How to use debounceTime in an angular component?

Question

My requirement is to perform reactive form field validations in such a way that the error messages are displayed only after the user stops typing.

How can I accomplish this using reactive forms and Rxjs debounceTime?

I'm looking for a solution that works with Reactive forms

Answer 1

The (or at least a) way to get this to work is to dynamically remove and add your validators as you go.

On your input(s), use a keydown binding that will strip away validators when the user starts to type, and a keyup binding that will run through a debounceTime pipe and then reapply the validators (but only after the specified debounce time has passed).

Code here:

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';
import { Subject } from 'rxjs';
import { debounceTime } from 'rxjs/operators';

@Component({
  selector: 'form-component',
  template: `
    <form [formGroup]="formGroup">
      <input type="text" formControlName="name" (keyup)="onKeyUp()" (keydown)="onKeyDown()" [ngClass]="{ 'invalid': formGroup.controls.name.invalid }">
    </form>
  `,
  styles: [
    '.invalid { border-color: red; color: red; }'
  ]
})
export class FormComponent implements OnInit {

  formGroup: FormGroup;
  subject: Subject<any> = new Subject();

  constructor(private formBuilder: FormBuilder) {}

  ngOnInit(): void {
    this.formGroup = this.formBuilder.group({
      name: [ '' ]
    });

    // Subscribe to the subject, which is triggered with each keyup
    // When the debounce time has passed, we add a validator and update the form control to check validity
    this.subject
      .pipe(debounceTime(500))
      .subscribe(() => {
          this.formGroup.controls.name.setValidators([ Validators.minLength(5) ]);
          this.formGroup.controls.name.updateValueAndValidity();
        }
      );
  }

  onKeyUp(): void {
    this.subject.next();
  }

  onKeyDown(): void {
    // When the user starts to type, remove the validator
    this.formGroup.controls.name.clearValidators();
  }

}

And StackBlitz here: https://stackblitz.com/edit/debounce-validator