When I read articles on software development, I often heard of the phrase "clean interface". People talked about clean interface for APIs, and classes.
How do you define "clean interface"? Is there any guideline for designing a system with clean interface?
A "clean interface" is one that doesn't leak implementation details. It's easy to understand, doesn't expose its private parts, and doesn't result in excessive coupling between you and the code you're interfacing with. It's more of a code smell than a strict definition...
Whole books have been written on this, and Framework Design Guidelines (.NET-centric) is one of my favorites. There's a lot to it, and no simple answer, but if I were pressed for a simple answer, I'd say design an API so that the simple stuff can be done without the dev having to read any extended documentation. That implies a straightforward, consistent interface with sensible, descriptive names. Don't make it hard to do the basic stuff.
Keep it simple and write lean and swift, self-explanatory code. One that is easy to read and to maintain.