fix(material/slider): handle ngModel initial null value

src/material/slider/slider-input.ts

@@ -243,11 +243,21 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
   _skipUIUpdate: boolean = false;
 
   /** Callback called when the slider input value changes. */
-  private _onChangeFn: (value: any) => void = () => {};
+  protected _onChangeFn: ((value: any) => void) | undefined;
 
   /** Callback called when the slider input has been touched. */
   private _onTouchedFn: () => void = () => {};
 
+  /**
+   * Whether the NgModel has been initialized.
+   *
+   * This flag is used to ignore ghost null calls to
+   * writeValue which can break slider initialization.
+   *
+   * See https://github.com/angular/angular/issues/14988.
+   */
+  protected _isControlInitialized = false;
+
   constructor(
     readonly _ngZone: NgZone,
     readonly _elementRef: ElementRef<HTMLInputElement>,
@@ -328,7 +338,7 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
   }
 
   _onInput(): void {
-    this._onChangeFn(this.value);
+    this._onChangeFn?.(this.value);
     // handles arrowing and updating the value when
     // a step is defined.
     if (this._slider.step || !this._isActive) {
@@ -422,7 +432,7 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
 
     this.value = value;
     this.valueChange.emit(this.value);
-    this._onChangeFn(this.value);
+    this._onChangeFn?.(this.value);
     this._slider._onValueChange(this);
     this._slider.step > 0
       ? this._updateThumbUIByValue()
@@ -500,7 +510,9 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
    * @docs-private
    */
   writeValue(value: any): void {
-    this.value = value;
+    if (this._isControlInitialized || value !== null) {
+      this.value = value;
+    }
   }
 
   /**
@@ -510,6 +522,7 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
    */
   registerOnChange(fn: any): void {
     this._onChangeFn = fn;
+    this._isControlInitialized = true;
   }
 
   /**
@@ -738,8 +751,10 @@ export class MatSliderRangeThumb extends MatSliderThumb implements _MatSliderRan
    * @docs-private
    */
   override writeValue(value: any): void {
-    this.value = value;
-    this._updateWidthInactive();
-    this._updateSibling();
+    if (this._isControlInitialized || value !== null) {
+      this.value = value;
+      this._updateWidthInactive();
+      this._updateSibling();
+    }
   }
 }

src/material/slider/slider.spec.ts

@@ -1168,6 +1168,21 @@ describe('MDC-based MatSlider', () => {
     }));
   });
 
+  describe('range slider w/ NgModel edge case', () => {
+    it('should initialize correctly despite NgModel `null` bug', fakeAsync(() => {
+      const fixture = createComponent(RangeSliderWithNgModelEdgeCase);
+      fixture.detectChanges();
+      const sliderDebugElement = fixture.debugElement.query(By.directive(MatSlider));
+      const slider = sliderDebugElement.componentInstance;
+      const startInput = slider._getInput(_MatThumb.START) as MatSliderRangeThumb;
+      const endInput = slider._getInput(_MatThumb.END) as MatSliderRangeThumb;
+      flush();
+      console.log('result: ', startInput.value);
+      checkInput(startInput, {min: -1, max: -0.3, value: -0.7, translateX: 90});
+      checkInput(endInput, {min: -0.7, max: 0, value: -0.3, translateX: 210});
+    }));
+  });
+
   describe('slider as a custom form control', () => {
     let fixture: ComponentFixture<SliderWithFormControl>;
     let slider: MatSlider;
@@ -1617,6 +1632,22 @@ class RangeSliderWithNgModel {
   endVal: number | undefined = 100;
 }
 
+@Component({
+  template: `
+  <mat-slider min="-1" max="0" step="0.1">
+    <input [(ngModel)]="startValue" matSliderStartThumb />
+    <input [(ngModel)]="endValue" matSliderEndThumb />
+  </mat-slider>
+
+`,
+  styles: SLIDER_STYLES,
+})
+class RangeSliderWithNgModelEdgeCase {
+  @ViewChild(MatSlider) slider: MatSlider;
+  startValue: number = -0.7;
+  endValue: number = -0.3;
+}
+
 @Component({
   template: `
   <mat-slider>

tools/public_api_guard/material/slider.md

@@ -218,6 +218,7 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
     // (undocumented)
     _initValue(): void;
     _isActive: boolean;
+    protected _isControlInitialized: boolean;
     _isFocused: boolean;
     _knobRadius: number;
     get max(): number;
@@ -232,6 +233,7 @@ export class MatSliderThumb implements _MatSliderThumb, OnDestroy, ControlValueA
     _onBlur(): void;
     // (undocumented)
     _onChange(): void;
+    protected _onChangeFn: ((value: any) => void) | undefined;
     // (undocumented)
     _onFocus(): void;
     // (undocumented)