convert-visibility-detector-to-scroll-spy

Convert visibility_detector to scroll_spy

visibility_detector reports per-widget paint-based visibility anywhere in the tree. scroll_spy is scoped to one scrollable and adds what VD lacks: a focus region, a single stable primary with anti-flicker rules, and rich per-item metrics with rebuild isolation. Convert when the detectors live inside a scrollable (feeds, lists, grids, pagers). Do NOT convert detectors that are not inside a scrollable (dialogs, overlays, static layouts) or that rely on paint-level occlusion (Opacity(opacity: 0), Offstage) - scroll_spy models scroll geometry, not painting; leave those on visibility_detector and say so.

API mapping

visibility_detector	scroll_spy replacement
VisibilityDetector(key:, onVisibilityChanged:, child:) per item	Wrap the scrollable once (ScrollSpyListView.builder, ScrollSpyGridView.builder, ScrollSpyCustomScrollView, ScrollSpyPageView.builder, or raw ScrollSpyScope<T>), then per item ScrollSpyItemLite<T>(id:, ...) or listener widgets
Key('card-$id') identity	the typed id: on the item (stable, unique per scope); keep any widget key separately
VisibilityInfo.visibleFraction	ScrollSpyItemFocus.visibleFraction (0..1, same meaning) via snapshot.items, itemFocusOf(id), or tryGetItemFocus(id)
onVisibilityChanged enter/exit (fraction > 0 / == 0)	per item: ScrollSpyItemVisibleListener(onChanged: (was, is) ...); or one screen-level controller.snapshot listener diffing visibleIds (better when you also need fraction thresholds; absence from snapshot.items while unmounted = fraction 0)
threshold logic (fraction >= 0.6)	scan snapshot.items.values in one snapshot listener with your own seen-set (dedup)
VisibilityDetectorController.instance.updateInterval (global throttle)	updatePolicy: on the scope/wrapper: perFrame (default; per-scroll precision), ScrollSpyUpdatePolicy.hybrid(ballisticInterval: ...) (throttled flings, closest analog), or onScrollEnd(debounce: ...) (events only at rest). Non-const; store in a field
VisibilityDetectorController.instance.notifyNow() / forget()	no equivalents needed: computes are scroll-driven; notifiers auto-evict
final 0.0 callback on unmount	items leaving tracking flip their boolean listenables to false; a snapshot diff also observes disposal. Flush any pending exits in your State.dispose

What has no VD equivalent (the usual reason to convert): isPrimary + ScrollSpyStability (stable winner for autoplay/featuring via ScrollSpyPrimaryListener), focusProgress/distanceToAnchorPx (effects), regions/policies, and boolean-only rebuild isolation.

Behavioral difference you must handle: covered routes

VD (paint-based) reports visibleFraction: 0 when an opaque route covers the screen, so VD code often gets exit events "for free" on navigation. scroll_spy freezes instead: no scrolling means no computes, so primaryId, focusedIds, and per-item state keep their last values while covered (verified behavior). If the analytics contract requires exits on navigation (or must suppress events while covered):

• Simplest: listen to TickerMode.getNotifier(context) from inside the route; it flips to disabled exactly when an opaque push transition completes and back on pop. Flush exits and set a _covered guard there.
• Navigation-explicit alternative: a RouteObserver + RouteAware (didPushNext = flush exits and pause; didPopNext = resync).

On pop, re-entry events for still-visible items only fire if you emit them yourself during resync; scroll_spy will not re-notify unchanged state.

Conversion order (keeps every commit compiling)

1. Add scroll_spy to pubspec (keep visibility_detector for now).
2. Introduce the scope: swap the scrollable for its ScrollSpy wrapper (or wrap with ScrollSpyScope<T>), with a ScrollSpyController<T> as a State field, disposed in dispose. Pick T = your id type.
3. Move the analytics/side-effect logic to controller listeners (snapshot scan or per-item listeners per the mapping) including the covered-route guard if required. Delete each VisibilityDetector wrapper as its logic moves; keep the child subtree in the item's child: slot.
4. Add the new capabilities that motivated the conversion (primary selection, stability, effects), typically via ScrollSpyPrimaryListener.
5. Remove the visibility_detector dependency and its controller tuning.

Verify

• flutter analyze; grep for visibility_detector returns nothing.
• Impression parity: scroll the converted screen and compare the event log with the pre-conversion behavior for: normal scroll, fast fling, push a covering route, pop back, and leave the screen. Fraction thresholds use the same 0..1 scale, so logged ids should match (per-frame detection may catch fast-fling crossings the old global throttle missed; call that out as an improvement, not a regression).
• For widget tests, the two-frame commit pipeline applies: see the diagnose-scroll-spy skill.

Report the mapping applied per removed API, the covered-route strategy chosen, and any detectors you deliberately left on visibility_detector.