Getter for opacity background layer and tests

src/services/backgroundlayermgr.js

@@ -162,6 +162,18 @@ ngeo.BackgroundLayerMgr.prototype.set = function(map, layer) {
   return previous;
 };
 
+/**
+ * Return the current opacity background layer of a given map. `null` is returned if
+ * the map does not have an opacity background layer.
+ * @param {ol.Map} map Map.
+ * @return {ol.layer.Base} layer The opacity background layer.
+ * @export
+ */
+ngeo.BackgroundLayerMgr.prototype.getOpacityBgLayer = function(map) {
+  const mapUid = ol.getUid(map).toString();
+  return mapUid in this.mapUids_ ? this.ngeoLayerHelper_.getGroupFromMap(map, gmf.BACKGROUNDLAYERGROUP_NAME).getLayers().item(1) : null;
+};
+
 /**
  * Set a background layer used by the opacity slider.
  * @param {ol.Map} map The map.

test/spec/services/backgroundlayermgr.spec.js

@@ -78,6 +78,20 @@ describe('ngeo.BackgroundLayerMgr', () => {
       const layer = ngeoBackgroundLayerMgr.get(map);
       expect(layer).toBe(expectedLayer);
     });
+
+    it('returns `null` if no opacity background layer', () => {
+      const layer = ngeoBackgroundLayerMgr.get(map);
+      expect(layer).toBe(null);
+    });
+
+    it('returns the current opacity background layer', () => {
+      const activeBgLayer = new ol.layer.Tile();
+      ngeoBackgroundLayerMgr.set(map, activeBgLayer);
+      const opacityBgLayer = new ol.layer.Tile();
+      ngeoBackgroundLayerMgr.setOpacityBgLayer(map, opacityBgLayer);
+      const layer = ngeoBackgroundLayerMgr.getOpacityBgLayer(map);
+      expect(layer).toBe(opacityBgLayer);
+    });
   });
 
 });