fix(slider): value not being rounded when using keyboard and decimal step (#11574)

When incrementing a slider using the keyboard, the value can end up looking like
33.300000000000004, if the consumer set a decimal step. We fixed this a while ago for dragging, but that didn't cover the keyboard interactions. These changes move the logic from the earlier fix so it can apply everywhere.

Relates to #10951.

Author: crisbeto

src/lib/slider/slider.spec.ts

@@ -490,6 +490,17 @@ describe('MatSlider', () => {
       expect(sliderInstance.value).toBe(33.3);
     });
 
+    it('should truncate long decimal values when using a decimal step and the arrow keys', () => {
+      fixture.componentInstance.step = 0.1;
+      fixture.detectChanges();
+
+      for (let i = 0; i < 3; i++) {
+        dispatchKeyboardEvent(sliderNativeElement, 'keydown', UP_ARROW);
+      }
+
+      expect(sliderInstance.value).toBe(0.3);
+    });
+
   });
 
   describe('slider with auto ticks', () => {

src/lib/slider/slider.ts

@@ -224,7 +224,15 @@ export class MatSlider extends _MatSliderMixinBase
   }
   set value(v: number | null) {
     if (v !== this._value) {
-      this._value = coerceNumberProperty(v);
+      let value = coerceNumberProperty(v);
+
+      // While incrementing by a decimal we can end up with values like 33.300000000000004.
+      // Truncate it to ensure that it matches the label and to make it easier to work with.
+      if (this._roundToDecimal) {
+        value = parseFloat(value.toFixed(this._roundToDecimal));
+      }
+
+      this._value = value;
       this._percent = this._calculatePercentage(this._value);
 
       // Since this also modifies the percentage, we need to let the change detection know.
@@ -638,17 +646,11 @@ export class MatSlider extends _MatSliderMixinBase
     } else if (percent === 1) {
       this.value = this.max;
     } else {
-      let exactValue = this._calculateValue(percent);
+      const exactValue = this._calculateValue(percent);
 
       // This calculation finds the closest step by finding the closest
       // whole number divisible by the step relative to the min.
-      let closestValue = Math.round((exactValue - this.min) / this.step) * this.step + this.min;
-
-      // If we've got a step with a decimal, we may end up with something like 33.300000000000004.
-      // Truncate the value to ensure that it matches the label and to make it easier to work with.
-      if (this._roundToDecimal) {
-        closestValue = parseFloat(closestValue.toFixed(this._roundToDecimal));
-      }
+      const closestValue = Math.round((exactValue - this.min) / this.step) * this.step + this.min;
 
       // The value needs to snap to the min and max.
       this.value = this._clamp(closestValue, this.min, this.max);