feat: Add dataDiff prop for incremental GeoJSON updates via updateData()

[samtalki]

Summary

Adds a dataDiff prop to GeoJSONSource (and RawSource) that calls MapLibre's GeoJSONSource.updateData(diff) instead of setData(). This resolves the TODO comment at RawSource.svelte line 95:

// TODO: support diffrential update ? (updateData)

Motivation

For applications rendering large GeoJSON datasets (thousands of features), every data prop change currently triggers a full setData() — which serializes the entire FeatureCollection to a web worker, re-runs geojson-vt tiling from scratch, and re-uploads all visible tiles to the GPU.

MapLibre GL JS v4+ provides updateData(diff) which accepts incremental diffs. When only feature properties change (not geometry), this skips the full re-tile entirely — only updating affected features in-place and reloading intersecting tiles.

Usage

<script>
  import { GeoJSONSource } from 'svelte-maplibre-gl';
  import type { GeoJSONSourceDiff } from 'maplibre-gl';

  let data = $state(initialGeoJSON);
  let dataDiff = $state<GeoJSONSourceDiff | undefined>(undefined);

  function updateColors() {
    // Incremental: only update properties, skip re-tiling
    dataDiff = {
      update: features.map(f => ({
        id: f.id,
        addOrUpdateProperties: [
          { key: 'color', value: computeColor(f) }
        ]
      }))
    };
  }
</script>

<GeoJSONSource {data} {dataDiff}>
  <CircleLayer paint={{ 'circle-color': ['get', 'color'] }} />
</GeoJSONSource>

• data → calls setData() (full replace) — use for structural changes
• dataDiff → calls updateData() (incremental) — use for property-only changes
• Both props can coexist: initial load via data, subsequent updates via dataDiff

Changes

• RawSource.svelte: Added dataDiff to Props type, new $effect that calls source.updateData(dataDiff) (same pattern as existing setData effect)
• GeoJSONSource.svelte: Added dataDiff to Props interface, passes through to RawSource

Checklist

• pnpm check:all — 0 errors, 0 warnings (11/11 packages)
• pnpm build:packages — all packages build
• pnpm test:unit -- --run — all tests pass
• Non-breaking: new optional prop, existing behavior unchanged
• Changeset included (minor version bump)

[changeset-bot]

🦋 Changeset detected

Latest commit: 3a8bd54

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[Copilot]

Pull request overview

Adds an optional dataDiff prop to enable incremental GeoJSON updates via MapLibre’s GeoJSONSource.updateData() for better performance on large datasets.

Changes:

• Extend RawSource and GeoJSONSource props with dataDiff?: GeoJSONSourceDiff
• Add a new reactive effect in RawSource that applies updateData(dataDiff) after initial mount
• Add a changeset to publish a minor version with the new API

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
svelte-maplibre-gl/src/lib/sources/RawSource.svelte	Adds dataDiff prop and a new $effect that calls GeoJSONSource.updateData()
svelte-maplibre-gl/src/lib/sources/GeoJSONSource.svelte	Wires dataDiff through to RawSource
.changeset/geojson-update-data.md	Announces new dataDiff prop and minor bump

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commit: 3a8bd54

[ciscorn]

@samtalki Thanks for the PR! It'd be nice to leverage Svelte's reactivity for updateData, but I'm not sure a reactive prop is the right fit. A diff feels more like a one-shot command than declarative state, which makes it tricky to decide when to re-apply it and how it should interact with data updates. Still figuring out what the right shape would be.

[samtalki]

Totally understand -- sorry for any inconvenience, I'm still learning frontend. Please let me know if there's anything I can do to improve this! @ciscorn