refactor(visualMap): do not make assumption of child class in parent

src/component/visualMap/PiecewiseModel.ts

@@ -244,6 +244,13 @@ class PiecewiseModel extends VisualMapModel<PiecewiseVisualMapOption> {
         // thisOption.selectedMode === 'multiple', default: all selected.
     }
 
+    /**
+     * @public
+     */
+    getItemSymbol(): string {
+        return this.get('itemSymbol');
+    }
+
     /**
      * @public
      */

src/component/visualMap/VisualMapModel.ts

@@ -39,7 +39,6 @@ import Model from '../../model/Model';
 import GlobalModel from '../../model/Global';
 import SeriesModel from '../../model/Series';
 import List from '../../data/List';
-import {PiecewiseVisualMapOption} from './PiecewiseModel';
 
 const mapVisual = VisualMapping.mapVisual;
 const eachVisual = VisualMapping.eachVisual;
@@ -237,6 +236,13 @@ class VisualMapModel<Opts extends VisualMapOption = VisualMapOption> extends Com
         );
     }
 
+    /**
+     * @public
+     */
+    getItemSymbol(): string {
+        return null;
+    }
+
     /**
      * @protected
      * @return {Array.<number>} An array of series indices.
@@ -484,9 +490,7 @@ class VisualMapModel<Opts extends VisualMapOption = VisualMapOption> extends Com
             const symbolSizeExists = (controller.inRange || {}).symbolSize
                 || (controller.outOfRange || {}).symbolSize;
             const inactiveColor = this.get('inactiveColor');
-            const itemSymbol = this.type === 'visualMap.piecewise'
-                ? (this as VisualMapModel<PiecewiseVisualMapOption>).get('itemSymbol')
-                : null;
+            const itemSymbol = this.getItemSymbol();
             const defaultSymbol = itemSymbol || 'roundRect';
 
             each(this.stateList, function (state: VisualState) {

src/component/visualMap/VisualMapView.ts

@@ -27,7 +27,6 @@ import GlobalModel from '../../model/Global';
 import ExtensionAPI from '../../core/ExtensionAPI';
 import VisualMapModel from './VisualMapModel';
 import { VisualOptionUnit, ColorString } from '../../util/types';
-import PiecewiseModel from './PiecewiseModel';
 
 type VisualState = VisualMapModel['stateList'][number];
 
@@ -118,7 +117,7 @@ class VisualMapView extends ComponentView {
 
         // Default values.
         if (visualCluster === 'symbol') {
-            visualObj.symbol = (visualMapModel as PiecewiseModel).get('itemSymbol');
+            visualObj.symbol = visualMapModel.getItemSymbol();
         }
         if (visualCluster === 'color') {
             const defaultColor = visualMapModel.get('contentColor');