Friday, 24 October 2014

Prototype-Driven Design

This is designed to be a normative approach based off the three [1, 2, 3] development approaches I wrote about earlier. Namely, how to take the use cases of a simple game to develop and refactor an extensible and versatile underlying code. This is a note to myself for next time.

Bootstrapping the framework
A game is a loop, and the loop has an exit condition. Hitting the exit condition exits the game. To be interesting, while the game is looping, the game has to do stuff. All that should be met with “duh”. What this means is that we have to organise an order of things to do in the loop. You need to create a window, load images, display, capture input, change game code, etc. Organising this basic pattern needs to come first..

The first thing is to get the libraries loading successfully. This may take more time than the task sounds like it would, and doing something slightly unusual (for example, choosing to develop on Windows using MinGW) might not have a single point of clear instructions. Don’t fret if this takes longer, it’s important to get this right.

There are a number of discrete systems that in concert make up a game. Many of these are “core”, and will be used every time in very nearly the same way. As such, each of these systems can be given a basic function (such as render updating the screen each frame), and each function placed as part of a list of functions that are always called. In mine, I called this the Engine fa├žade, where the update() loop took care of everything the engine needed to do with a single call from the game loop.

At this stage, it’s okay to use scenario-based development. “A screen should appear on start-up”, “a button press should quit the game”, etc. Indeed, there’s little more one can do at this stage. The important point is not to overdo this, however, because you a) cannot account for every scenario, and b) what you think will be right scenarios may not be the case. Overdo it at this stage, and prepare to refactor.

The prototype approach
Take a simple game like Snake. It’s a fairly simple game so it won’t take much to work out how to translate what happens on screen into code. It’s also a fairly-well defined game, so there’s not a lot of effort in setting the win / loss conditions and how that translates to a loop. It also tests out what you need in a game – to capture input, to update the game world accordingly, to trigger off sounds, play music, and to update the screen.

The general approach goes like this.
  1. Try to build the game artefact to do what you want it to do.
  2. If the engine can’t support the functionality, add the functionality.
  3. If the engine only partially supports the functionality, enhance what’s there.
  4. If the engine encourages a lot of writing the same code multiple times, refactor.

Such a system will also feed back into itself, where later development will override earlier development. The important point is not to get hung up on a decision made earlier if a better decision comes along, after a short while you should have a working game and hence a platform to immediately test out how well the system works.

Like the bootstrap framework, this might take some time to set up. There’s a lot to do going from nothing at all to even a simple working game (without taking every possible shortcut). But what it allows for is very quick refactoring and enhancement. Once you have a fully functioning product, it should be a matter of hours or even minutes to enhance it for the better.

The development phase is similar to Test-Driven Design, though in this case a piece of game functionality is the unit test. You write game code to do something, but it won’t work until you then write the engine enhancement that will make it work.

The allure of design
The prototype approach requires a continual awareness of the allure of design. When making a component, it’s easy to come up with potential uses for that component – all of which takes time to design, develop, and debug. Furthermore, enhancing the system in such a way doesn't mean the work will actually achieve anything. The prototype-driven design in this sense is a way of keeping the developer honest and focused, and not wasting time on the mere potentiality of a function.

This is not a call, however, for the abandonment of design. It’s a call to prevent overdesign, to limit areas where splurges in design can lose sight of the bigger picture. The idea of a chaotic mess of code is what you end up with even with the best of intentions, so there’s no need to add to it if it can be avoided. What I am talking about here is a different kind of chaotic mess – a bloat of code of which only some applies to the end result.

The allure of design is part of the creativity that enables development in the first place, and to some extent it’s necessary for consideration for any potential solution. Otherwise we hit the other extreme – a constant rewriting of the same basic code whenever a new requirement exists. It’s a balancing act, yes, and how the prototype-driven design approach works should be to limit the potential for overdesign.

I suppose the difference I'm trying to articulate is the difference between extensible and bulletproof.

Scope-creep is a good indication that you are moving away from the PD-D and being drawn back into the allure of design. If a problem is getting unwieldy, it’s worth reminding yourself of what the component was meant to achieve in the first place. If it wasn't something with a game-related end product, then that should be a strong indication of overdesign and grounds to stop.

A guideline
The approach sketch above isn't much of a methodology, but a guideline to stay focused. It’s a way to catch that intuitive aesthetic about how code ought to be done and give it a focal point. Furthermore, it’s a guideline that as far as I can tell applies to one person: me. Or to put it another way, this is a sketch of an approach that keeps my distracted-self focused on why I'm doing this in the first place. The desire to “get it right” is coming at the expense of “getting it done”, and this approach seems promising as a way of somewhat alleviating that.

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