fix(material/datepicker): don't invoke change handler when filter is swapped out if result is the same

src/material/datepicker/date-range-input.spec.ts

@@ -20,6 +20,7 @@ import {BACKSPACE} from '@angular/cdk/keycodes';
 import {MatDateRangeInput} from './date-range-input';
 import {MatDateRangePicker} from './date-range-picker';
 import {MatStartDate, MatEndDate} from './date-range-input-parts';
+import {Subscription} from 'rxjs';
 
 describe('MatDateRangeInput', () => {
   function createComponent<T>(
@@ -317,6 +318,57 @@ describe('MatDateRangeInput', () => {
     expect(end.errors?.matDatepickerFilter).toBeTruthy();
   });
 
+  it('should should revalidate when a new date filter function is assigned', () => {
+    const fixture = createComponent(StandardRangePicker);
+    fixture.detectChanges();
+    const {start, end} = fixture.componentInstance.range.controls;
+    const date = new Date(2020, 2, 2);
+    start.setValue(date);
+    end.setValue(date);
+    fixture.detectChanges();
+
+    const spy = jasmine.createSpy('change spy');
+    const subscription = new Subscription();
+    subscription.add(start.valueChanges.subscribe(spy));
+    subscription.add(end.valueChanges.subscribe(spy));
+
+    fixture.componentInstance.dateFilter = () => false;
+    fixture.detectChanges();
+    expect(spy).toHaveBeenCalledTimes(2);
+
+    fixture.componentInstance.dateFilter = () => true;
+    fixture.detectChanges();
+    expect(spy).toHaveBeenCalledTimes(4);
+
+    subscription.unsubscribe();
+  });
+
+  it('should not dispatch the change event if a new filter function with the same result ' +
+    'is assigned', () => {
+      const fixture = createComponent(StandardRangePicker);
+      fixture.detectChanges();
+      const {start, end} = fixture.componentInstance.range.controls;
+      const date = new Date(2020, 2, 2);
+      start.setValue(date);
+      end.setValue(date);
+      fixture.detectChanges();
+
+      const spy = jasmine.createSpy('change spy');
+      const subscription = new Subscription();
+      subscription.add(start.valueChanges.subscribe(spy));
+      subscription.add(end.valueChanges.subscribe(spy));
+
+      fixture.componentInstance.dateFilter = () => false;
+      fixture.detectChanges();
+      expect(spy).toHaveBeenCalledTimes(2);
+
+      fixture.componentInstance.dateFilter = () => false;
+      fixture.detectChanges();
+      expect(spy).toHaveBeenCalledTimes(2);
+
+      subscription.unsubscribe();
+    });
+
   it('should throw if there is no start input', () => {
     expect(() => {
       const fixture = createComponent(RangePickerNoStart);

src/material/datepicker/date-range-input.ts

@@ -122,8 +122,19 @@ export class MatDateRangeInput<D> implements MatFormFieldControl<DateRange<D>>,
   @Input()
   get dateFilter() { return this._dateFilter; }
   set dateFilter(value: DateFilterFn<D>) {
+    const start = this._startInput;
+    const end = this._endInput;
+    const wasMatchingStart = start && start._matchesFilter(start.value);
+    const wasMatchingEnd = end && end._matchesFilter(start.value);
     this._dateFilter = value;
-    this._revalidate();
+
+    if (start && start._matchesFilter(start.value) !== wasMatchingStart) {
+      start._validatorOnChange();
+    }
+
+    if (end && end._matchesFilter(end.value) !== wasMatchingEnd) {
+      end._validatorOnChange();
+    }
   }
   private _dateFilter: DateFilterFn<D>;
 

src/material/datepicker/datepicker-input-base.ts

@@ -152,8 +152,7 @@ export abstract class MatDatepickerInputBase<S, D = ExtractDateTypeFromSelection
   private _filterValidator: ValidatorFn = (control: AbstractControl): ValidationErrors | null => {
     const controlValue = this._dateAdapter.getValidDateOrNull(
       this._dateAdapter.deserialize(control.value));
-    const dateFilter = this._getDateFilter();
-    return !dateFilter || !controlValue || dateFilter(controlValue) ?
+    return !controlValue || this._matchesFilter(controlValue) ?
         null : {'matDatepickerFilter': true};
   }
 
@@ -392,6 +391,12 @@ export abstract class MatDatepickerInputBase<S, D = ExtractDateTypeFromSelection
     return false;
   }
 
+  /** Gets whether a value matches the current date filter. */
+  _matchesFilter(value: D | null): boolean {
+    const filter = this._getDateFilter();
+    return !filter || filter(value);
+  }
+
   // Accept `any` to avoid conflicts with other directives on `<input>` that
   // may accept different types.
   static ngAcceptInputType_value: any;

src/material/datepicker/datepicker-input.ts

@@ -112,8 +112,12 @@ export class MatDatepickerInput<D> extends MatDatepickerInputBase<D | null, D>
   @Input('matDatepickerFilter')
   get dateFilter() { return this._dateFilter; }
   set dateFilter(value: DateFilterFn<D | null>) {
+    const wasMatchingValue = this._matchesFilter(this.value);
     this._dateFilter = value;
-    this._validatorOnChange();
+
+    if (this._matchesFilter(this.value) !== wasMatchingValue) {
+      this._validatorOnChange();
+    }
   }
   private _dateFilter: DateFilterFn<D | null>;
 

src/material/datepicker/datepicker.spec.ts

@@ -1490,6 +1490,45 @@ describe('MatDatepicker', () => {
         expect(cells[0].classList).toContain('mat-calendar-body-disabled');
         expect(cells[1].classList).not.toContain('mat-calendar-body-disabled');
       });
+
+      it('should revalidate when a new function is assigned', fakeAsync(() => {
+        const classList = fixture.debugElement.query(By.css('input'))!.nativeElement.classList;
+        testComponent.date = new Date(2017, JAN, 1);
+        testComponent.filter = () => true;
+        fixture.detectChanges();
+        flush();
+        fixture.detectChanges();
+
+        expect(classList).not.toContain('ng-invalid');
+
+        testComponent.filter = () => false;
+        fixture.detectChanges();
+        flush();
+        fixture.detectChanges();
+
+        expect(classList).toContain('ng-invalid');
+      }));
+
+      it('should not dispatch the change event if a new function with the same result is assigned',
+        fakeAsync(() => {
+          const spy = jasmine.createSpy('change spy');
+          const subscription = fixture.componentInstance.model.valueChanges?.subscribe(spy);
+          testComponent.filter = () => false;
+          fixture.detectChanges();
+          flush();
+          fixture.detectChanges();
+
+          expect(spy).toHaveBeenCalledTimes(1);
+
+          testComponent.filter = () => false;
+          fixture.detectChanges();
+          flush();
+          fixture.detectChanges();
+
+          expect(spy).toHaveBeenCalledTimes(1);
+          subscription?.unsubscribe();
+        }));
+
     });
 
     describe('datepicker with change and input events', () => {
@@ -2325,8 +2364,9 @@ class DatepickerWithMinAndMaxValidation {
 })
 class DatepickerWithFilterAndValidation {
   @ViewChild('d') datepicker: MatDatepicker<Date>;
+  @ViewChild(NgModel) model: NgModel;
   date: Date;
-  filter = (date: Date) => date.getDate() != 1;
+  filter = (date: Date | null) => date?.getDate() != 1;
 }