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Consuming OPML aggregated content

Outline Processor Markup Language (OPML) describes a format for storing outlines in XML. The purpose of this format is to provide a way to exchange information between outliners and Internet services. OPML has also become popular as a format for exchanging subscription lists between feed readers and aggregators. An outline is a tree, where each node contains a set of named attributes with string values. The framework handles OPML as just another syndication resource, meaning it implements the ISyndicationResource interface and has an API that should be familiar to those who have used the other framework syndication resource entities.

By default, when you consume OPML outlines using the framework API classes, any syndication extension that is natively supported by the framework is automatically retrieved from the OPML syndication data source. The framework also attempts to strip invalid XML hexadecimal characters from malformed outlines and will auto-detect XML language encodings if specified by the data source.

Example #1: Consuming an external OPML outline

The simplest way to consume an external OPML outline is to utilize the static Create method that the OpmlDocument class exposes. This method is provided to handle the common scenario of consuming an outline that is available via a Uri.

How to consume an OPML document

using Argotic.Syndication;

OpmlDocument document = OpmlDocument.Create(new Uri("http://blog.oppositionallydefiant.com/opml.axd"));

Example #2: Consuming an OPML outline from multiple data sources

OPML outlines may also consumed using the overloaded Load method, which is defined on the ISyndicationResource interface. This method's overloads provide a means of consuming outlines from a variety of data sources, including IXPathNavigable, Stream, XmlReader, and Uri.

How to load an OPML outline from multiple data sources

using System.IO;
using System.Xml;
using System.Xml.Xpath;
using Argotic.Syndication;

OpmlDocument document   = new OpmlDocument();

using (FileStream stream = new FileStream("SimpleBlogroll.xml", FileMode.Open, FileAccess.Read))
{
    document.Load(stream);
}

document    = new OpmlDocument();
using (FileStream stream = new FileStream("SimpleBlogroll.xml", FileMode.Open, FileAccess.Read))
{
    XmlReaderSettings settings  = new XmlReaderSettings();
    settings.IgnoreComments     = true;
    settings.IgnoreWhitespace   = true;

    using (XmlReader reader = XmlReader.Create(stream, settings))
    {
        document.Load(reader);
    }
}

document    = new OpmlDocument();
using (FileStream stream = new FileStream("SimpleBlogroll.xml", FileMode.Open, FileAccess.Read))
{
    document.Load(new XPathDocument(stream));
}

Sample file that can be used with the above code example: SimpleBlogroll.xml

Example #3: Specifying the set of features to support when loading syndicated content

The framework provides a means of specifying the set of features that are supported when loading a syndication resource via the SyndicationResourceLoadSettings class. This class is passed as an argument to the Create and Load methods of the syndication resource in order to configure how the load operation is performed.

The SyndicationResourceLoadSettings class exposes the following properties:
  • AutoDetectExtensions - Indicates if syndication extensions supported by the load operation are automatically determined based on the XML namespaces declared on the syndication resource.
  • CharacterEncoding - Indicates the character encoding to use when parsing a syndication resource.
  • RetrievalLimit - Determines the maximum number of resource entities to retrieve from a syndication resource.
  • SupportedExtensions - Indicates the custom syndication extensions that you wish to support.
  • Timeout - Specifies the amount of time after which asynchronous load operations will time out.

How to limit the number of OPML outlines that are processed

using Argotic.Common;
using Argotic.Syndication;

SyndicationResourceLoadSettings settings    = new SyndicationResourceLoadSettings();
settings.RetrievalLimit                     = 5;

Uri documentUrl         = new Uri("http://blog.oppositionallydefiant.com/opml.axd");
OpmlDocument document   = OpmlDocument.Create(documentUrl, settings);

foreach (OpmlOutline outline in document.Outlines)
{
    // The outlines have been limited to the first 5 in the OPML document
}

For more information about OPML

http://www.opml.org
http://en.wikipedia.org/wiki/OPML

Last edited Mar 11, 2008 at 7:30 PM by Oppositional, version 7

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