1. Galitz, W.: The Essential Guide to User Interface Design: An Introduction to GUI Design Principles and Techniques. Wiley Publishing, Indianapolis (2007)
2. Android Developers.: Build an App Widget.
https://developer.android.com/guide/topics/appwidgets/overview
. Accessed 11 Dec 2018
3. Android Developers: Accessory Development Kit.
https://stuff.mit.edu/afs/sipb/project/android/docs/tools/adk/index.html
. Accessed 23 Dec 2018
4. University of Birmingham: How much does it cost to build a mobile app?
https://intranet.birmingham.ac.uk/collaboration/mobile-apps/faqs/budget.aspx
5. Alcaraz-Valencia, P., Gaytán-Lugo, L., Hernández-Gallardo, S., Maciel, R.: The role of technology in audio texts’ comprehension for English as a second language. In: Andujar, A. (ed.) Recent Tools for Computer-and Mobile-Assisted Foreign Language Learning. IGI-Global, USA (2019)