title: How can I control username input in the ParseLoginUI?
body: <p>I'm using the ParseLoginUI to allow users to create accounts with, and sign in to, my app. As seen here: 
  <a href="http://blog.parse.com/2014/06/25/login-love-for-your-android-app/" rel="nofollow">http://blog.parse.com/2014/06/25/login-love-for-your-android-app/</a></p>
  
  <p>However, as it's currently configured, users can create usernames like "a b  " and Parse thinks that's perfectly fine. Most customizations with the ParseLoginUI can be done via <code>meta-data</code> in the <code>AndroidManifest.xml</code> but I haven't found anything in the documentation for controlling input. Is there a way to control this input? example: disallowing space characters</p>