Angular 5 Debounce Custom Async Validator

Question

I've created a custom Async validator that uses a service to validate emails against a server. However, this means the server is hit every time a character is entered which is no good. I've followed several answers on here that I haven't been able to get working.

My Validator:

import {FormControl, NG_ASYNC_VALIDATORS, Validator} from 
'@angular/forms';
import { Http } from '@angular/http';
import {Directive, forwardRef} from "@angular/core";
import {ValidateEmailService} from "../services/validate-email.service";
import {UserService} from "../services/user.service";

@Directive({
  selector: '[appEmailValidator]',
  providers: [
    { provide: NG_ASYNC_VALIDATORS, useExisting: forwardRef(() => EmailValidator), multi: true }
  ]
})
export class EmailValidator implements Validator {
  public validateEmailService: ValidateEmailService;

  constructor(
    private _http: Http,
    private _userService: UserService
  ) {
    this.validateEmailService = new ValidateEmailService(this._http, this._userService);
  }

  validate(c: FormControl) {
    return new Promise(resolve => {
      this.validateEmailService.validateEmail(c.value)
        .subscribe((res) => {
          console.log(res);
          if (res.valid) {
            resolve(null);
          } else {
            resolve({
              valid: {
                valid: false
              }
            });
          }
        });
      })
    }
}

It works well by itself but as soon as I try to add some form of debounce to it, I end up breaking it.

I've tried the answers from this question and I get errors along the lines of Type X is not assignable to type 'Observable<any>' etc.

I got close by using a setTimeout but all that ended up doing was halting the functionality.

My end goal is to only run the validator when the input hasn't been changed for about 600 ms, but would settle for only validating once every 600-2000 ms.

For additional clarity, the validateEmail method from the ValidateEmailService:

public validateEmail(email: string) {

  let validateEmail = new ValidateEmail(email);

  return this._http.get(
    this.getUrl(validateEmail),
    this.getOptionArgs())
    .map((response: Response) => Object.assign(new UserEmailVerification(), response.json().UserEmailVerification));

}

Answer 1

I haven't seen an async validator implemented as a Directive, but rather as a validator function assigned to a form control.

Here's an example validator I use for a similar case:

import { Injectable } from '@angular/core';
import { AbstractControl, AsyncValidatorFn } from '@angular/forms';
import { Observable, timer, of } from 'rxjs';
import { switchMap, map } from 'rxjs/operators';

import { MembershipsService } from '@app/memberships/memberships.service';

@Injectable()
export class MembershipsValidators {

  constructor (
    private membershipsService: MembershipsService,
  ) {}

  checkMembershipExists(email?: string): AsyncValidatorFn {
    return (control: AbstractControl): Observable<{ [key: string]: any } | null> => {
      if (control.value === null || control.value.length === 0) {
        return of(null);
      }
      else if (email && email === control.value) {
        return of(null);
      }
      else {
        return timer(500).pipe(
          switchMap(() => {
            return this.membershipsService.lookupMember(control.value).pipe(
              map(member => {
                if (!member) {
                  return { noMembership: { value: control.value } };
                }

                return null;
              })
            );
          })
        );
      }
    };
  }

}

That gets imported and applied to a form control as such:

this.form = this.formBuilder.group({
  memberEmail: new FormControl('', {
    validators: [ Validators.required, Validators.pattern(regexPatterns.email) ],
    asyncValidators: [ this.membershipsValidators.checkMembershipExists() ],
  }),
});

This way, the async validator doesn't fire until the synchronous validators are satisfied.

Answer 2

You could create an Observable within your promise to accomplish the debounce.

This logic may not be cut and paste but should get you close.

import {distinctUntilChanged, debounceTime, switchMap} from 'rxjs/operators';

 validate(c: FormControl) {
  return new Promise(resolve => {
    new Observable(observer => observer.next(c.value)).pipe(
      debounceTime(600),
      distinctUntilChanged(),
      switchMap((value) => { return this.validateEmailService.validateEmail(value) })
    ).subscribe(
      (res) => {
        console.log(res);
        if (res.valid) {
          resolve(null);
        } else {
          resolve({
            valid: {
              valid: false
            }
          });
        }
      }
    )
  })
}