Quiz: Checked Attributes (Step 1)

Назад: Kernel#eval

Дальше: Quiz: Checked Attributes (Step 2)

Where you take your first step toward solving the boss’ challenge, with Bill looking over your shoulder.

You and Bill look back at the first two steps of your development plan:

Write a Kernel Method () named add_checked_attribute using eval to add a super-simple validated attribute to a class.
Refactor add_checked_attribute to remove eval.

Focus on the first step. The add_checked_attribute method should generate a reader method and a writer method, pretty much like attr_accessor does. However, add_checked_attribute should differ from attr_accessor in three ways. First, while attr_accessor is a Class Macro (), add_checked_attribute is supposed to be a simple Kernel Method (). Second, attr_accessor is written in C, while add_checked_attribute should use plain Ruby (and a String of Code ()). Finally, add_checked_attribute should add one basic example of validation to the attribute: the attribute will raise a runtime exception if you assign it either nil or false. (You’ll deal with flexible validation down the road.)

These requirements are expressed more clearly in a test suite:


	require 'test/unit'

	class Person; end

	class TestCheckedAttribute < Test::Unit::TestCase
	def setup
	add_checked_attribute(Person, :age)
	@bob = Person.new
	end

	def test_accepts_valid_values
	@bob.age = 20
	assert_equal 20, @bob.age
	end

	def test_refuses_nil_values
	assert_raises RuntimeError, 'Invalid attribute' do
	@bob.age = nil
	end
	end

	def test_refuses_false_values
	assert_raises RuntimeError, 'Invalid attribute' do
	@bob.age = false
	end
	end
	end

	# Here is the method that you should implement.
	def add_checked_attribute(klass, attribute)
	# ...
	end

(The reference to the class in add_checked_attribute is called klass because class is a reserved word in Ruby.)

Can you implement add_checked_attribute and pass the test?

Before You Solve This Quiz…

You need to generate an attribute like attr_accessor does. You might appreciate a short review of attr_accessor, which we talked about first in . When you tell attr_accessor that you want an attribute named, say, :my_attr, it generates two Mimic Methods () like the following:

	def my_attr
	@my_attr
	end

	def my_attr=(value)
	@my_attr = value
	end

Quiz Solution

Here’s a solution:

	def add_checked_attribute(klass, attribute)
*	eval "
*	class #{klass}
*	def #{attribute}=(value)
*	raise 'Invalid attribute' unless value
*	@#{attribute} = value
*	end
*
*	def #{attribute}()
*	@#{attribute}
*	end
*	end
*	"
	end

Here’s the String of Code () that gets evaluated when you call add_checked_attribute(String, :my_attr):

	class String
	def my_attr=(value)
	raise 'Invalid attribute' unless value
	@my_attr = value
	end

	def my_attr()
	@my_attr
	end
	end

The String class is treated as an Open Class (), and it gets two new methods. These methods are almost identical to those that would be generated by attr_accessor, with an additional check that raises an exception if you call my_attr= with either nil or false.

“That was a good start,” Bill says. “But remember our plan. We only used eval to pass the unit tests quickly; we don’t want to stick with eval for the real solution. This takes us to step 2.”

Назад: Kernel#eval

Дальше: Quiz: Checked Attributes (Step 2)