Reading State
A single value.
$val = \Drupal::state()->get('key');
Multiple key/value pairs.
$pairs = \Drupal::state()->getMultiple($keys);
Writing State
A single value.
\Drupal::state()->set('key','value');
Multiple values.
\Drupal::state()->setMultiple($keyvalues);
Removing State
Delete a value
\Drupal::state()->delete('key');