Why are MVC & TDD not employed more in game architecture?

问：

I will preface this by saying I haven't looked a huge amount of game source, nor built much in the way of games.

But coming from trying to employ 'enterprise' coding practices in web apps, looking at game source code seriously hurts my head: "What is this view logic doing in with business logic? this needs refactoring... so does this, refactor, refactorrr"

This worries me as I'm about to start a game project, and I'm not sure whether trying to mvc/tdd the dev process is going to hinder us or help us, as I don't see many game examples that use this or much push for better architectural practices it in the community.

The following is an extract from a great article on prototyping games, though to me it seemed exactly the attitude many game devs seem to use when writing production game code:

Mistake #4: Building a system, not a game

...if you ever find yourself working on something that isn’t directly moving your forward, stop right there. As programmers, we have a tendency to try to generalize our code, and make it elegant and be able to handle every situation. We find that an itch terribly hard not scratch, but we need to learn how. It took me many years to realize that it’s not about the code, it’s about the game you ship in the end.

Don’t write an elegant game component system, skip the editor completely and hardwire the state in code, avoid the data-driven, self-parsing, XML craziness, and just code the damned thing.

... Just get stuff on the screen as quickly as you can.

And don’t ever, ever, use the argument “if we take some extra time and do this the right way, we can reuse it in the game”. EVER.

is it because games are (mostly) visually oriented so it makes sense that the code will be weighted heavily in the view, thus any benefits from moving stuff out to models/controllers, is fairly minimal, so why bother?

I've heard the argument that MVC introduces a performance overhead, but this seems to me to be a premature optimisation, and that there'd more important performance issues to tackle before you worry about MVC overheads (eg render pipeline, AI algorithms, datastructure traversal, etc).

Same thing regarding TDD. It's not often I see games employing test cases, but perhaps this is due to the design issues above (mixed view/business) and the fact that it's difficult to test visual components, or components that rely on probablistic results (eg operate within physics simulations).

Perhaps I'm just looking at the wrong source code, but why do we not see more of these 'enterprise' practices employed in game design? Are games really so different in their requirements, or is a people/culture issue (ie game devs come from a different background and thus have different coding habits)?

回答1:

Why are MVC & TDD not employed more in game architecture?

Warning: opinion ahead.

MVC isn't used (much) because:

1. MVC is a relatively new approach and games are typically built on old code. Most people making MVC apps are building them from nothing each time. (I know MVC itself is actually decades-old; what is new is the widespread interest in it, which essentially came from web apps like RoR). In the business software I worked on that was based on legacy code, there was no use for MVC there either. It is a culture thing, but it's not game-specific.
2. MVC is essentially a GUI pattern for event-driven programs. Games are neither GUI-dominated nor event-driven, hence the applicability is not as great as it is for web apps or other form-based applications. MVC is typically the Observer pattern with some well-known roles, and games often don't have the luxury of waiting to observe something interesting - they have to update everything every frame (or some variation on that) whether or not anybody has clicked anything.

That's not to say games can't learn a lot from MVC. I always try and separate the model from the view in games I make. The traditional idea of the view and controller being 2 parts of the same (widget) object doesn't really work though, so you have to be a bit more abstract about it. I agree with you that performance is unlikely to be a real problem here. If anything performance can benefit from keeping visual stuff and logic stuff separate as that is more amenable to cache coherency, parallelism, etc.

TDD isn't used (much) because:

1. It's really awkward to use in games. Again, it's fine for event-driven software where inputs come in and outputs come out and you can compare what you saw against what you expected and tick it off as correct. For simulations with large amounts of shared state it's significantly more awkward.
2. There's no indisputable evidence that it helps anything at all. Just a lot of claims, and some figures often based on self-reporting and appeals to authority. Perhaps it helps poor programmers become mediocre but holds back good programmers. Perhaps the time spent writing tests could be better spent learning SOLID principles, or designing the correct interface in the first place. Perhaps it gives a false sense of security to have tests that pass without any real proof that you have enough tests to ensure your object does the right thing.

Again, as a caveat, I think unit tests are great, but I don't think "test first" is either beneficial or sensible. I also don't think it's practical to test the main simulation when there is so much shared state changing many times per second. I limit my tests to my low level and library code, generally.

However, as you can probably guess, I am largely biased against "'enterprise' coding practices" as enterprises are well-known for overrunning timescales and budgets. "So do games developers!" I hear you say - well, yes. I don't think anybody really knows the best way to develop software right now and I'm not convinced that adopting regimented practices in mainstream software is at all a step up from the freer practices in entertainment software. In fact I suspect it's primarily of benefit to those who rely on commodity Java programmers where these approaches are not a ladder to climb to greater heights but a safety net to stop them falling too low.

回答2：

Here's my original answer to a similar question on SO from a while back, at least concerning the MVC part of your question:

It's rarely used in games. It took me a while to figure out why, but here's my thoughts:

MVC exists to make a distinction between two representations. The Model is the abstract representation of your data. It's how the machine views the state of your application. The View (and Controllers) represent a more concrete visible instantiation of that system in a way that's meaningful to humans.

In most business apps, these two worlds are pretty different. For example, a spreadsheet's model is simply a 2D grid of values. It doesn't have to think about how wide the cells are in pixels, where the scrollbars are, etc. At the same time, the spreadsheet view doesn't know how cell values are calculated or stored.

In a game, those two worlds are much closer to each other. The game world (model) is typically a set of entities positioned in some virtual space. The game view is also a set of entities positioned in some virtual space. Bounding volumes, animation, position, etc., all things you would consider part of the "view" are also directly used by the "model": animation can affect physics and AI, etc.

The end result is that the line between model and view in a game would be arbitrary and not helpful: you'd end up duplicating a lot of state between them.

Instead, games tend to decouple things along domain boundaries: AI, physics, audio, rendering, etc. will be kept as separate as possible.

回答3：

Because MVC doesn't fit in the architecture of a game. The dataflow for a game is entirely different than that of a enterprice application, because it's not as event driven and there is often a (very) tight millisecond budget in which to perform these operations. There are a lot of things that need to happen in 16.6 milliseconds so it's more beneficial to have a 'fixed' and rigid data pipeline that processes the data you need on screen in exactly that time.

Also, the separation is there; most of the time it's just not wired the same way as the MVC pattern. There is a rendering engine (View), user input handling (Controller) and the rest such as gameplay logic, ai, and physics (Model). Think about it; if you're fetching user input, running the ai and rendering the image 60 times per second, then why would you need an Observer between the model and the view to tell you what has changed? Don't interpret this as me saying that you don't need the Observer pattern in games, you do, but in this case you really don't. You're doing all that work, every frame, anyway.

Although TDD is hardly ever used in development studios, we do use Agile practices towards software development and SCRUM seems to be the popular choice at least here in Europe. The reason is simple, the changes come from everywhere. The artists might want more texture budget, larger worlds, more trees while the designers want a richer story (more content on disk), a streaming world and the publishers want you to finish on time, and on budget and so does the marketing department. And apart from that, the "state of the art" keeps changing rapidly.

That doesn't mean that we don't do testing either, most studios have large Q&A departments, run loads of regression tests and do unit tests on a regular basis. However, there's hardly any point doing unit tests upfront because a large part of the bugs are art/graphics related (holes in collision meshes, wrong textures whatever, glitch in the depth of field shader) that unit tests can't cover. And besides working code, the most important factor of any game is that it's fun, and there's no unit testing that.

Also remember that in the console world this is even different still, because you're programming more for the hardware then for anything else. This generally (PS2/PS3) means that the flow of the data is way more important then the flow of the code in nearly every case; due to performance considerations. Which nullifies the use of TDD as an architectural tool in most cases. Good OOP code generally has bad dataflow, making it hard to vectorize and parallelize.

回答4：

As the quote says, many programmers make the mistake of (trying to) build a system, not a game. Typically that system keeps ballooning out of control until it's so complex that theoretically it can handle anything, but in practicality all you have is a big bundle of code. Or more often, before you even get to a working stage, you are so tangled up in code that doesn't run that you lose focus (if you had any to begin with) and motivation (since nothing is truly working).

Prototypes and iteration tend to work much better. In the end, a great design might come out of it, but more often something more simple and refined comes out of it. KISS and YAGNI come to mind.

I personally believe there needs to be a balance. If there's a core mechanic of your game, work on it. You still need to iterate, but you do need to refine it. Hint: organization of your code is not a core mechanic of your game.

Case in point: Peggle, by PopCap Games. A core mechanic of the game is the ball physics. They perfected it! I'm sure they spent quite a lot of time making sure it was absolutely perfect, because it is what makes the game. But at the same time, I can totally picture an early prototype of their game that maybe just draws sprites to the screen and does some type of more primitive collision detection and bouncing, just to see if the game idea is fun. Then once they found out that shooting a ball and watching it bounce can actually be fun, they refined the bouncing of the ball. _{(this is all just speculation, of course)}

It also depends on your technical requirements, which you should nail down early on (not your game design, just the technical requirements). The platform that your game runs on should not change, or if it should be allowed to change, you need to know exactly the extent that you plan to allow it to change, no more and no less. Design on that. If you're developing a game for OpenGL, and you don't care about DirectX, then really, don't care about it. That means that if it's more convenient for you to have each entity draw itself and not worry about Factories and other design patterns like that, then do that. It's okay, because it meets the requirements. You should not have to change it later, despite what you tell yourself. And really, worst case scenario? Refactor later. It takes time later but it lets you focus on the now, getting a working game on one platform even if it means it can't simultaneously and automatically run on your toaster.

Test driven design, however, is a more opinionated topic. I am of the belief that game developers should do more of it. I also think game developers have some of the most rigorous, tight schedules, and they think they can't afford to spend time on TDD when they just want to get a game going. Also, again with the motivation, TDD is a lot slower and you get to see a lot less of a functioning game (in the beginning at least). This can have serious negative effects on programmer motivation.

I think it's also just a general lack of knowledge and practice. I don't think TDD is prevalent in other areas either, but like agile development, I think it's spreading. You might say it's ahead of its time (or maybe not, as the case may be years from now). More important than TDD is "RDD" - Requirements Driven Development. _{I just made that up.} What is your goal? To make a game. Everything else comes second. If you can prove that TDD increases productivity and helps teams reach deadlines, then don't you think everyone would be using it? And maybe that's the case. But right now, our industry is more competitive than ever, there are harder and sooner deadlines, and stuff just needs to work. Construction workers don't build scaffolding first; they lay a foundation, then they raise some walls and floors, and only then do they build selective bits of scaffolding to do specific tasks which scaffolding makes more convenient. I think the same applies to software development.

Sorry for such a long post. I hope you've gained some bits of wisdom from it. I am just a student, talking about my observations, with very limited industry experience but a lot of reading from industry experts. So take my words with a grain of salt.

And hey, do what you think will work. You can always change it or scrap it and start over. That's the cool thing about any form of engineering; if at first you don't succeed, try something different. (or something like that :-P ) You're not pouring concrete; software is malleable.

---

By the way, I've been asking this same type of question and researching these types of design principles for some time. Here are some questions, both here and in Stack Overflow, that you might find relevant:

• "How can you organize the code for a game to fit the MVC pattern?"
• "How to design a game's software such that it is easy to unit test?"
• "Component based game engine design"
• "Any good tutorials or resources for learning how to design a scalable and “component” based game 'framework'?"
• "Seperation of game- and rendering logic"
• "Is unit testing viable in game programming?"
• "Is the MVC design pattern used in commercial computer games"
• "Is test driven development a normal approach in game development?"
• "Do any of you use Scrum for game development?"
• "Architecture for Game - To couple or not to couple?"
• "OO game design question"