Option to disable form control change event emit from user (HTML) actions

[m-radzikowski]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

When changing reactive form control value programmatically, we can disable emitting value change event by passing {emitEvent: false} as a second setValue() argument.
But when the form control is bound to HTML component in the template and user performs a change, there is no way I could found to prevent value change event from emitting.

Expected behavior

Allow enabling/disabling change event emitting as an option for form control. It should be possible when declaring form control (so probably in a AbstractControlOptions) and also as a AbstractControl method.
Form control with disabled event emitting should not emit change event after user changes in a view, and it's to discuss if it should by default also not emit events after programmatic changes by setValue() method.

Minimal reproduction of the problem with instructions

https://stackblitz.com/edit/angular-gitter-jduydj
In a console you can see, that after choosing variant from the select change event is fired twice:

• firstly with a new variant value, but old other controls values,
• secondly with new other controls values, fired by updateValueAndValidity() call.

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

1. Reduce number of fired events.
2. In more complicated scenarios, firing event with incosistent data may lead to errors or wrong app behaviors, so it must be handled separately and introduces redundant logic to handle this special cases.

Only other option I can see to not fire that event is to not bind that control to template. But in this case we need to handle this control value separately, which also introduces redundant logic.

[manklu]

You can use the following snippet to avoid the double emit in your case

import {debounceTime} from 'rxjs/operators'

...

this.form.valueChanges.pipe(debounceTime(0)).subscribe(data => {
  console.log('form value change', data);
});

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