Preload tiles around a marker

using System.Collections.Generic;
using UnityEngine;

public class PreloadTilesAroundMarker:MonoBehaviour
{
    public int countTilesX = 3; 
    public int countTilesY = 3;
    public int zoomOffset = 2;

    private Dictionary<ulong, OnlineMapsTile> tiles;
    private OnlineMapsMarker marker;
    private OnlineMaps map;

    private void OnGUI()
    {
        if (GUILayout.Button("Preload"))
        {
            StartPreloading();
        }
    }

    private void Start()
    {
        map = OnlineMaps.instance;
        marker = map.AddMarker(map.position);

        tiles = new Dictionary<ulong, OnlineMapsTile>();
    }

    private void StartPreloading()
    {
        foreach (KeyValuePair<ulong, OnlineMapsTile> pair in tiles)
        {
            pair.Value.Unblock(this);
        }

        tiles.Clear();

        double lng, lat;
        marker.GetPosition(out lng, out lat);

        int zoom = map.zoom - zoomOffset;
        if (zoom < 3) zoom = 3;

        double tx, ty;
        map.projection.CoordinatesToTile(lng, lat, zoom, out tx, out ty);

        int itx = (int) tx;
        int ity = (int) ty;

        int sx = itx - (countTilesX - 1) / 2;
        int sy = ity - (countTilesY - 1) / 2;

        int max = 1 << zoom;

        for (int x = sx; x < sx + countTilesX; x++)
        {
            itx = x;
            if (itx < 0) itx += max;
            else if (itx >= max) itx -= max;

            for (int y = sy; y < sy + countTilesY; y++)
            {
                if (y < 0 || y > max) continue;

                ulong key = OnlineMapsTile.GetTileKey(zoom, itx, y);
                OnlineMapsTile parentTile = OnlineMapsTile.GetTile(zoom - 1, itx / 2, y / 2);
                OnlineMapsTile tile = new OnlineMapsTile(itx, y, zoom, map, parentTile);
                tile.Block(this);
                tiles[key] = tile;
            }
        }
    }
}