Why registerOnValidatorChange in Validator instances overwrites current onChange handler?

There is Validator interface in Angular. There are a lot of predefined validators in Angular (like RequiredValidators and stuff).

Why registerOnValidatorChange in Validator instances overwrites current onChange handler?

Here is example of the code: https://github.com/angular/angular/blob/4.4.6/packages/forms/src/directives/validators.ts#L95

So what was the motivation?

The problem is: I am trying to watch changing required state of some control. I used registerOnValidatorChange for that. After that I tried it on another control (inside ng-form element) and I realised, that my watcher is not working. It is so, since ngForm redefined onChange handler by calling registerOnValidatorChange method again.

registerOnValidatorChange

ng-form

ngForm

registerOnValidatorChange

1 Answer
1

You have to notify the form when a validator's internal state has changed.

When you create a validator as a directive. That directive can have @Input() bindings that provide data for calculating the validator result.

@Input()

If that @Input() binding is changed, then the form does not know that the validator has been modified.

@Input()

So registerOnValidatorChange is called to pass a callback function to the directive. You keep a reference to that callback and call it when ever the directive is changed. This tells the form to check the validator again.

registerOnValidatorChange

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