[Feature Request] Allow loadTileAtPath prop for LocalTile

[mellet]

A great feature would be the possibility to pass a loadTileAtPath prop that takes a function that allows you to load tiles with whatever means you want.

In some cases there is no direct access to the filesystem, or you want to get tile tiles from a local database etc.

A example would be when developing in Expo, as they only expose their own filesystem api that does not work when passed in to LocalTile.

Example usage

fetchTileExpo => async ({z,x,y}) => {
   const path = `${Expo.FileSystem.documentDirectory}tiles/${z}/${x}/${y}.png`
   const imageData = await Expo.FileSystem.readAsStringAsync(path)
   const image = new Image()
   image.src = `data:image/png;base64,${imageData}`
   return image
}

<MapView region={this.state.region}>
  <LocalTile
    loadTileAtPath={this.fetchTileExpo}
    tileSize={256}
  />
</MapView>

Looking at the source code the LocalTile looks like it already uses the native version of this, but just fetches the tile from the filesystem.

Would be great to be able to pass our own functions here that returns the tile, then the developer can customise how to fetch the tile if there is some special way that it needs to be done (Expo filesystem, database, some map service that needs some math done to x, y, z to get the correct tile etc).

[mellet]

I actually managed to get offline maps working in Expo.
It looks like the Expo Filesystem is handled as if it was a url. So I can actually pass it to the MapTile.UrlTile.

However, this feature might still be good for some use cases.

[markmssd]

Great article, thanks for taking the time to explain your solution in details :) I will probably use it in a near future.

[h3ll0w0rld123]

Awesome work! This saved me, I was working for hours and couldn't figure out why localtile didn't work.

I had a question about your tile fetching though. It looks like you make a separate request for each tile. For performance, I'm hoping to download a compressed file, then uncompress in one go. I see expo doesn't have that feature and using a different library would require a detachment. Do you have any thoughts on this? Did you have to worry about performance?

[h3ll0w0rld123]

Also, looking at the source code, I believe the idea you suggested is not possible because the loading has to be done on the native side, not javascript. If so, I think this issue can be closed?