From: cremes.devlist@...
Date: 2006-06-06T00:14:25+09:00
Subject: Need help with TDD, stubs and mocks

I'm writing a script to process a directory structure and some files  
within it. The process depth-first searches the directory tree from  
some root and looks for a Manifest file. When it finds it, it  
processes the file (XML plist, containing keys for filenames and  
paths) and records some information about the files referenced in the  
manifest. This recorded information then gets posted to a web service  
via SOAP.

Actually, I already wrote this using my usual top-down style. First,  
I wrote the code to depth-first search the directories. This included  
code to test all entries in a directory (using #file? and  
#directory?). Next I wrote the code to detect the manifest file. Then  
I used the plist gem to read it into memory (great gem). Then I wrote  
the code to do my magic on the files referenced in the plist. Etc.

I just iterated through my requirements until I had a finished product.

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.

So now I'd like to rewrite it using TDD.

I need some advice from the TDD experts out there.

1. Is it fair to say that I should rewrite bottom-up? That is, start  
with methods that are independent of others. For example, assume no  
subdirs and just get a list of entries from the current directory.  
Then write the method to iterate those entries. Then write one to  
test if the entry is a file or a directory. Etc.

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)?

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

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.

Instead of treating these as global to the class instance, should I  
be passing them as method parameters?


I appreciate any and all insight. I'm hoping to continually improve  
as a programmer so I can *answer* these questions instead of always  
asking them. :-)

cr