Use OAuth 2 with App Engine by requesting a cookie

oauth.scala

// this is kind of a hack since App Engine doesn't support OAuth 2, but that's the
// only version of OAuth that's natively supported by Android
val token = settings.getString("oauth2_token", "")
// send OAuth token to appspot login page
// response will contain a valid cookie
val req = new HttpGet("https://x-allow-oauth-dot-google-showy.appspot.com/_ah/login?continue=http://localhost/&auth=" + token);
val client = new DefaultHttpClient()
// avoid redirects - we are only interested in the cookie, not any content
client.getParams.setBooleanParameter(ClientPNames.HANDLE_REDIRECTS, false)
// we are not interested in the result, so no need to save it
client.execute(req);

import scala.collection.JavaConversions._
val cookies = client.getCookieStore().getCookies()
// if the response contains the right cookie, then we successfully authenticated
if(cookies.exists(_.getName() == "SACSID")) {
  // enable redirects again to make the request we initially wanted
  // we are reusing the same HttpClient, so the cookie is still set
  client.getParams.setBooleanParameter(ClientPNames.HANDLE_REDIRECTS, true)
  val req = new HttpGet(url)
  req.setHeader("X-Same-Domain", "true")
  val resRaw = client.execute(req)
  val res = (resRaw.getStatusLine().getStatusCode(), io.Source.fromInputStream(resRaw.getEntity().getContent()).mkString(""))
  log("Response Status Code: " + res._1);
  log("Response body:" + res._2);
} else {
// otherwise, something was wrong with our OAuth token - maybe it expired?
  runOnUiThread { showPopup("Authentication failed") }
  (500, "auth failed")
}