# This file provides a few new assertions and class methods for expressing
# contracts in a comfortable way that does not require manually writing test
# methods.
#
# See Contract.provides.


class Contract < Test::Unit::TestCase
  # Tests that the tested Object provides the specified methods with the 
  # specified behavior.
  #
  # If a block is supplied it will be evaluated in the context of the
  # contract so <code>@object</code> will refer to the object being tested.
  #
  # This can be used like this:
  #   class ListContract < Contract
  #     provides :size do
  #       assert(@object.size >= 0, "#size should never be negative.")
  #     end
  #
  #     provides :include? 
  #
  #     provides :each do
  #       count = 0
  #       @object.each do |item|
  #         assert(@object.include?(item),
  #           "#each should only yield items that the list includes.")
  #         count += 1
  #       end
  #       assert_equal(@object.size, count,
  #         "#each should yield #size items.")
  #     end
  #   end
  def self.provides(*symbols, &block) # :yields:
    symbols.each do |symbol|
      define_method("test_provides_#{symbol}".intern) do
        assert_respond_to(@object, symbol)
        instance_eval(&block) if block
      end
    end
  end
end