For ease of reference the summary of the kata is repeated here;
A roulette wheel has numbers from 0 to 36. A spin of the wheel takes 20 seconds and leaves the ball on a random number.
Design and implement a roulette wheel using TDD. Think about how using time and random numbers affects your design. Consider how much functionality each test covers and what responsibilities you are testing.
I wanted to drive this in a way where I did not need to mock out clocks or random number generators. To do this I thought that a pattern I have seen used extensively before would be suitable; where the generation of tick events was the responsibility of one class, whilst another had the job of responding to those events. Secondly I planned to going with the model of “Object’s send messages to share data” so I knew straight away that I would be looking at a tell don’t ask approach.
The first notes I made on the kata were:
This is event based so avoid “World” stuff in test/codelets and to try for public methods are have void return types.
That about constituted the amount of “up front design I did before hitting the keyboard.
My very first failing, as opposed to not compiling, test looked like this;
Note that at this point I am not taking any interest in time at all. Only that the observer is notified that the wheel has stopped after the spin.
I questioned where to go next and in writing my summary from the kata on cyber-dojo I think I went the harder route to start with. I took on verifying random number outputs, this took several goes round the red/green/refactor cycle and the final test I ended up with was a bit of a behemoth;
To push the design forward my next test looked like this;
This as you can see led to some changes in the interface. Which I didn’t jump straight to but my RouletteWheel class was starting to take shape;
There were quite a few more evolutions of this but the next interesting bit of work to talk about on a TickProvider class, something to generate those tick events
Again we are getting the basic communication right without worrying about timing.
to get to that we eventually ended up with;
Finally we got to the point where we needed to wire up an assembly of the RouletteWheel and the TickProvider. I did this by testing in TTDAIYMI (TDD as if you meant it) style writing the code directly in the test before extracting to a static method on RouletteWheel
The TDD approach has led my final code I think to show good separation of concerns between the generator of tick events (TickProvider), the consumer of those events (RouletteWheel) and the consumer of the stopped event that gives the final location of the ball (WheelObserver). This last one I particularly like as it totally separates presentation from the model, its also easy to envisage how the code could be adapted to publish intermediate events whilst the wheel was “spinning”.
No doubt someone will point out some horrendous threading (or other) bug, please note this is a Kata or practice and I didn’t pair on it. But if you do spot one can I suggest you respond by proposing a test that would expose it? Remember TDD is not about passing when software works, its about improving your design and as a bonus you get some tests that should fail when it doesn’t behave as you intended. I hope I showed that here.
For the full gist including test classes see below;
You can also see my working on the excellent cyber-dojo here I hope no-one will make further changes at the time of writing my work goes up to revison 106. Note that revisions after that may not be mine.
You’ll see (as is my typical experience of TDD and coding in general) that progress is not quite as linear as some explanations will have led you to believe, including here 😉
The total time I spent on this was around the 3 hour mark so higher end of allotted time (I took a break in the middle to teach my nephew some TDD 🙂
I enjoyed the Kata, thanks to @jjefries1 for suggesting it. I tackles some of the more challenging things in TDD but I think it also shows that if TDD is followed it does give you that nudge to a better final design.