Simplify events and store listeners only in one place

src/ol/layer/Group.js

@@ -80,9 +80,7 @@ class LayerGroup extends BaseLayer {
      */
     this.listenerKeys_ = {};
 
-    listen(this,
-      getChangeEventType(Property.LAYERS),
-      this.handleLayersChanged_, this);
+    this.addEventListener(getChangeEventType(Property.LAYERS), this.handleLayersChanged_);
 
     if (layers) {
       if (Array.isArray(layers)) {

src/ol/layer/Heatmap.js

@@ -1,7 +1,6 @@
 /**
  * @module ol/layer/Heatmap
  */
-import {listen} from '../events.js';
 import {getChangeEventType} from '../Object.js';
 import {createCanvasContext2D} from '../dom.js';
 import VectorLayer from './Vector.js';
@@ -83,9 +82,7 @@ class Heatmap extends VectorLayer {
      */
     this.gradient_ = null;
 
-    listen(this,
-      getChangeEventType(Property.GRADIENT),
-      this.handleGradientChanged_, this);
+    this.addEventListener(getChangeEventType(Property.GRADIENT), this.handleGradientChanged_);
 
     this.setGradient(options.gradient ? options.gradient : DEFAULT_GRADIENT);
 

src/ol/layer/Layer.js

@@ -121,9 +121,7 @@ class Layer extends BaseLayer {
       this.setMap(options.map);
     }
 
-    listen(this,
-      getChangeEventType(LayerProperty.SOURCE),
-      this.handleSourcePropertyChange_, this);
+    this.addEventListener(getChangeEventType(LayerProperty.SOURCE), this.handleSourcePropertyChange_);
 
     const source = options.source ? /** @type {SourceType} */ (options.source) : null;
     this.setSource(source);