class ViewModel::GarbageCollection

def collect_references(tree, &block)
  return enum_for(__method__, tree) unless block_given?
  case tree
  when Hash
    if tree.size == 1 && (ref = tree[ViewModel::REFERENCE_ATTRIBUTE])
      block.(ref)
    else
      tree.each_value { |t| collect_references(t, &block) }
    end
  when Array
    tree.each { |t| collect_references(t, &block) }
  end
end