@angular/forms

Validator

interface

An interface implemented by classes that perform synchronous validation.

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

Provide a custom validator

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

          @Directive({  selector: '[customValidator]',  providers: [{provide: NG_VALIDATORS, useExisting: CustomValidatorDirective, multi: true}]})class CustomValidatorDirective implements Validator {  validate(control: AbstractControl): ValidationErrors|null {    return {'custom': true};  }}

validate

ValidationErrors

Method that performs synchronous validation against the provided control.

@paramcontrolAbstractControl<any, any>

The control to validate against.

@returnsValidationErrors

registerOnValidatorChange

void

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

@paramfn() => void

The callback function

@returnsvoid