I have found PowerApps a very handy way to prototype screens, but some additional features are needed to support incremental development. The process might proceed as follows:
1. Create a screen mock-up with no back-end data (entering data directly into the value properties).
2. Incorporate changes to the screen per user feedback.
3. Create a temporary data source to support adding some features and showing variations in the data conditions.
4. Begin adding permanent data sources. Tie the lookups for the data to screen events such as selection of something in a combo box or NEXT/PREVIOUS buttons. Link screen components to columns in the data sources.
I have been told that data sources need to be incorporated at the beginning of building an app, which severly limits the tool's usability for prototyping and incremental development. As I understand it, I presently need to completely rebuild the app if I'm changing data sources.