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API

    
      interface AsyncValidator extends Validator {}

Method that performs async validation against the provided control.

@paramcontrolAbstractControl<any, any, any>

The control to validate against.

@returnsPromise<ValidationErrors | null> | Observable<ValidationErrors | null>

A promise or observable that resolves a map of validation errors if validation fails, otherwise null.

Registers a callback function to call when the validator inputs change.

@paramfn() => void

The callback function

@returnsvoid

Usage Notes

Provide a custom async validator directive

The following example implements the AsyncValidator interface to create an async validator directive with a custom error key.

import { of } from 'rxjs';

@Directive({
  selector: '[customAsyncValidator]',
  providers: [{provide: NG_ASYNC_VALIDATORS, useExisting: CustomAsyncValidatorDirective, multi:
true}]
})
class CustomAsyncValidatorDirective implements AsyncValidator {
  validate(control: AbstractControl): Observable<ValidationErrors|null> {
    return of({'custom': true});
  }
}

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