An HttpHandler for use with Facebook

In my SocialMine Facebook application, which uses MIT’s Simile Exhibit, I needed to fetch Facebook FQL query results from the Exhibit client-side JavaScript. While it may not sound that complicated it’s not something directly supported by the Facebook Developer Toolkit (FDT). I created an HttpHandler to respond to the Exhibit requests for the JSON data which populates the Exhibit. The HTTP request includes a Referrer header that contains the necessary data, fb_sig_user and fb_sig_session_key to construct an instance of facebook.Components.FacebookService. For completeness here is a list of the queryparams contained in the Referrer URL:

auth_token=1379094a2a42e1c0037a33048912a875
fb_sig_in_iframe=1
fb_sig_locale=en_US
fb_sig_in_new_facebook=1
fb_sig_time=1245301045.1412
fb_sig_added=1
fb_sig_profile_update_time=1241328269
fb_sig_expires=0
fb_sig_user=719571200
fb_sig_session_key=c93185e145713ce98d72dfc2-719571700
fb_sig_ss=57abbef0284354d029fe8a19fa00dfce
fb_sig_ext_perms=offline_access,email,auto_publish_recent_activity
fb_sig_api_key=33c52d0a0c9867f677a1f05154c28478
fb_sig_app_id=5012360767
fb_sig=9ad664af23910ca18aeb203203c366cd

The HttpHandler is an abstract class with a single abstract method called FacebookRequest descendents override to handle the request. As you can see below the handler is quite simple, parsing the queryparams into a Dictionary, creates an instance of the FacebookService and calls FacebookRequest passing the service instance and the HttpContext passed to the handler.

public abstract class FacebookHandler : IHttpHandler
{
    private const string REQUEST_SESSION_KEY = "fb_sig_session_key";
    private const string REQUEST_USER_ID = "fb_sig_user";

    #region IHttpHandler Members

    public bool IsReusable
    {
        get { return false; }
    }

    void IHttpHandler.ProcessRequest(HttpContext ctxt)
    {
        facebook.Components.FacebookService f = new facebook.Components.FacebookService();
        f.ApplicationKey = WebConfigurationManager.AppSettings["APIKey"];
        f.Secret = WebConfigurationManager.AppSettings["Secret"];

        Dictionary<string, string> dic = new Dictionary<string, string>();

        string[] split = ctxt.Request.UrlReferrer.Query.Split(new char[] { '?', '&', '=' });
        // Skip the first item since it’s the "?"
        for (int i = 1; i < split.Length - 1; i += 2)
        {
            if(i < split.Length)
                dic.Add(split[i], split[i + 1]);
        }

        f.SessionKey = dic[REQUEST_SESSION_KEY];
        f.uid = Convert.ToInt64(dic[REQUEST_USER_ID]);
        FacebookRequest(f, ctxt);
    }
    #endregion

    protected abstract void FacebookRequest(facebook.Components.FacebookService fbservice, HttpContext ctxt);
}

The JavaScript of Simile Exhibit requests the JSON data using the following LINK tag:

<link href="friendsjson.aspx" type="application/json" rel="exhibit/data" />    

The reference to friendsjson.aspx isn’t actually an ASPX as it’s handled by a descendent of this HttpHandler which is registered in the httpHandlers section of the web.config as follows:

<add verb="*" path="friendsjson.aspx" type="FBFQL.JSONHandler" />

In my application the FacebookRequest method queries using FQL for profile data and returns a JSON result that populates the Exhibit.

Btw, if you're looking for help on Facebook development be sure to read my wiki article.