angular 5 template forms detect change of form validity status

If you want to get only the status and not the value you can use statusChanges:

export class Component {

    @ViewChild('myForm') myForm;

    this.myForm.statusChanges.subscribe(
        result => console.log(result)
    );
}

If you even want data changes, you can subscribe to the valueChanges of the form and check the status of the form using this.myForm.status:

export class Component {

    @ViewChild('myForm') myForm;

    this.myForm.valueChanges.subscribe(
        result => console.log(this.myForm.status)
    );
}

Possible values of status are: VALID, INVALID, PENDING, or DISABLED.

Here is the reference for the same