Leaflet扩展：层

 L.TileLayer.Kitten = L.TileLayer.extend({
    getTileUrl: function(coords) {
        var i = Math.ceil( Math.random() * 4 );
        return "http://placekitten.com/256/256?image=" + i;
    },
    getAttribution: function() {
        return "<a href='http://placekitten.com/attribution.html'>PlaceKitten</a>"
    }
});

L.tileLayer.kitten = function() {
    return new L.TileLayer.Kitten();
}

L.tileLayer.kitten().addTo(map);