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@@ -15,6 +15,33 @@ module ThoughtBot # :nodoc:
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           end
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           fixtures *all_fixtures
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         end
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+
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+        # Macro that creates a test asserting the numeric difference between the
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+        # return value of an expression that is run before and after the current
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+        # setup block is run.  This is similar to Active Support's
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+        # <tt>assert_difference</tt> assertion.
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+        #
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+        # Example:
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+        #
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+        #   context "Creating a post"
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+        #     setup do 
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+        #       Post.create
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+        #     end
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+        #
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+        #     should_differ "Post.count", :by => 1
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+        #   end
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+        #
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+        # While not shown in this example, the <tt>:by</tt> option is optional
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+        # and defaults to <tt>1</tt>.
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+        def should_differ(expression, options = {})
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+          difference = options[:by] || 1
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+          expression_evaluation = lambda { eval(expression) }
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+          before = lambda { @_should_differ_value = expression_evaluation.bind(self).call }
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+          should "differ '#{expression}' by #{difference}", :before => before do
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+            assert_equal @_should_differ_value + difference, expression_evaluation.bind(self).call
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+          end
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+        end
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+        
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       end
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       # Prints a message to stdout, tagged with the name of the calling method.

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@@ -76,5 +76,22 @@ class HelpersTest < Test::Unit::TestCase # :nodoc:
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         assert_same_elements([@a, "abc"].flatten, [3, 3, "def", "abc"])
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       end
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     end
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+    
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+    context "after adding another value" do
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+      setup do
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+        @a.push("another")
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+      end
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+      
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+      # testing the default :by option of 1
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+      should_differ "@a.length" 
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+    end
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+    
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+    context "after adding 3 more values" do
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+      setup do
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+        @a.push(1, 2, 3)
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+      end
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+      
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+      should_differ "@a.length", :by => 3
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+    end
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   end
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 end