Working with multiple values

This example shows how to set and display multiple values for a computed field.

to populate a multi-valued field, a single value must still be returned, but your function will be called multiple times for each of your values (or "deltas"). 
So it's necessary to populate the full delta-keyed array, but then only return the value requested (for each delta).

function computed_field_FIELD_NAME_compute($entity_type_manager, $entity, $fields, $delta) {
  // Fill in array values with data.  The keys are optional in this case as they are the defaults.
  $values = [
    0 => 'Dog',
    1 => 'Cat',
    2 => 'Mouse',
  ];

  // Return the single value currently requested by the system.
  return $values[$delta];
}

getting the value/values

when having single value field this line will return the single value:

$val = $node->your-field-item->getValue();

but if you having a multi-value field, the same line will return an array:

To get all values of a multi-value field:

$node = $variables['node'];
$values = array();
foreach ($node->field_name->getValue() as $value) {
  if (!empty($value['value'])) {
    $values[] = $value['value'];
  }
}

or

$node = $variables['node'];
$values = array();
foreach ($node->field_name as $item) {
  if (!empty($item->value)) {
    $values[] = $item->value;
  }
}

setting the multi value field:

$node->field_name[LANGUAGE_NONE][] = new_value;

More shortcuts and ways to update multi-value field programmatically

$entity->field_name_muti->value = ['foo', 'bar', 'baz']; 
$entity->field_name_multi->target_id = [1, 2, 3]
$entity->field_name_multi->target_id = [$another_entity1, $another_entity2, $another_entity3]

this will also work:

$entity->field_name_muti = ['foo', 'bar', 'baz']; 
$entity->field_name_multi = [1, 2, 3]
$entity->field_name_multi = [$another_entity1, $another_entity2, $another_entity3]