Quiz: Checked Attributes (Step 3)

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Where you sprinkle some flexibility over today’s project.

To solve the boss’ challenge, you and Bill still need to implement a few important features. One of these features is described in the third step of your development plan: “validate attributes through a block.” Right now, your generated attribute raises an exception if you assign it nil or false. But it’s supposed to support flexible validation through a block.

Because this step changes the interface of add_checked_attribute, it also calls for an update of the test suite. Bill replaces the two test cases that checked for nil or false attributes with a single new test case:


	require 'test/unit'

	class Person; end

	class TestCheckedAttribute < Test::Unit::TestCase
	def setup
*	add_checked_attribute(Person, :age) {\|v\| v >= 18 }
	@bob = Person.new
	end

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

*	def test_refuses_invalid_values
*	assert_raises RuntimeError, 'Invalid attribute' do
*	@bob.age = 17
*	end
*	end
	end

*	def add_checked_attribute(klass, attribute, &validation)
	# ... (The code here doesn't pass the test. Modify it.)
	end

Can you modify add_checked_attribute so that it passes the new tests?

Quiz Solution

You can pass the tests and solve the quiz by changing a couple of lines in add_checked_attribute:

*	def add_checked_attribute(klass, attribute, &validation)
	klass.class_eval do
	define_method "#{attribute}=" do \|value\|
*	raise 'Invalid attribute' unless validation.call(value)
	instance_variable_set("@#{attribute}", value)
	end

	define_method attribute do
	instance_variable_get "@#{attribute}"
	end
	end
	end

“Step 3 was quick,” Bill notes. “Let’s move on to step 4.”

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