From: Jim Weirich <jim@...>
Date: 2006-06-06T00:57:11+09:00
Subject: Re: Need help with TDD, stubs and mocks

cr wrote:
> When I finished I decided to write tests to verify each method.
> However, the main entry point into the script starts off by doing the
> directory traversal which then feeds into every other step. I can't
> call the method without running all of its dependent methods (which
> essentially runs the whole program). This strikes me as my first clue
> that I probably did this wrong.

Code not done test first will often be hard to test.  I think we just 
discovered that.

> 1. Is it fair to say that I should rewrite bottom-up? That is, start
> with methods that are independent of others. 

When I do TDD, I start with very simple cases and work toward more 
general cases.  Both bottom-up and top-down work, I tend to do 
outside-in (i.e. start at both the top and bottom and work toward the 
middle).

> 2. Do you generally keep all methods in a class independent of each
> other? How do you write, and then test, methods that have
> dependencies on each other (and therefore cause side effects)?

Dependencies?  Like "You Must Call Method A before Method B"?  I try to 
avoid those kinds of dependencies when possible.  Sometimes the 
semantics of the object naturally introduce them (e.g. you usually have 
to push something in order to pop something from a stack).


> 3. How does one mock or stub out a SOAP call? What's the difference
> between a stub and a mock?

Is there an object that you use to perform the soap call?   If so, just 
replace the object with a Stub or Mock.

Stub VS Mock:  (Terminalogy varies, but the following is a common 
distinction) Mocks check their calls for correctness, stubs don't. 
Stubs are used to provide external behavior that might not exist in a 
test environment.  Mocks are used to verify that you are actually 
calling particular methods with paricular arguments.

Note that using this terminology allows a single object to mock some 
methods and stub others.  I do this alot.  If you divide methods into 
two camps: those that return values (queries) and those that do things 
(commands), I usually stub queries and mock commands.

> 4. How do you test a method that utilizes an instance variable that
> was set somewhere else? E.g. #methodA calculates @result. #methodB
> retrieves @second_result. #methodC compares @result and @second_result.

Construct your test case so that methodA and methodB are called before 
methodC does its work.

Example:

   def test_comparison
     thing = Thing.new
     thing.methodA
     thing.methodB
     assert_equal ExpectedResult, thing.methodC
   end


-- Jim Weirich

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