SunMate

Sunscreen advice is usually a fixed number: reapply every two hours. That number ignores the actual UV, your skin, whether you are in the shade and whether you have been swimming. SunMate replaces it with something that reacts.

You give it your Fitzpatrick skin type once. From the live UV index it works out how long you have before you burn, and when you log sunscreen with an SPF, the burn time and the reapply reminder both shift accordingly. Log a reapply early and the timer moves with you. Today's UV curve shows each protected window as a flattened overlay, so you can see what the sunscreen actually bought you.

It is built to be read without being opened. Home screen and lock screen widgets in several sizes, a morning briefing, an alert shortly before UV crosses your threshold. A background job refreshes hourly and repaints the widgets.

The alert logic is a set of pure functions, which is the only reason it is testable at all: no MediaQuery, no clock, no network in the background isolate. UV data comes from currentuvindex.com under CC BY 4.0; the clear-sky comparison curve is computed on the device, so the app can say how much the clouds are taking off today's peak without asking anyone.