Up to this point, editing code on CircuitPython devices has sometimes been tough to get set up and working. Some editors have trouble with not writing the code to the board immediately and cache it instead and some will just show everything as one color, making coding a little bit more difficult.
Runestone is a streamlined and lightweight Text and Code Editor that runs on iOS devices such as the iPhone and iPad with color coding that supports CircuitPython. It is a free app, that has an option to unlock premium features such as line numbers, page guides, app icon customization, and displaying invisible characters.
Runestone does an excellent job of giving you exactly what you need in order to code on the go without too many bells and whistles which can make the learning curve much steeper. Although it doesn't have some features such as tabs, this works for CircuitPython because saving the file will cause the code to reload anyways. This guide should help get you up and running so you can start editing on iOS devices.