The framework used by any In.de app has responsive design already built in. You can define how fields have to adapt if the user changes screen size and the framework does the rest. You can even build an app that runs on a PC screen, on a tablet and on a smartphone.
When you build a mobile app the framework does even more. For instance if the app was designed for a tablet and you run it on a smartphone, the framework decides automatically which fields to hide if there are too many in a list layout.
If you, programmer, decide that the automatic algorithm is not sufficient you can implement the Initial event and, using the browserInfo method you can detect what kind of device the user is using. Than you can read the screenWidth and screenHeigth properties and decide on your own. If your view is too complex and does not fit well on a smartphone you can design a view for a tablet and a view for a smartphone and with fiew lines of code you can detect the device used by the user and open the right view.
Try reading the mobile manual and see how it works:
http://doc.instantdeveloper.com/eng/inde-mobile-users-guide.pdf