Generation of web feeds and podcasts

using System;
using System.Collections.Generic;
using System.Web;
using System.Xml;

namespace Uncas.Web
{
    /// <summary>
    /// Represents a web feed
    /// </summary>
    public class Feed
    {
        #region Constructor

        // TODO: consider another constructor with the fields required for an iTunes podcast

        /// <summary>
        /// Creates a feed with the provided title and description. Valid RSS.
        /// </summary>
        /// <param name="title">The title of the feed</param>
        /// <param name="description">The description of the feed</param>
        public Feed(string title, string description)
        {
            CheckRequiredValue(title, "title");
            CheckRequiredValue(description, "description");
            // Sets the properties:
            this.Title = title;
            this.Description = description;
            // Initializes the list of items:
            this.Items = new List<FeedItem>();
        }

        /// <summary>
        /// Creates a feed with the provided title, link, description, and author. Valid RSS and Atom.
        /// </summary>
        /// <param name="title">The title of the feed</param>
        /// <param name="description">The description of the feed</param>
        /// <param name="author">The author of the feed</param>
        public Feed(string title, string description, string author)
            : this(title, description)
        {
            CheckRequiredValue(author, "author");
            this.Author = author;
        }

        #endregion

        #region Public properties

        /// <summary>
        /// The title of the feed
        /// </summary>
        public string Title { get; set; }
        /// <summary>
        /// The description of the feed
        /// </summary>
        public string Description { get; set; }
        /// <summary>
        /// The author of the feed
        /// </summary>
        public string Author { get; set; }
        /// <summary>
        /// Alternate url, e.g. of a blog web page with the same content
        /// </summary>
        public string AlternateLink { get; set; }
        /// <summary>
        /// Copyright information
        /// </summary>
        public string Copyright { get; set; }
        /// <summary>
        /// The language of the feed
        /// </summary>
        public string Language { get; set; }
        /// <summary>
        /// Email of the managing editor
        /// </summary>
        public FeedEmailAddress ManagingEditor { get; set; }
        /// <summary>
        /// Email of the webmaster
        /// </summary>
        public FeedEmailAddress WebMaster { get; set; }
        /// <summary>
        /// Categories
        /// </summary>
        public string[] Categories { get; set; }

        /// <summary>
        /// Title of a header image
        /// </summary>
        public string ImageTitle { get; set; }
        /// <summary>
        /// Url to header image
        /// </summary>
        public string ImageUrl { get; set; }

        /// <summary>
        /// The list of items for the feed
        /// </summary>
        public List<FeedItem> Items { get; set; }

        #endregion Public properties

        #region Private properties

        /// <summary>
        /// The total number of enclosures in the feed items of this feed
        /// </summary>
        /// <remarks>
        /// Only when this number is larger than 0, the RSS feed displays iTunes info.
        /// </remarks>
        private int NumberOfEnclosures
        {
            get
            {
                int numberOfEnclosures = 0;
                foreach (FeedItem item in this.Items)
                    if (item.Enclosure != null)
                        numberOfEnclosures++;
                return numberOfEnclosures;
            }
        }

        #endregion

        #region Public methods

        /// <summary>
        /// Writes a RSS feed to the HttpResponse of the HttpContext
        /// </summary>
        /// <param name="context">The HttpContext</param>
        public void WriteRSS(HttpContext context)
        {
            HttpResponse response = context.Response;
            string link = context.Request.Url.AbsoluteUri;

            #region Creates the document

            // Sets the content-type:
            response.Clear();
            response.ContentType = "application/rss+xml";

            // Creates an XmlTextWriter to write the XML data to a string:
            XmlTextWriter writer = new XmlTextWriter(response.OutputStream, System.Text.Encoding.UTF8);
            writer.Formatting = Formatting.Indented;

            // Writes the document:
            writer.WriteStartDocument();

            writer.WriteStartElement("rss");

            if (this.NumberOfEnclosures > 0)
            {
                writer.WriteAttributeString("xmlns:itunes",
                    "http://www.itunes.com/dtds/podcast-1.0.dtd");
                writer.WriteAttributeString("xmlns:media",
                    "http://search.yahoo.com/mrss/");
                writer.WriteAttributeString("xml:lang", this.Language);
            }
            writer.WriteAttributeString("version", "2.0");

            // Creates the channel:
            writer.WriteStartElement("channel");

            #endregion

            #region Feed channel info

            #region Required fields

            writer.WriteElementString("title", this.Title);
            writer.WriteElementString("link", link);
            writer.WriteElementString("description", this.Description);

            #region atom:link
            writer.WriteStartElement("atom:link");
            writer.WriteAttributeString("xmlns:atom", "http://www.w3.org/2005/Atom");
            writer.WriteAttributeString("rel", "self");
            writer.WriteAttributeString("href", link);
            writer.WriteAttributeString("type", "application/rss+xml");
            writer.WriteEndElement();
            #endregion atom:link

            #endregion Required fields

            #region Fields with fixed values (fixed by Ole L. Sørensen)

            writer.WriteElementString("ttl", "60");
            writer.WriteElementString("docs"
                , "http://cyber.law.harvard.edu/rss/rss.html");

            DateTime now = DateTime.UtcNow;
            writer.WriteElementString("pubDate", now.ToString("r"));
            writer.WriteElementString("lastBuildDate", now.ToString("r"));

            #endregion Fields with fixed values

            #region Optional fields

            if (this.Copyright != null)
                writer.WriteElementString("copyright", this.Copyright);
            if (this.Language != null)
                writer.WriteElementString("language", this.Language);
            if (this.ManagingEditor != null)
                writer.WriteElementString("managingEditor", this.ManagingEditor.ToString());
            if (this.WebMaster != null)
                writer.WriteElementString("webMaster", this.WebMaster.ToString());

            // TODO: include rating info?
            // TODO: include cloud info?
            if (this.Categories != null)
            {
                foreach (string category in this.Categories)
                {
                    writer.WriteElementString("category", category);
                }
            }

            #region iTunes info

            if (this.NumberOfEnclosures > 0)
            {
                // TODO: consider itunes:explicit, maybe via an enumeration...
                // Possible values for itunes:explicit: yes, clean, no
                // HACK: I use itunes:explicit="no"
                writer.WriteElementString("itunes:explicit", "no");

                writer.WriteElementString("itunes:subtitle", this.Description);
                writer.WriteElementString("itunes:summary", this.Description);
                if (this.Author != null)
                    writer.WriteElementString("itunes:author", this.Author);

                // TODO: does itunes:owner equal ManagingEditor?
                if (this.ManagingEditor != null)
                {
                    writer.WriteStartElement("itunes:owner");
                    writer.WriteElementString("itunes:name", this.ManagingEditor.RealName);
                    writer.WriteElementString("itunes:email", this.ManagingEditor.EmailAddress);
                    writer.WriteEndElement();
                }

                if (this.ImageUrl != null)
                {
                    writer.WriteStartElement("itunes:image");
                    writer.WriteAttributeString("href", this.ImageUrl);
                    writer.WriteEndElement();
                }

                if (this.Categories != null)
                {
                    foreach (string category in this.Categories)
                    {
                        writer.WriteStartElement("itunes:category");
                        writer.WriteAttributeString("text", category);
                        writer.WriteEndElement();
                    }
                }
            }

            #endregion iTunes info

            #region Image info

            if (this.ImageUrl != null)
            {
                writer.WriteStartElement("image");
                writer.WriteElementString("title", this.ImageTitle);
                writer.WriteElementString("url", this.ImageUrl);
                writer.WriteElementString("link", link);
                writer.WriteEndElement();
            }

            #endregion Image info

            #endregion Optional fields

            #endregion Feed channel info

            #region Feed items

            foreach (FeedItem item in this.Items)
            {
                writer.WriteStartElement("item");

                writer.WriteElementString("title", item.Title);

                #region Optional fields

                if (item.Link != null)
                {
                    writer.WriteElementString("link", item.Link);
                    // TODO: use something else for item guid???
                    writer.WriteElementString("guid", item.Link);
                }
                else
                {
                    // TODO: validation recommendation: "item should contain a guid element"
                }

                if (item.Description != null)
                    writer.WriteElementString("description", item.Description);
                if (item.Author != null)
                    writer.WriteElementString("author", item.Author.ToString());
                if (item.Category != null)
                    writer.WriteElementString("category", item.Category);
                if (item.PublishDate.HasValue)
                    writer.WriteElementString("pubDate", item.PublishDate.Value.ToString("r"));

                #region Enclosure info

                if (item.Enclosure != null)
                {
                    // e.g. pod cast:
                    writer.WriteStartElement("enclosure");
                    writer.WriteAttributeString("url", item.Enclosure.Url);
                    writer.WriteAttributeString("type", item.Enclosure.MimeType);
                    writer.WriteAttributeString("length", item.Enclosure.Length.ToString());
                    writer.WriteEndElement();

                    #region iTunes info
                    if (item.Description != null)
                        writer.WriteElementString("itunes:subtitle", item.Description);
                    if (item.Author != null)
                        writer.WriteElementString("itunes:author", item.Author.EmailAddress);
                    if (item.Description != null)
                        writer.WriteElementString("itunes:summary", item.Description);

                    if (item.ImageUrl != null)
                    {
                        writer.WriteStartElement("itunes:image");
                        writer.WriteAttributeString("href", item.ImageUrl);
                        writer.WriteEndElement();
                    }

                    writer.WriteStartElement("media:content");
                    writer.WriteAttributeString("url", item.Enclosure.Url);
                    writer.WriteAttributeString("fileSize"
                        , item.Enclosure.Length.ToString());
                    writer.WriteAttributeString("type", item.Enclosure.MimeType);
                    writer.WriteEndElement();
                    #endregion iTunes info
                }

                #endregion Enclosure info

                #endregion Optional fields

                writer.WriteEndElement();
            }
            #endregion Feed items

            #region Finishes the document

            writer.WriteEndElement();
            writer.WriteEndElement();
            writer.Close();
            response.End();

            #endregion
        }

        /// <summary>
        /// Writes an Atom feed to the HttpResponse of the HttpContext
        /// </summary>
        /// <param name="context">The HttpContext</param>
        public void WriteAtom(HttpContext context)
        {
            HttpResponse response = context.Response;
            string link = context.Request.Url.AbsoluteUri;

            #region Creates the xml document

            // Set the content-type
            response.Clear();
            response.ContentType = "application/atom+xml";

            // Use an XmlTextWriter to write the XML data to a string...
            XmlTextWriter writer = new XmlTextWriter
                (response.OutputStream, System.Text.Encoding.UTF8);
            writer.Formatting = Formatting.Indented;

            // document info
            writer.WriteStartDocument();
            writer.WriteStartElement("feed");
            writer.WriteAttributeString("xmlns", "http://www.w3.org/2005/Atom");

            #endregion Creates the xml document

            #region Feed info

            #region Required fields

            writer.WriteStartElement("title");
            writer.WriteAttributeString("type", "html");
            writer.WriteString(this.Title);
            writer.WriteEndElement();

            writer.WriteStartElement("link");
            writer.WriteAttributeString("rel", "self");
            writer.WriteAttributeString("type", "application/atom+xml");
            writer.WriteAttributeString("href", link);
            writer.WriteEndElement();

            writer.WriteStartElement("subtitle");
            writer.WriteAttributeString("type", "html");
            writer.WriteString(this.Description);
            writer.WriteEndElement();

            // TODO: use another id than the link?
            writer.WriteElementString("id", link);

            #endregion Required fields

            #region Optional fields

            DateTime now = DateTime.UtcNow;
            writer.WriteElementString("updated"
                , XmlConvert.ToString(now, XmlDateTimeSerializationMode.Utc));

            if (this.Author != null)
            {
                writer.WriteStartElement("author");
                writer.WriteElementString("name", this.Author);
                writer.WriteEndElement();
            }

            if (this.AlternateLink != null)
            {
                writer.WriteStartElement("link");
                writer.WriteAttributeString("rel", "alternate");
                writer.WriteAttributeString("type", "text/html");
                writer.WriteAttributeString("href", this.AlternateLink);
                writer.WriteEndElement();
            }

            #endregion Optional fields

            #endregion Feed info

            #region Writes feed items

            foreach (FeedItem item in this.Items)
            {
                #region item info
                writer.WriteStartElement("entry");

                writer.WriteElementString("title", item.Title);

                #region Optional fields

                if (item.Link != null)
                {
                    writer.WriteStartElement("link");
                    writer.WriteAttributeString("href", item.Link);
                    writer.WriteEndElement();

                    // TODO: should another method for generating a unique item id be used?
                    writer.WriteElementString("id", item.Link);
                }

                // TODO: validation warning: "Two entries with the same value for atom:updated"
                // TODO: do not use datetime.now, but require a publish date?
                DateTime updated = DateTime.UtcNow;
                if (item.PublishDate.HasValue)
                    updated = item.PublishDate.Value;
                writer.WriteElementString("updated"
                    , XmlConvert.ToString(updated
                    , XmlDateTimeSerializationMode.Utc));

                if (item.Description != null)
                {
                    writer.WriteStartElement("content");
                    writer.WriteAttributeString("type", "html");
                    writer.WriteString(item.Description);
                    writer.WriteEndElement();
                }
                else
                {
                    // TODO: consider this (required) element, when there is no description...
                    writer.WriteElementString("content", this.Title);
                }

                if (item.Enclosure != null)
                {
                    writer.WriteStartElement("link");
                    writer.WriteAttributeString("rel", "enclosure");
                    writer.WriteAttributeString("type", item.Enclosure.MimeType);
                    writer.WriteAttributeString("length", item.Enclosure.Length.ToString());
                    writer.WriteAttributeString("href", item.Enclosure.Url);
                    writer.WriteEndElement();
                }

                #endregion Optional fields

                writer.WriteEndElement();
                #endregion
            }
            #endregion

            #region Finishes the document

            writer.WriteEndElement();
            writer.Close();
            response.End();

            #endregion Finishes the document
        }

        #endregion

        #region Public static methods

        /// <summary>
        /// Common method that handles a required string parameters
        /// </summary>
        /// <param name="value">The string parameter</param>
        /// <param name="nameOfValue">The name of the parameter</param>
        public static void CheckRequiredValue(string parameter, string nameOfParameter)
        {
            if (string.IsNullOrEmpty(parameter))
                throw new ArgumentNullException(nameOfParameter,
                    string.Format("It is required that {0} has a value", nameOfParameter));
        }

        #endregion
    }
}