Tradier customer, Utsav Shah, recently open-sourced his Android SDK making it easier than ever to build mobile applications connected with Tradier. It implements all REST-based API functionality including: trading, accounts and market data. It also includes some steps to help integrate the complete OAuth flow into your application.
You can find the Android SDK on Github at: https://github.com/Utsav2/AndroidTradier
Special thanks to Utsav for building this helpful kit for Tradier/Android customers. Happy Developing!