TileLayer.js

/*
 * L.TileLayer is used for standard xyz-numbered tile layers.
 */

L.TileLayer = L.GridLayer.extend({

	options: {
		maxZoom: 18,

		subdomains: 'abc',
		errorTileUrl: '',
		zoomOffset: 0,

		maxNativeZoom: null, // Number
		tms: false,
		zoomReverse: false,
		detectRetina: false,
		crossOrigin: false
	},

	initialize: function (url, options) {

		this._url = url;

		options = L.setOptions(this, options);

		// detecting retina displays, adjusting tileSize and zoom levels
		if (options.detectRetina && L.Browser.retina && options.maxZoom > 0) {

			options.tileSize = Math.floor(options.tileSize / 2);
			options.zoomOffset++;

			options.minZoom = Math.max(0, options.minZoom);
			options.maxZoom--;
		}

		if (typeof options.subdomains === 'string') {
			options.subdomains = options.subdomains.split('');
		}

		// for https://github.com/Leaflet/Leaflet/issues/137
		if (!L.Browser.android) {
			this.on('tileunload', this._onTileRemove);
		}
	},

	setUrl: function (url, noRedraw) {
		this._url = url;

		if (!noRedraw) {
			this.redraw();
		}
		return this;
	},

	createTile: function (coords, done) {
		var tile = document.createElement('img');

		L.DomEvent.on(tile, 'load', L.bind(this._tileOnLoad, this, done, tile));
		L.DomEvent.on(tile, 'error', L.bind(this._tileOnError, this, done, tile));

		if (this.options.crossOrigin) {
			tile.crossOrigin = '';
		}

		/*
		 Alt tag is set to empty string to keep screen readers from reading URL and for compliance reasons
		 http://www.w3.org/TR/WCAG20-TECHS/H67
		*/
		tile.alt = '';

		tile.src = this.getTileUrl(coords);

		return tile;
	},

	getTileUrl: function (coords) {
		var invertedY = this._globalTileRange.max.y - coords.y;

		return L.Util.template(this._url, L.extend({
			r: L.Browser.retina ? '@2x' : '',
			s: this._getSubdomain(coords),
			x: coords.x,
			y: this.options.tms ? invertedY : coords.y,
			'-y': invertedY,
			z: this._getZoomForUrl()
		}, this.options));
	},

	_tileOnLoad: function (done, tile) {
		// For https://github.com/Leaflet/Leaflet/issues/3332
		if (L.Browser.ielt9) {
			setTimeout(L.bind(done, this, null, tile), 0);
		} else {
			done(null, tile);
		}
	},

	_tileOnError: function (done, tile, e) {
		var errorUrl = this.options.errorTileUrl;
		if (errorUrl) {
			tile.src = errorUrl;
		}
		done(e, tile);
	},

	getTileSize: function () {
		var map = this._map,
		    tileSize = L.GridLayer.prototype.getTileSize.call(this),
		    zoom = this._tileZoom + this.options.zoomOffset,
		    zoomN = this.options.maxNativeZoom;

		// increase tile size when overscaling
		return zoomN !== null && zoom > zoomN ?
				tileSize.divideBy(map.getZoomScale(zoomN, zoom)).round() :
				tileSize;
	},

	_onTileRemove: function (e) {
		e.tile.onload = null;
	},

	_getZoomForUrl: function () {

		var options = this.options,
		    zoom = this._tileZoom;

		if (options.zoomReverse) {
			zoom = options.maxZoom - zoom;
		}

		zoom += options.zoomOffset;

		return options.maxNativeZoom !== null ? Math.min(zoom, options.maxNativeZoom) : zoom;
	},

	_getSubdomain: function (tilePoint) {
		var index = Math.abs(tilePoint.x + tilePoint.y) % this.options.subdomains.length;
		return this.options.subdomains[index];
	},

	// stops loading all tiles in the background layer
	_abortLoading: function () {
		var i, tile;
		for (i in this._tiles) {
			if (this._tiles[i].coords.z !== this._tileZoom) {
				tile = this._tiles[i].el;

				tile.onload = L.Util.falseFn;
				tile.onerror = L.Util.falseFn;

				if (!tile.complete) {
					tile.src = L.Util.emptyImageUrl;
					L.DomUtil.remove(tile);
				}
			}
		}
	}
});

L.tileLayer = function (url, options) {
	return new L.TileLayer(url, options);
};