Call alert method when Validate form

I’m using reactiveform.
When the user clicks on the button to enter data, I would like to call a method if the data entered by him are invalid.

How can I do?

This is the form

    this.userForm ={
      email: ['', [Validators.required,]],
      psw: ['', [Validators.required, Validators.minLength(8), Validators.maxLength(16),]]

    <form [formGroup]="userForm" novalidate>
  <ion-item padding>
    <ion-label><ion-icon name="person"></ion-icon></ion-label>
    <ion-input type="email" placeholder="Email" formControlName="email"></ion-input>

  <ion-item padding>
      <ion-label><ion-icon name="key"></ion-icon></ion-label>
      <ion-input type="password" placeholder="Password" formControlName="psw"></ion-input>

    <button ion-button full (click)="login()" [disabled]="!userForm.valid">Login</button>