example-1.ts

const nameControl = new FormControl('');

nameControl.patchValue('John');

control.value; // => 'John'
control.errors; // => null
control.touched; // => false

control.markTouched(true);

control.touched; // => true

const validator: ValidatorFn = control => {
  if (control.value.length < 2) {
    return {
      tooShort: 'Must be at least 2 characters',
    };
  }

  return null;
};

control.setValidators(validator);

// get the current value as well as changes
control.observe('value').subscribe(value => {
  // do stuff
});

// just get changes
control.observeChanges('errors').subscribe(errors => {
  // do stuff
});