Wheel picker

The selection line does not move. That is the whole trick of a wheel picker: a fixed band sits across the middle of the control, the values scroll under it, and whatever comes to rest inside the band is the answer. Neighbouring values stay on screen above and below, usually dimmed and foreshortened so the drum reads as a cylinder rather than a list, which means the reader can see where they are in the sequence without opening anything. Several wheels side by side compose one value out of parts, which is why a time is three drums (hour, minute, meridiem) rather than one long list of every minute in the day.

It earns its place on touch. The whole control is one large target sitting under the thumb, and picking a value costs a flick rather than a journey to a menu and back, so the hand never leaves the place it is already resting. That is also the boundary of its usefulness. A drum is only good for a short, ordered, well-known sequence, since the reader cannot see more than a few values at once and cannot jump: choosing 1987 from a list of years is a long spin, and a wheel of countries is a joke. When the set is long or unordered, a select or a search field is the honest control, and a calendar month laid out as a grid beats a wheel for a date the reader is picking against weekdays, which is why iOS ships both and its own date picker defaults to the grid.

On the web the mechanism is usually scroll snap: a scrolling column with snap points at each item and a mask over the ends, which gets the physics and the keyboard for free from the platform. That is worth knowing because the illusion is easy to half build. A drum with no snap leaves a value straddling the line, and a drum whose neighbours are not visible is just a cropped list. The rest of the craft is the part people forget, and it is all accessibility: the wheel needs a real name, its options need to be reachable by keyboard and by the arrow keys once focused, and the value has to be announced when it changes, because a control whose whole state is a position is invisible to anyone not looking at it.

The specimen here is a three column drum with a lit band across the middle. Drag a column and the value under the band changes with it, snapping at release. It snaps at rest rather than flinging, since a scripted drag lets go with no velocity; a real drum spun by a thumb carries momentum and coasts to a stop, which is the one part of the feel a specimen like this cannot show. Compare the time picker, the same job done as two lists of cells, where nothing is hidden but nothing is under the thumb either.