public enum TraitUnit

A unit of measure for a Trait value.

Conversion is affine, with one honest exception

canonical = value * scale + offset, the same model com.codename1.health.HealthUnit uses, because temperature needs the offset. Converting between units of different TraitUnitDimensions throws IllegalArgumentException: that is a bug in the calling code, not a condition to report through an AsyncResource.

Mireds and Kelvin are reciprocal, not affine, so Kelvin is deliberately not a constant here – it cannot be expressed in the table, and forcing it in would produce a colour temperature that is silently wrong rather than obviously wrong. Use miredToKelvin(double) and kelvinToMired(double), or TraitValue.getColorTemperatureKelvin(), which are named so that stepping outside the conversion table is a visible act.

And the trap that catches everyone once: a higher mired value is a warmer light, because mireds are the reciprocal of Kelvin. 153 mireds is a cold blue-white; 400 is candlelight.

Why this is an enum where HealthUnit is a class

HealthUnit is an interned final class because its symbol is the wire format Apple’s HKUnit(from:) parses, and because the set grows with the data types. Neither is true here: the set is small, closed and defined by what the two backends can actually express, and nothing about a home accessory will need a unit that is not already on this list. An enum gets switch and exhaustive reasoning for free.

The one thing an enum does not get for free is a stable wire form – ordinal() shifts the moment a constant is inserted in the middle – so each carries an explicit getWireId() and forWireId(int) resolves it.