feat: Initial RasterLayer

[kylebarron]

Change list

• Initial RasterLayer
• User passes in an async callback for fetching tile data and a sync callback for rendering given each tile
• deck.gl makes request for each tile. That request is sent to Python over the Jupyter comm. Then the layer receives the tile request, calls each callback, and sends data back to the frontend.
• For now this only supports web mercator tiling, but that will be fixed in a future PR
• In the future,
  • the from_async_geotiff method will define its own fetch_tile callback before passing to cls()
  • We should ensure that the abort signal is sent to cancel any Python requests.
  • We may want per-tile caching on the Python side?

Example with OSM data:

Screen.Recording.2026-02-04.at.2.54.54.PM_1080p.mov

from lonboard import viz, Map
from lonboard import RasterLayer
from aiohttp import ClientSession

OSM_URL = "https://tile.openstreetmap.org/{z}/{x}/{y}.png"
headers = {
    "User-Agent": "Lonboard/0.1 (+https://developmentseed.org/lonboard/latest/)"
}

session = ClientSession(headers=headers)

async def fetch_tile(x, y, z):
    url = OSM_URL.format(x=x, y=y, z=z)
    resp = await session.get(url)
    return await resp.read()

def render_tile(data):
    return data

layer = RasterLayer(
    fetch_tile=fetch_tile,
    render=render_tile
)

Map(layer)