validatorChange triggers valueChanges emit even if model stays the same

[pkozlowski-opensource]

I'm submitting a...

[x] Bug report  

Current behavior

When a control is implementing the Validator interface and informs forms infrastructure about validator condition changes (through registerOnValidatorChange) the valueChanges is emitting each time validator change is called, even if model stays the same.

Expected behavior

valueChanges should emit only if the underlying model value changes, regardless of the validation changes.

Minimal reproduction of the problem with instructions

https://stackblitz.com/edit/angular-value-change-e79u33?file=app%2Fapp.component.ts

What is the motivation / use case for changing the behavior?

valueChanges should not emit when the model is not changing - otherwise its basic contract is broken.

Environment

Angular version: ^6.0.0


Browser: ALL

[kara]

@pkozlowski-opensource The valueChanges observable actually emits whenever the value is re-calculated, not necessarily when it changes (the same is true for statusChanges). If you call setValue() with the same value 10 times, you will get 10 valueChanges events. The observable's name is a bit misleading, but this is how it's intended to work. If you need only new values, you can add a distinctUntilChanged operator.

That said, it doesn't seem necessary to re-calculate the value when the validator changes. We only technically need to re-calculate the validation status, so perhaps that would be the way forward here (though breaking).

[popec]

Regarding #18574
How can I emit valueChanges on formControl programatically for same value now ?

[csvn]

This seems to be the root cause for angular/components#9295, where changing the min validator on MatDateRangeInput causes two new valueChanges to emit, when just changing the validation. The linked issue can likely not be solved without first fixing this one. Could it maybe be included for v13?