Test if a value exists in $form_state

Question

If I wanted to rebuild a form in Drupal 7, conditional on the state of the form, I would do something like this:

if (isset($form_state['values']['foo'])) {
  $form['...
}

Now, $form_state is an object in Drupal 8. What would be the best equivalent?

Answer 1

Pretty much $form_state->getValue('foo');.

See the docs page for getValue.

Answer 2

I belive that the equivalent to isset() is

if ($form_state->hasValue('foo')) {
  ...
}