when_constant

gistfile1.rb

# usage:
# when_constant "Foo::BAR", 'baz' do
#  # do something
# end

def when_constant(constant_string, new_value)
  class_or_module = nil
  constant_name = nil
  
  if constant_string =~ /::/
    values = constant_string.split("::")
    class_or_module = values[0..-2].join("::").constantize
    constant_name = values[-1]
  else
    class_or_module = Object
    constant_name = constant_string
  end
  
  old_value = class_or_module.const_get constant_name
  class_or_module.send(:remove_const, constant_name)
  class_or_module.const_set constant_name, new_value
  yield
  class_or_module.send(:remove_const, constant_name)
  class_or_module.const_set constant_name, new_value
end