Asynchronous validation of a form in Angular

Here is an example of how you could add asynchronous validation to a form in Angular:

1. First, you’ll need to import the FormControl and FormGroup classes from the @angular/forms module, as well as any other modules that you'll need for your validation logic.

import { FormControl, FormGroup } from '@angular/forms';

2. Next, you’ll need to create a new FormControl for the input field that you want to add asynchronous validation to. You can do this by passing an initial value to the FormControl constructor.

this.email = new FormControl('', [Validators.email]);

3. Now you will add the async validation logic in a async validator,

emailChecker(control: FormControl) {
   return new Promise((resolve, reject) => {
     setTimeout(() => {
       if (control.value === 'existing@email.com') {
         resolve({ emailTaken: true });
       } else {
         resolve(null);
       }
     }, 2000);
   });
 }

4. Next you will add this async validator to form control and create a form group

this.email.setAsyncValidators(this.emailChecker);
this.form = new FormGroup({email: this.email});

5. Now, you can use this form in your template and use the built-in ng-invalid and ng-valid classes to apply styles to the input field based on its validation status

<form [formGroup]="form">
  <input type="text" formControlName="email">
  <div *ngIf="email.invalid && email.errors.emailTaken">
    Email already taken
  </div>
</form>

6. Finally you can use the form.statusChanges, form.valueChanges or form.status to listen to the changes and to check the validation status.

this.form.statusChanges.subscribe(status => {
      console.log(status)
    });

This is just an example, and you’ll likely need to adapt it to suit the specific needs of your application. Additionally this is a very basic example, you can use more complex ways to check for the errors using services and APIs.