Swift 3. OSMTileLayer for Google Maps iOS SDK. With argument retina=true in constructor, tile layer automatically doubles the tiles resolution. It makes OSM tiles more readable on retina-devices.

OSMTileLayer.swift

import Foundation
import GoogleMaps
import UIImage_ResizeMagick

/**
 Custom GMSTileLayer which allows us to double tile size for OSM.
 Available size of OSM tiles is 256, but for retina devices we need 512x512.
*/
class OSMTileOverlay: GMSTileLayer {
  var retina: Bool = false
  var retinaTileSize: Int = 512

  init(retina: Bool, retinaTileSize: Int = 512) {
    super.init()
    
    self.retina = retina
    self.retinaTileSize = retinaTileSize

    if self.retina {
      self.tileSize = self.retinaTileSize
    }
  }

  override func requestTileFor(x: UInt, y: UInt, zoom: UInt, receiver: (GMSTileReceiver!)) {
    let urlStr = "http://irs.gis-lab.info/?layers=osm&request=GetTile&z=\(zoom)&x=\(x)&y=\(y)"

    var request = URLRequest(url: URL(string: urlStr)!)
    request.httpMethod = "GET"

    let session = URLSession.shared
    session.dataTask(with: request, completionHandler: {(data, response, error) in
      DispatchQueue.main.async(execute: {
        if error != nil {
          print("Error downloading tile")
          receiver.receiveTileWith(x: x, y: y, zoom: zoom, image: kGMSTileLayerNoTile)
        }
        else {
          if let image = UIImage(data: data!) {
            if self.retina {
              let doubledImage = image.resizedImage(byMagick: "\(self.retinaTileSize)x\(self.retinaTileSize)!")
              receiver.receiveTileWith(x: x, y: y, zoom: zoom, image: doubledImage)
            } else {
              receiver.receiveTileWith(x: x, y: y, zoom: zoom, image: image)
            }
          } else {
            receiver.receiveTileWith(x: x, y: y, zoom: zoom, image: kGMSTileLayerNoTile)
          }
        }

      })
    }).resume()
  }
}