working draft for a MBTileSource that reads tiles from a given MBTiles container at a given path

MBTileSource.js

import XYZ from 'ol/source/XYZ'
import TileState from 'ol/TileState'
import Database, { Integer } from 'better-sqlite3'

const MBTileSource = options => {
  /* instance of the SQLite db */
  const db = new Database(options.path, { readonly: true })

  const metadata = name => {
    const result = db.prepare('SELECT value FROM metadata WHERE name=?').get(name)
    return (result ? result.value : null)
  }

  const imageFormat = metadata('format')
  if (!imageFormat || imageFormat === 'pbf') throw new Error('unsupported tile image format')

  const getTile = (z, x, y /* , pixelRatio, projection */) => {
    // console.log(`fetching tile for ${z}/${x}/${y}`)
    const tile = db.prepare(`
        SELECT tile_data FROM tiles 
        WHERE zoom_level = :z AND tile_column = :x AND tile_row = :y`
    ).get({
      z: Integer(z),
      x: Integer(x),
      y: Integer((1 << z) - 1 - y) // flip y for TMS tiling scheme
    })
    return tile ? tile.tile_data : null
  }

  const source = new XYZ()

  /* no URL required, returns just the z,x,y coords as an array */
  source.tileUrlFunction = (coordinates /*, p2, projection */) => {
    return coordinates
  }

  /* loading the tiles by coords */
  source.tileLoadFunction = (tile, coords) => {
    const image = getTile(coords[0], coords[1], coords[2])
    if (image) {
      tile.getImage().src = `data:image/${imageFormat};base64,${Buffer.from(image).toString('base64')}`
    } else {
      tile.setState(TileState.ERROR)
    }
  }

  return source
}

export default MBTileSource