bug(mat-datepicker): value change firing on filter re assign

[pgg]

Reproduction

https://stackblitz.com/edit/angular-ivy-eaktsw?devtoolsheight=33&file=src/app/app.component.ts

Steps to reproduce:
open stackblitz, look at console output, value change triggered incorrectly

Expected Behavior

value change to fire only when value has changed, not when filter changes

Actual Behavior

value change firing for same values

Environment

• Angular: 10.2.1
• CDK/Material: 10.2.7
• Browser(s): Version 86.0.4240.111 (Official Build) (x86_64)
• Operating System (e.g. Windows, macOS, Ubuntu): macOS

[crisbeto]

We can't do a whole lot here, because we only tell the validator to revalidate and then it determines on its own whether to dispatch the change event. I've submitted #20970 which will prevent the event from firing if a new function with the same result is called, but that won't fix your example, because the validation state changes on each assignment (false -> true -> false).

[pgg]

still does not make sense that a change event is fired for the same value regardless of validation result? fyi (dateChange) event is not triggered for the same. as a side note: [matDatepickerFilter] does not work with the async pipe: Type '((date: Date | null) => boolean) | null' is not assignable to type 'DateFilterFn<Date | null>'

[crisbeto]

We have control over dateChange so we can dispatch it only when it changes, but the valueChange event is handled by @angular/forms so all we can do is tell it that the validation state has changed and then it decides which events to trigger. You can file an issue on angular/angular if you think the behavior should be changed.

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