@angular/forms

AsyncValidator

interface

An interface implemented by classes that perform asynchronous validation.

  
    interface AsyncValidator {  optional override registerOnValidatorChange(fn: () => void): void;}}

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});  }}

validate

Promise<ValidationErrors> | Observable<ValidationErrors>

Method that performs async validation against the provided control.

@paramcontrolAbstractControl<any, any>

The control to validate against.

@returnsPromise<ValidationErrors> | Observable<ValidationErrors>

registerOnValidatorChange

void

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

@paramfn() => void

The callback function

@returnsvoid