WebResourceProvider VB.NET style

• Download source files - 21 Kb

Introduction

First of all I have to thank Ravi Bhavnani for his excellent WebResourceProvider article.

I just took the opportunity to translate the code into VB.NET and tried to use as much .NET Framework functionality as possible.

Please refer to Ravi Bhavnani's article for more in-depth information about the idea of WebResourceProviders.

The source code provided with this article is a Visual Studio .NET project with the base class WebResourceProvider and a demo application including two derived classes:

• stock quotes
• the translation of a piece of text (which uses also a POST request)

How it Works

You use WebResourceProvider by deriving your own resource provider class from it, and overriding any of these virtual methods:

• Init
• ConstructUrl
• IsPost()
• GetPostData()
• ParseContent()
• MoreAvailable()

WebResourceProvider provides an assortment of methods to help parse downloaded content. They are:

Method		Purpose
At		Checks whether current location is at a string
AtExact		Case sensitive version of at()
SkipTo		Advances current location to next occurence of a string
SkipToExact		Case sensitive version of SkipTo()
SkipBackTo		Advances current location to previous occurence of a string
SkipBackToExact		Case sensitive version of SkipBackTo()
ExtractTo		Extracts text from current location to the start of a string
ExtractToExact		Case sensitive version of ExtractTo()
ExtractToEnd		Extracts text from current location to end of content
GetIndex		Returns current location
GetLinks		Returns HREF and IMG links in content
ResetIndex		Sets current location to start of content
ResetIndex		Sets current location to start of content
StoreIndex		Stores current location
RestoreIndex		Restores the prevoiusly stored location
RemoveComments		Removes comments from content
RemoveScripts		Removes scripts from content
RemoveEnclosingAnchorTag		Removes anchor tag enclosing a string
RemoveEnclosingQuotes		Removes quotes enclosing a string
RemoveHtml		Removes HTML from a string
Trim		Removes leading and trailing whitespace from a string
ReplaceSpecialChars		Replace special HTML characters like ampersand, umlaut, etc. to its character representation

Sample Resource Providers

There are two sample resource providers in the source code:

The QuoteProvider class works by posting a request to Yahoo's basic stock quote form and parsing the returned information.

The TranslationProvider class works by posting a request to Google's translation engine and parsing the returned information.  The request includes the translation mode. The sample performs a reverse translation and presents it along with the original text for comparison purposes.

Using WebResourceProvider

To use WebResourceProvider do the following:

1. Start building any VB.NET project in Visual Studio .NET
2. Add WebResourceProvider.vb to your project
3. Create a new class which inherits from WebResourceProvider
4. Override the ConstructUrl() method in your derived class. This method specifies the URL to be downloaded.
5. Override the ParseContent() method in your derived class. This method extracts information from the downloaded content and stores it in the derived class' member variables.
6. Optionally override other WebResourceProvider virtual methods. See the source code of the sample resource providers included in the demo project for examples.
7. Run and debug your code from within Visual Studio .NET

Acknowledgement

WebResourceProvider uses the following code written by others:

• WebResourceProvider (in C++) by Ravi Bhavnani
• Case-Insensitive String Replace by Uwe Keim

Revision History

• 11 June 2002 Initial version of article.
• 27 August 2002 Some bugs fixed, added functions (SkipBackTo, SkipBackToExact, StoreIndex, RestoreIndex, ReplaceSpecialChars)