peter
d065902404
Review follow-up. The viewport-scaled lookahead was unbounded, so a tall window prefetched several feed pages on every mount / filter change before any scroll — wasted work on the app's heaviest query. Clamp it to MAX_LOOKAHEAD_ROWS, and move the policy into a pure readAhead.ts (readAheadLookahead / shouldPrefetchNext) with unit tests covering the floor, the scaling, the cap boundary, and the fetch trigger — the core of the feed-scroll fix now has coverage instead of only manual E2E.
31 lines
1.7 KiB
TypeScript
31 lines
1.7 KiB
TypeScript
// Infinite-scroll read-ahead policy for VirtualGrid, extracted as a pure function so the decision is
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// unit-testable without a layout engine.
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//
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// The next page is pulled once the last rendered row comes within `lookahead` rows of the loaded end,
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// where lookahead scales with the number of rows the viewport currently renders (so it adapts to
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// window height / column count / row height) but is clamped to a floor and a ceiling:
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// - floor PREFETCH_ROWS: the finite-list / first-paint case where nothing is rendered yet.
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// - ceiling MAX_LOOKAHEAD_ROWS: a tall window renders many rows, and an uncapped lookahead would
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// eagerly prefetch several pages on every mount / filter change — wasted work on the app's
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// heaviest query (the feed). The cap bounds that while still buffering a couple of screens ahead.
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export const PREFETCH_ROWS = 4;
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export const PREFETCH_VIEWPORTS = 2;
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export const MAX_LOOKAHEAD_ROWS = 40;
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/** How many rows ahead of the last rendered row to keep buffered, for a viewport rendering
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* `renderedRows` rows. Clamped to [PREFETCH_ROWS, MAX_LOOKAHEAD_ROWS]. */
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export function readAheadLookahead(renderedRows: number): number {
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return Math.min(MAX_LOOKAHEAD_ROWS, Math.max(PREFETCH_ROWS, renderedRows * PREFETCH_VIEWPORTS));
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}
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/** Whether to fetch the next page: true once the last rendered row (`lastIndex`) is within the
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* read-ahead window of the loaded end (`rowCount` total loaded rows). Because the caller re-runs
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* this as each page settles, it chains — refilling the buffer as the user consumes it. */
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export function shouldPrefetchNext(
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lastIndex: number,
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rowCount: number,
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renderedRows: number,
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): boolean {
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return lastIndex >= rowCount - 1 - readAheadLookahead(renderedRows);
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}
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