Michael Friis' Blog

Layar is a really great meta-app for iPhone and Android that lets you see a lot of third-party geo-based augmented reality layers on your phone. A “layar” consists of a JSON webservice that provides Points of Interest to users. There is HttpHandler implementation available for .Net, but the Layar specification is so simple (in the good sense of the word) that I decided to just whip up my own in a MVC controller. Computing distances is pretty akward using LinqtoSQL and SQL Server, I use the DistanceBetween function described here. It is used by the FindNearestEvents stored procedure in the code below.

public class LayarController : Controller
{
    public ActionResult GetPOIs(string lat, string lon, 
        string requestedPoiId, string pageKey)
    {
        var db = new DatabaseDataContext();

        int? page = null;
        if (!string.IsNullOrEmpty(pageKey))
        {
            page = int.Parse(pageKey);
        }

        var eventssp = db.FindNearestEvents(
            float.Parse(lat, NumberStyles.Float, CultureInfo.InvariantCulture),
            float.Parse(lon, NumberStyles.Float, CultureInfo.InvariantCulture),
            20, page ?? 0);

        var events = eventssp.Select(e => new POI()
        {
            lat = e.Lat.Value.ToLayarCoord(),
            lon = e.Lng.Value.ToLayarCoord(),
            distance = e.Distance.Value,
            id = e.PermId,
            title = e.Title,
            line2 = e.BodyText,
            attribution = "Ekstra Bladet Krimikort"
        }).ToList();

        return this.Json(
            new Response
            {
                radius = (int)(events.Max(e => e.distance) * 1000),
                nextPageKey = page != null ? page + 1 : 1,
                morePages = events.Count() == 20,
                hotspots = events,
            }, JsonRequestBehavior.AllowGet
            );
    }
}

public class Response
{
    public string layer { get { return "krimikort"; } }
    public int errorCode { get { return 0; } }
    public string errorString { get { return "ok"; } }
    public IEnumerable hotspots { get; set; }
    public int radius { get; set; }
    public int? nextPageKey { get; set; }
    public bool morePages { get; set; }
}

public class POI
{
    public object[] actions { get { return new object[] { }; } }
    public string attribution { get; set; }
    public float distance { get; set; }
    public int id { get; set; }
    public string imageUrl { get; set; }
    public int lat { get; set; }
    public int lon { get; set; }
    public string line2 { get; set; }
    public string line3 { get; set; }
    public string line4 { get; set; }
    public string title { get; set; }
    public int type { get; set; }
}

public static class Extensions
{
    public static int ToLayarCoord(this double coord)
    {
        return (int)(coord * 1000000);
    }
}

Here is how I generate sitemaps using the XDocument API and a ContentResult. The entries are events that come out of the EventRepository, please substitute as needed. Note that it would be vastly more elegant to use ActionLinks in some way. Note also that the first entry is a link to a Google Earth KMZ file (more here).

[OutputCache(Duration = 12 * 3600, VaryByParam = "*")]
public ContentResult Sitemap()
{
    string smdatetimeformat = "yyyy-MM-dd";

    var erep = new EventRepository();
    var events = (from e in erep.GetGeocodedEvents()
                    where e.IncidentTime.HasValue
                select new {e.Title, e.PermId, e.IncidentTime}).ToList();

    XNamespace sm = "http://www.sitemaps.org/schemas/sitemap/0.9";
    XNamespace geo = "http://www.google.com/geo/schemas/sitemap/1.0";
            
    XDocument doc = new XDocument(
        new XElement(sm + "urlset",
            new XAttribute("xmlns", "http://www.sitemaps.org/schemas/sitemap/0.9"),
            new XAttribute(XNamespace.Xmlns + "geo", 
                "http://www.google.com/geo/schemas/sitemap/1.0"),
            new XElement(sm + "url",
                new XElement(sm + "loc", "http://krimikort.ekstrabladet.dk/gearth.kmz"),
                new XElement(sm + "lastmod", DateTime.Now.ToString(smdatetimeformat)),
                new XElement(sm + "changefreq", "daily"),
                new XElement(sm + "priority", "1.0"),
                new XElement(geo + "geo",
                    new XElement(geo + "format", "kmz")
                )
            )
            ,
            events.Select(e => 
                new XElement(sm + "url",
                    new XElement(sm + "loc", EventExtensions.AbsUrl(e.Title, e.PermId)),
                    new XElement(sm + "lastmod", e.IncidentTime.Value.ToString(smdatetimeformat)),
                    new XElement(sm + "changefreq", "monthly"),
                    new XElement(sm + "priority", "0.5")
                )
            )
        )
    );

    return Content(doc.ToString(), "text/xml");
}