464. A slider says when a gesture ends

Date: 2026-09-23

Status

Accepted. Adds one hook to slider in :widgets (docs/core-widgets.md §3), asked for by goldberry-media’s seek bar (docs/goldberry-media.md §3, “Seeking”).

Context

slider reports every step of a drag through onChange, snapped and clamped, which is what a thumb that follows the finger needs. A media seek bar needs a second fact: when the user lets go. §3 scrubs to keyframes while the bar is dragged, because an exact seek per step would decode from the keyframe every time, and asks for one exact seek on release. With only onChange, every drag step was an exact seek (the phase 2 audio-player did exactly that, and the Engine coalesced them), and there was no way to resume a player paused for the drag at the right moment.

Decision

Slider gains onCommit, a twelfth component and a wither (slider.onCommit(value -> …)), and commit= in markup. It is told the same snapped, clamped value onChange would be:

• once when a press or a drag on the slider is released, from the release’s position, which is where the last move put it;
• after every key step, since a key press is a whole gesture.

The eleven-component constructor stays, so no existing slider changes.

Alternatives considered

• onChangeStart and onChangeEnd, as Flutter has. The start is the first onChange of a gesture, which a caller can tell for itself; one hook is enough.
• Commit only on pointer release. Then a keyboard user would never commit, and a seek bar driven by arrows would scrub for ever.
• Let the media widget watch the pointer itself. It would duplicate the slider’s own capture and travel arithmetic, and get the release position slightly differently wrong.

Consequences

• media-controls, audio-player and media-player scrub while dragging and seek exactly on release (§7, S2).
• A slider in a form can save on commit rather than on every step.
• The record’s equality now includes one more lambda, which a markup-built and a Java-built slider only share when both have none: §11’s parity comparison still holds for every slider that does not set it.