Dynamically populating Advanced Custom Fields values

Populating fields dynamically in ACF is simple using the acf/load_field hook. An example:

//Populate the Select field field_5587fccf24f38 with dynamic values
add_filter('acf/load_field/key=field_5587fccf24f38', function($field) {
  //These can be dynamically generated using any PHP code
  $field['choices']['one'] = 'Choice one';
  $field['choices']['two'] = 'Choice two';

  return $field;
});

But if you also use ACF Local JSON, you’ll notice that your dynamic field values get exported whenever you re-resave one of your field groups. Not ideal.

Using dynamic values with ACF Local JSON

We need a way to remove the dynamic values just before they are exported to Local JSON. Luckily, there’s an undocumented filter called acf/prepare_field_for_export that will let us do this! Example:

//Don't export dynamic values via Local JSON
add_filter('acf/prepare_field_for_export', function($field) {

  //If we're at the correct field
  if(isset($field['key']) && $field['key'] === 'field_5587fccf24f38') {

    //Blank out the select options with an empty array
    $field['choices'] = array();
  }

  return $field;
});

Updated 2016-01-15
There was another filter called acf/prepare_fields_for_export (note fields instead of field). This filter was removed in ACF 5.3.3.