Create conditionals in fields.

Maybe #1221892: Context module integration could help?

A common use case will be something along the lines of a controlling checkbox that hides/reveals a set of child fields.
Is that within the scope or would it be better to use a dedicated module like conditional fields?

I've only ever used it previously in a D6 site (without DS) and have no idea how well it plays with DS.
If it works with DS then that'd be great. Ensuring it does could probably be all that is required.

I'm not sure if it does that kind of switch case behaviour that your described in #3 though.

sub

FYI, a recent commit to conditional fields added: "Enhance effects system and add two new States: Filled with value / Emptied."

Which sounds like the type of condition you're after.

I know the issue is closed as an duplicate but a temporary solution for solving this is writing a custom field formatter (which is a solution for 99% of my problems, thanks for the tip swentel).

This is a crapy code feel free to make it better ;) The magic happens in kg_formatter_field_formatter_view()

function kg_formatter_field_formatter_info() {
  return array(
    // the key must be unique, so it's best to prefix with your module's name.
    'kg_formatter' => array(
      // the label is is what is displayed in the select box in the UI.
      'label' => t('kg formatter'),   
      // field types is the important bit!! List the field types your formatter is for.
      'field types' => array('text'), 
    ),
  );
}

/**
 * Implements hook_field_formatter_settings_summary().
 */
function kg_formatter_field_formatter_settings_summary($field, $instance, $view_mode) {
  $display = $instance['display'][$view_mode];
  $summary = t('simple kg formatter'); // we use t() for translation and placeholders to guard against attacks
  return $summary;
}

/**
 * Implements hook_field_formatter_view().
 */
function kg_formatter_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {

 // here happens the magic: return the element when specs has a value! The whole entity object is available.
  if (!empty($items[0]['value']) && $entity->field_specs['und'][0]['value'] != 0 ){
    $kg = $items[0]['value']." kg"; 
    $element[0]['#markup']=$kg;     
    return $element;
  }
}