class ViewModel::Migration::Builder

def build!
  migration = Class.new(@superclass)
  migration.define_method(:up, &@up_block) if @up_block
  migration.define_method(:down, &@down_block) if @down_block
  migration
end

def check_signature!(block)
  unless block.arity == 2
    raise RuntimeError.new('Illegal signature for migration method, must be (view, references)')
  end
end

def down(&block)
  check_signature!(block)
  @down_block = block
end

def initialize(superclass = ViewModel::Migration)
  @superclass = superclass
  @up_block = nil
  @down_block = nil
end

def up(&block)
  check_signature!(block)
  @up_block = block
end