Module scroll 

LVGL-style scroll state + math, living next to layout.

Widgets in zest are transient: the Runtime rebuilds the whole tree from view(&self) every frame, so a container cannot itself remember a drag origin or a fling velocity. All cross-frame scroll/gesture state therefore lives in a single host-owned value — ScrollState — which the screen stores as a field and passes by reference into view. Containers only read it during measure/arrange/ draw and emit ScrollMsg in handle_touch; the owned copy is mutated only in update() via ScrollState::apply and ScrollState::tick.

Touch events carry no timestamp (see event), so velocity for fling is sampled in update() from embassy_time::Instant deltas the caller passes as now_ms, never inside handle_touch. Momentum is driven by a self-rescheduling tick_task (a 16 ms Timer-delayed Task), not a subscription, so the animation cleanly stops when it settles.

The math deliberately avoids libm: the tap-vs-scroll threshold uses Manhattan distance, and friction is a per-frame-constant multiply (no powf/sqrt), valid because the tick dt is approximately constant at 16 ms.

Structs

ScrollState

Cross-frame scroll/gesture/velocity state owned by the host screen.

Enums

GesturePhase

Gesture / animation phase of a ScrollState.

ScrollDirection

Which axes a container scrolls on.

ScrollMsg

Message a scrollable container emits in handle_touch; the host applies it in update(). Carries the geometry (and snap lines on release) so the owned ScrollState can integrate without touching layout.

ScrollbarMode

When the scrollbar thumb is visible.

SnapMode

How scrolling settles to content boundaries.

Constants

FRICTION

Velocity multiplier applied each animation frame during a fling.

MAX_FLING_V

Maximum fling speed (px/s, per axis) the release sample is capped to.

MIN_FLING_V

Minimum fling speed (px/s, per axis) below which a fling becomes a spring.

RUBBER_C

Rubber-band resistance coefficient: larger → stiffer over-scroll.

SCROLL_THRESHOLD

Pixel distance a finger must travel before a press is reinterpreted as a scroll (Manhattan distance, |dx| + |dy|).

SPRING_K

Spring stiffness: fraction of the remaining distance closed each frame.

TICK_MS

Animation frame period in milliseconds (~60 fps).

Functions

tick_task

Self-rescheduling animation clock: a Task that waits one frame (~16 ms) and then yields msg, so the host’s ScrollTick arm can call ScrollState::tick and re-arm while ScrollState::is_animating.