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:
- Start building any VB.NET project in Visual Studio .NET
- Add
WebResourceProvider.vb to your project
- Create a new class which inherits from
WebResourceProvider
- Override the
ConstructUrl() method in your derived class.
This method specifies the URL to be downloaded.
- 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.
- Optionally override other
WebResourceProvider virtual
methods. See the source code of the sample resource providers included in the
demo project for examples.
- Run and debug your code from within Visual Studio .NET
Acknowledgement
WebResourceProvider uses the following code written by
others:
Revision History
- 11 June 2002 Initial version of article.
- 27 August 2002 Some bugs fixed, added functions (SkipBackTo, SkipBackToExact, StoreIndex, RestoreIndex, ReplaceSpecialChars)
