L.TileLayer.PouchDBCached for nuxt.js

Allows all Leaflet TileLayers to cache into PouchDB in nuxt project for offline use, in a transparent fashion.

Dependencies

Works with Leaflet 1.0.0 (or newer) and PouchDB 6 or 7.
There is a problem that prevents this to work with leaflet 1.3.2 or 1.3.3 (issue Leaflet#6245).
However, this has been fixed in leaflet 1.3.4, so everything works fine from versions >= 1.3.4 again.

You probably want to load Leaflet, PouchDB and Leaflet.TileLayer.PouchDB like so:

npm install --save nuxt-leaflet
npm install --save pouchdb
npm install --save nuxt-leaflet-pouchdb

If you are still using Leaflet 0.7.x, the latest compatible release is v0.1.0.

Usage

The plugin modifies the core L.TileLayer class, so it should be possible to cache any tile layer.

To use, add the option useCache with a value of true when instantiating your layer. You probably want to use Leaflet's crossOrigin option, like so:

<template>
  ...
        <l-tile-layer
          :url="url"
          :attribution="attribution"
          :options="options"
        />
  ...
</template>
<script>
import L from 'leaflet'
import { LMap, LTileLayer, LPopup, LMarker } from 'vue2-leaflet'
import '@/helpers/filters'
import 'nuxt-leaflet-pouchdb'

export default {
...
  data() {
    return {
      options: {
        useCache: true,
        crossOrigin: true
      }
    }
  },
...
}

Options available are as follows:

• dbName: Name of the pouchDB the cached tiles are stored in. Defaults to 'offline-tiles'.
• useCache: Whether the cache is enabled. This option must be set at initialization time.
• saveToCache: Whether to save new tiles to the cache or not. Defaults to true.
• useOnlyCache: Whether to fetch tiles from the network or not. Defaults to false.
• cacheMaxAge: Time (in milliseconds) for any given tile to be considered 'fresh'. Tiles older than this value will be re-requested from the network. Defaults to 24 hours.

Functions available are as follows:

• seed: Starts seeding the cache for a given bounding box (a L.LatLngBounds), and between the two given zoom levels.

Events available are as follows:

• tilecachehit: Fired when a tile has been found in the tile cache. The event includes data as per http://leafletjs.com/reference.html#tile-event
• tilecachemiss: Like tilecachehit, but is fired when the tile has not been found in the cache.
• tilecacheerror: Fired when there was an error trying to save a tile in the cache. The event data includes:
• tile: A reference to the failed tile
• error: The error message, probably related to CORS.
• seedstart: Fired when a layer cache has started seeding. The event data includes:
• bbox: bounding box for the seed operation, as per the L.TileLayer.seed() function call.
• minZoom and maxZoom: zoom levels the seed operation, as per the L.TileLayer.seed() function call.
• queueLength: (integer) Total number of tiles to be loaded during the seed operation.
• seedend: Fired when a layer cache has finished seeding.
• seedprogress: Fired every time a tile is cached during a seed operation
• remainingLength: (integer) How many tiles are left in the seed queue. Starts with a value of queueLength and drops down to zero.

Cross-Origin Resource Sharing

Due to the tile images being parsed and stored by the browser (technically, extracting data from a canvas in which a external image has been loaded into), the tiles must come from a tile server which allows CORS (Cross-Origin Resource Sharing) on the tiles. So tiles must have a CORS header allowing them to be loaded in the document where you're using this caching layer.

In other words: if chrome shows a grey map, and displays CORS-related messages in the console, make sure that your tileserver adds this header to all tiles:

Access-Control-Allow-Origin: *

Underlying cache structure

This plugin uses an instance of PouchDB, named offline-tiles by default. PouchDB is a key-value store, so the key is the URL of a tile, and the value is a plain object containing a timestamp and the base64-encoded image.

License and stuff

Under MIT license.

Up until version 0.4.0 this plugin was maintained by Iván Sánchez Ortega (MazeMap) while starting with version 0.5.0 it is maintained by Nikolaus Krismer.

Heavily inspired by https://github.com/tbicr/OfflineMap