MapCache Swift

The missing part of MapKit: A simple way to cache map tiles and support offline browsing of maps.

Current features:

• Automatically save tiles in a disk cache as user browses the map.
• Generation of interpolated tiles beyond maximum server zoom level ({z})
• Cache capacity. Once the cache is full it will use a LRU (Least Recently Used) algorithm.
• Get Current cache size
• Clear existing cache
• Download a full region of the map (experimental)
• Available reference documentation

Installation

MapCache is available through CocoaPods. To install it, simply add the following line to your Podfile:

#Podfile
pod 'MapCache'

Then in the folder where you have the Podfile run the command

pod install

How to use MapCache?

In the view controller where you have a MKMapView, import MapCache:

import MapCache

Then, within the ViewController add:

// ViewController.swift
class ViewController: UIViewController {
  @IBOutlet weak var map: MKMapView!

  override func viewDidLoad() {
    super.viewDidLoad()

    //...

    map.delegate = self

    //...

    // First setup the config of our cache. 
    // The only thing we must provide is the url template of the tile server.
    // (All other config options are explained below in the section MapCache Configuration)
    let config = MapCacheConfig(withUrlTemplate: "https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png")

    // initialize the cache with our config
    let mapCache = MapCache(withConfig: config)

    // We tell the MKMapView to use the cache
    // So whenever it requires a tile, it will be requested to the 
    // cache 
    map.useCache(mapCache)

    //...
}

Finally, tell the map delegate to use mapCacheRenderer:

//ViewController.swift

// Assuming that ViewController is the delegate of the map
// add this extension:
extension ViewController : MKMapViewDelegate {
    func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
        return mapView.mapCacheRenderer(forOverlay: overlay)
    }
}

After setting up the map cache, browsed areas of the map will be kept on disk. If the user browses again, that area it will use the local version.

Note that in the current version, the cache does not support expiration dates, so if you need to get an updated version of the tiles you must call clear() which will wipe out the whole cache.

mapCache.clear() {
    // do something after clear
    print("cache cleared!")
}

To get current cache size:

mapCache.calculateDiskSize()

You can take a look at the Example/ folder to see a complete implementation.

Reference documentation is also available.

MapCache configuration

Configurating the map cache is pretty straight forward. Typically, you will only need to set urlTemplate and, optionally, the subdomains.

These are the options:

var config = MapCacheConfig()

// Set the URL template.
// For Open Street Maps you can chose: https://wiki.openstreetmap.org/wiki/Tile_servers
// It defaults to OpenStreetMaps servers
//
// Below we set cartoDB Base map server (https://github.com/CartoDB/cartodb/wiki/BaseMaps-available)
config.urlTemplate: "https://${s}.basemaps.cartocdn.com/base-light/${z}/${x}/${y}.png"


// In the urlTemplate ${s} stands for subdomains, which allows you to balance
// the load among the
// different tile servers.
// Default value is ["a","b","c"].
config.subdomains = ["a", "b"]


// Cache Name is basically is the subfolder name in which the tiles are store.
// Default value is "MapCache"
config.cacheName = "Carto"


// Max zoom supported by the tile server
// Default value is 19
config.maximumZ = 21

// Minimum zoom can also be set.
// config.minimumZ = 0

// Continues to show map tiles even beyond maximumZ
// config.overZoomMaximumZ = true


// Capacity of the cache in bytes. Once the cache is full it uses a LRU algorithm
// (Least Recently Used), that is, it removes the tiles last used a lot of time ago.
// Each time a tile is retrieved from the cache it is updated the value of last time used.
// Default value of the capacity is unlimited.
config.capacity = 200 * 1024 * 1024 // 200 Megabytes

If you need to use MapCache in different controllers, be sure to use the same values in the config to avoid issues.

How does MapCache work behind the scenes

If you need to build something on top of MapCache, read the following.

MapCache is a hack of MapKit, the map framework from Apple, used on their iOS, macOS, tvOS, and watchOS platforms.

Understanding MapCache bootstrap

As explained in How to use MapCache? section, in order to bootstrap MapCache, we have to call this method:

map.useCache(mapCache)

Where map is an instance of MKMapView, the main class used to display a map with MapKit. What MapCache does through the MKMapView+MapCache extension is to add a new method useCache that tells MKMapView to display a new tile layer on top of the default layers. Because of this, while the tiles are loaded you may see the tiles of the default Apple Maps.

This extension also adds a variable in the MKMapView to keep the cache config.

A layer in the map is called overlay in MapKit terminology. MapCache uses tile based overlay implemented in the class CachedTileOverlay, which is a subclass of MKTileOverlay.

Overlays have associated renderers that are the actual classes that draw the content of an overlay on the screen. For example, there are rendererers for points, lines, polygons, and tiles. When MapView needs to display an overlay, it calls the delegate with the overlay it is going to render and you need to provide the renderer to use. In order to do that, We added a mapCacheRenderer method that returns the default MKTileOverlayRenderer when the class of the overlay passed as the argument is of the type CachedTileOverlay. That is why we need to add this code on the application in the delegate of the map view (MKMapViewDelegate) :

extension ViewController : MKMapViewDelegate {
    func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
        return mapView.mapCacheRenderer(forOverlay: overlay)
    }
}

CachedTileOverlay and MapCacheProtocol

As mentioned earlier, CachedTileOverlay is a tile based layer that is implemented as a subclass of MKTileOverlay. Basically, the only thing that it does is to override two methods of the parent class:

1. func url(forTilePath path: MKTileOverlayPath) -> URL. The goal of this method is to return the URL of the tile. We need to overwrite it to be able to use the tile server of our preference.

2. func loadTile(at path: MKTileOverlayPath, result: @escaping (Data?, Error?) -> Void). This method is the one that returns the actual Tile.

If you take a look at the implementation of CachedTileOverlay, you will notice that it only forwards the request to the method with the same signature of a variable called mapCache which is an instance of a class that implements MapCacheProtocol

override public func url(forTilePath path: MKTileOverlayPath) -> URL {
    return mapCache.url(forTilePath: path)
}

The MapCacheProtocol definition is pretty simple, as it just requires to have a config variable instance of a MapCacheConfig and an implementation of the two methods that are called from CachedTileOverlay:

public protocol MapCacheProtocol {

    var config: MapCacheConfig { get set }

    func url(forTilePath path: MKTileOverlayPath) -> URL

    func loadTile(at path: MKTileOverlayPath, result: @escaping (Data?, Error?) -> Void)
    
}

If you need to create a custom implementation of the cache, you need to create a class that implements this protocol and initialize the cache using map.useCache(myCustomCacheImplementationInstance). The implementation provided by the library is on the MapCache class.

Something that may also be useful is the DiskCache class.

If you need further information you can take a look at

Reference documentation of MapCache.

Development

Clone the repo

git clone https://github.com/merlos/MapCache

With Xcode Open the file Example/MapCache.xcworkspace

To check if the pod is ready to be released run the command in the root folder folder

pod lib lint MapCache.podspec

Generate the documentation

This project uses jazzy for generating the reference documentation)

To install jazzy

gem install jazzy --verbose

In the root folder of the project run

jazzy

It will generate the html in docs/

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License - MIT

Copyright (c) 2019-2020 Juan M. Merlos @merlos and contributors.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.