fix(slider): Update both thumbs' value indicator UI's if layout is invoked with undefined thumb.

PiperOrigin-RevId: 345445209

Author: joyzhong

packages/mdc-slider/foundation.ts

@@ -642,15 +642,20 @@ export class MDCSliderFoundation extends MDCFoundation<MDCSliderAdapter> {
 
   /**
    * Updates value indicator UI based on current value.
-   * @param thumb Thumb whose value indicator to update.
+   * @param thumb Thumb whose value indicator to update. If undefined, all
+   *     thumbs' value indicators are updated.
    */
   private updateValueIndicatorUI(thumb?: Thumb) {
-    if (!this.isDiscrete || !thumb) return;
+    if (!this.isDiscrete) return;
 
     const value =
         this.isRange && thumb === Thumb.START ? this.valueStart : this.value;
     this.adapter.setValueIndicatorText(
         value, thumb === Thumb.START ? Thumb.START : Thumb.END);
+
+    if (!thumb && this.isRange) {
+      this.adapter.setValueIndicatorText(this.valueStart, Thumb.START);
+    }
   }
 
   /**

packages/mdc-slider/test/foundation.test.ts

@@ -211,6 +211,17 @@ describe('MDCSliderFoundation', () => {
       expect(mockAdapter.setThumbStyleProperty).not.toHaveBeenCalled();
       expect(mockAdapter.setTrackActiveStyleProperty).not.toHaveBeenCalled();
     });
+
+    it('#layout with no thumbs updates both thumbs\' value indicators', () => {
+      const {foundation, mockAdapter} = setUpAndInit(
+          {isRange: true, isDiscrete: true, valueStart: 5, value: 10});
+
+      foundation.layout();
+      expect(mockAdapter.setValueIndicatorText)
+          .toHaveBeenCalledWith(10, Thumb.END);
+      expect(mockAdapter.setValueIndicatorText)
+          .toHaveBeenCalledWith(5, Thumb.START);
+    });
   });
 
   describe('#destroy', () => {