC_Init(ByVal sender As Object, _
ByVal e As System.EventArgs) _
Handles Me.Init
If Not RssFeed Is String.Empty Then
mNews = New net.webservicex.www.RSStoHTML
mMessage = mNews.GetHTML(RssFeed)
End If
End Sub
#End Region
The initialization event handler checks to see if the RssFeed property is empty and, if it is not, it evokes the web services GetHTML web method to populate the message member variable with the HTML version of the information returned by the service.
The next region in the project is called “Properties” and it contains the two properties used by the control, RssFeed and RssFeedTitle. The properties update or read from the content of private member variables to manage their content. The RssFeed property is used to contain the path to the RSS feed while the RssFeedTitle property allows the user to key a title for the control.
#Region "Properties"
<Category("RSS Feed")> _
<Browsable(True)> _
<Description("Enter the URL for the RSS Feed.")> _
Public Property RssFeed() As String
Get
Return mRssUrl
End Get
Set(ByVal value As String)
mRssUrl = value
End Set
End Property
<Category("RSS Feed")> _
<Browsable(True)> _
<Description("Enter the title for the RSS Feed.")> _
Public Property RssFeedTitle() As String
Get
Return mRssTitle
End Get
Set(ByVal value As String)
mRssTitle = value
End Set
End Property
#End Region
The last detail to tend to is the rendering; here RenderContents is overridden and, within a Try – Catch block, a set of nested Divs are created. Within the main Div there are two separate Divs, one to hold the user defined title for the feed, and one to display the HTML. In both instances, the content is rendered by a label control added to the Div. If the rendering operation fails, the control will display “RSS Feed Offline” to the user.
#Region "Rendering"
Protected Overrides Sub RenderContents(ByVal output As HtmlTextWriter)
Try
output.AddAttribute(HtmlTextWriterAttribute.Align, "center")
output.AddAttribute(HtmlTextWriterAttribute.Valign, "top")
output.AddAttribute(HtmlTextWriterAttribute.Width, "100%")
output.RenderBeginTag(HtmlTextWriterTag.Div)
output.AddAttribute(HtmlTextWriterAttribute.Align, "left")
output.AddAttribute(HtmlTextWriterAttribute.Valign, "top")
output.AddAttribute(HtmlTextWriterAttribute.Width, "50%")
output.RenderBeginTag(HtmlTextWriterTag.Div)
Dim lbl As New Label
lbl.ID = "lblRssTitle"
lbl.Text = "<b>" & Me.RssFeedTitle.ToString() & "</b>"
lbl.RenderControl(output)
output.RenderEndTag()
output.AddAttribute(HtmlTextWriterAttribute.Align, "left")
output.AddAttribute(HtmlTextWriterAttribute.Valign, "top")
output.AddAttribute(HtmlTextWriterAttribute.Width, "50%")
output.RenderBeginTag(HtmlTextWriterTag.Div)
Dim lbl2 As New Label
lbl2.ID = "lblRssContent"
lbl2.Text = mMessage.ToString()
lbl2.RenderControl(output)
output.RenderEndTag()
output.RenderEndTag()
Catch
output.RenderBeginTag(HtmlTextWriterTag.Div)
output.Write("RSS Feed Offline")
output.RenderEndTag()
End Try
End Sub
#End Region
Summary
This project demonstrates a very easy way to display RSS feed content on a web page through the use of custom server controls and web services. The control will only display the current or last feed from the RSS feed’s site; it would be necessary to capture and store multiple feed messages over time in order to display a group of them from any given RSS feed using this service. The same web service web method could be used to accomplish this task by capturing and saving the messages to a database, these stored strings containing the HTML version of the feed content could be used to populate labels in a similar manner to that used in this demonstration.
