A DD4T.net Implementation - Extending the LinkFactory

In my current DD4T .net implementation, I encountered the requirement of having Rich Text Format fields resolved in a custom way. Namely, I had to check whether the type of a RTF Component link is of a wrapper around the actual Component to link to, and if so, resolve the link to the wrapped Component.

An elegant way of doing to is to extend the default DD4T.ContentModel.Factories.LinkFactory and create a new method ResolveRTFLink(string componentUri):

public interface IMyLinkFactory : ILinkFactory
{
    string ResolveRTFLink(string componentUri);
}

The implementing class would extend DD4T LinkFactory and implement IMyLinkFactory:

public class MyLinkFactory : LinkFactory, IMyLinkFactory
{
    [Inject]
    public virtual IComponentFactory ComponentFactory { get; set; }

    [Inject]
    public virtual IModelFactory ModelFactory { get; set; }

At the same time the class must provide an implementation for ResolveRTFLink method:

public string ResolveRTFLink(string componentUri)
{
    string key = string.Format("Link_{0}", componentUri);
    string link = (string)CacheAgent.Load(key);

    if (link == null)
    {
        link = GetRTFUrl(componentUri);
        if (link == null)
        {
            return ResolveLink(componentUri);
        }
        CacheAgent.Store(key, "Link", link);
    }

    return "UnresolvedLink".Equals(link) ? null : link;
}

private string GetRTFUrl(string componentUri)
{
    ModelBase model = ModelFactory.TryGetModel<ModelBase>(componentUri);
    // custom logic to resolve link to 'model'

    return resolvedLink;

}

The mapping of interface to factory singleton instance is specified using Ninject, in class DD4TNinjectModule.cs:

Bind<IMyLinkFactory>().ToMethod(context => new MyLinkFactory()
{
    LinkProvider = context.Kernel.Get<IMyLinkProvider>()
}).InSingletonScope();

Next, we need to call the method ResolveRTFLink. We do this from a helper class that I shamelessly copied from the DD4T libraries. The only modification is to call my own custom resolve method. As you can see this is an extension method for class string:

public static MvcHtmlString ResolveRichText(this string value)
{
    XmlDocument document = new XmlDocument();
    XmlNamespaceManager namespaceManager = new XmlNamespaceManager(document.NameTable);
    namespaceManager.AddNamespace("xhtml", XHTML_NAMESPACE_URI);
    namespaceManager.AddNamespace("xlink", XLINK_NAMESPACE_URI);
    document.LoadXml(string.Format("<xhtmlroot>{0}</xhtmlroot>", value));

    foreach (XmlNode node in document.SelectNodes("//xhtml:a[@xlink:href[starts-with(string(.),'tcm:')]][@xhtml:href='' or not(@xhtml:href)]", namespaceManager))
    {
        string componentUri = node.Attributes["xlink:href"].Value;
        string url = LinkFactory.ResolveRTFLink(componentUri);

        if (string.IsNullOrEmpty(url))
        {
            foreach (XmlNode childNode in node.ChildNodes)
            {
                node.ParentNode.InsertBefore(childNode.CloneNode(true), node);
            }
            node.ParentNode.RemoveChild(node);
        }
        else
        {
            XmlAttribute href = document.CreateAttribute("xhtml:href");
            href.Value = url;
            node.Attributes.Append(href);
            foreach (XmlAttribute attribute in node.SelectNodes("//@xlink:*", namespaceManager))
            {
                node.Attributes.Remove(attribute);
            }
        }
    }

    foreach (XmlNode node in document.SelectNodes("//*[@xlink:*]", namespaceManager))
    {
        foreach (XmlAttribute attribute in node.SelectNodes("//@xlink:*", namespaceManager))
        {
            node.Attributes.Remove(attribute);
        }
    }

    return new MvcHtmlString(RemoveNamespaceReferences(document.DocumentElement.InnerXml));
}

Finally, all I'm left to do is to call the string extension method whenever resolving an RTF field is required. Namely, this is done in one place only -- in extension methods in class IFieldSetExtenstionMethods.cs as described in post IFieldSet Extension Methods:

public static IList<string> ResolveRichTexts(this IFieldSet fieldSet, string fieldName)
{
    return StringValues(fieldSet, fieldName).Select(x => x.ResolveRichText().ToString()).ToList();
}

public static string ResolveRichText(this IFieldSet fieldSet, string fieldName)
{
    string value = StringValues(fieldSet, fieldName).FirstOrDefault<string>();
    return value == null ? null : value.ResolveRichText().ToString();
}