Download tiles

FetchTiles.js

function async fetchTiles(tileGrid, rootFolder, tileServerUrl) {
  /*
   * For Expo to be able to download the tiles,
   * the directories have to be created first.
   */
  const create_directories = tileGrid.map((tile) => {
    const folder = `${rootFolder}/${tile.z}/${tile.x}`
    return FileSystem.makeDirectoryAsync(folder, {
      intermediates: true,
    })
  })
  await Promise.all(create_directories)
  
  
  const tile_downloads = tiles.map(tile => {
    const fetchUrl = `${tileServerUrl}/${tile.z}/${tile.x}/${tile.y}.png`
    const fsLocation = `${rootFolder}/${tile.z}/${tile.x}/${tile.y}.png`
    
    return FileSystem.downloadAsync(fetchUrl, fsLocation)
  })

  const fileStatuses = await Promise.all(tile_downloads)
  
  /*
  // Debug downloaded files
  fileStatuses.forEach(file => {
    console.log('Downloaded, file.uri)
  })
  */
}