Substantially improve Photon results with map-derived parameters. (#339)

The Photon geocoder returns poor results unless the center point and
zoom of the map are included as query parameters.

src/control.ts

@@ -316,9 +316,10 @@ export class GeocoderControl extends EventedControl {
     const event: StartGeocodeEvent = { input: value };
     this.fire(suggest ? 'startsuggest' : 'startgeocode', event);
 
+    const context = { map: this._map };
     const results = suggest
-      ? await this.options.geocoder!.suggest!(value)
-      : await this.options.geocoder!.geocode(value);
+      ? await this.options.geocoder!.suggest!(value, context)
+      : await this.options.geocoder!.geocode(value, context);
 
     if (requestCount === this._requestCount) {
       const event: FinishGeocodeEvent = { input: value, results };

src/geocoders/photon.ts

@@ -22,8 +22,17 @@ export class Photon implements IGeocoder {
     L.Util.setOptions(this, options);
   }
 
-  async geocode(query: string): Promise<GeocodingResult[]> {
+  async geocode(query: string, context?: { map?: L.Map }): Promise<GeocodingResult[]> {
     const params = geocodingParams(this.options, { q: query });
+    const center = context?.map?.getCenter?.();
+    if (center) {
+      params.lat = center.lat;
+      params.lon = center.lng;
+    }
+    const zoom = context?.map?.getZoom?.();
+    if (zoom) {
+      params.zoom = zoom;
+    }
     const data = await getJSON<any>(this.options.serviceUrl, params);
     return this._parseResults(data);
   }