Ruby: What is at_exit and how to write tests for it.

Ruby has a neat feature called at_exit which takes a block and then executes the contents of this block when the program ends. There are a couple of VERY important details:

<ol> <li>It takes a block and converts it into a Proc object at the time of parsing. This means that the data has to be available in the binding, or you'll run into errors. Example: instance variables need to be set before you can use them in that block. Better idea: don't use instance variables in there at all.</li> <li>You can 'chain' at_exit calls, and they will be resolved in a First In, Last Out (FILO) order.</li> </ol>

Once you know this, using at_exit and writing tests for it becomes a little easier:

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class Piddler
   def initialize
     create_pid_file
   end

   private

   def create_pid_file
     pid_file = '/tmp/piddler/my_pid' #Simplified for example purposes
     File.new(pid_file, 'w')
     at_exit { FileUtils.rm_f pid_file }
   end
end

What you'll notice is that the at_exit block is defined RIGHT AFTER I create what I will need to resolve/undo/finish - not separately, right inside the method.

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def test_clears_pid_file_when_it_exits
   at_exit { assert_equal 0, Dir['/tmp/piddler/*'].size}
   Piddler.new
end

The advantage of that is that I know exactly when it gets defined. For this example, it gets defined at the end of the 'initialize > create_pid_file' call. This means that any at_exit blocks defined BEFORE that will be resolved AFTER.

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