A code review of PewPew by Mike Chambers
A very rare event has occurred - the chance to look at, discuss and play with the full source for a Flash game! I've worked with the source to dozens of games by different developers, but I'm normally under a non-disclosure agreement so I cannot divulge the full horrors of what I found there. But Mike Chambers from Adobe has kindly released the full source to his game under MIT open source license, and I thought this could be a great spring-board to open up discussion on the architecture and best practice of making Flash games.
The source is here: http://github.com/mikechambers/pewpew/archives/master
but you also need to download is framework from here: http://github.com/mikechambers/Simple-Game-Framework/archives/master
Mike, hasn't made a game since Flash 4, so if you read this Mike, please take it in the scholarly spirit in which it is intended :)
and to everyone else, remember I am just one guy with some strong opinions, weakly held.
Ok here's my thoughts...
Repository - Mike has not included his framework in the game repository. I'd consider this a bad idea for 2 reasons.
- A developer coming to the project cold cannot compile it. I believe that the repo of a project should contain everything you need to get started.
- There is a strong chance that API changes could be made to the framework code that break the build of the game. You really want to use a stable version of a framework rather than having to fix errors every time an update is made to the framework. Then if you want to update to the latest version of the framework you grab the new version and fix all the problems at once. You don't want to come back to make a quick change to your game in 6 months, only to find that you can longer compile because you have changed the API of your framework.
- Mike has chosen to compile from the Flash IDE, which is fine with me and most design-driven agencies. Some developers won't like it, but who cares.
- Mike has unticked the "include hidden layers" option in the Flash publish settings tab. This is one of my absolute no-nos as it means you can break the build just by hiding and unhiding layers while you're looking around the timeline. If I hadn't encountered this lunacy before, I might have spent hours figuring out why it suddenly wasn't compiling. In the end I had to revert to the original version because I couldn't remember exactly what should and shouldn't be hidden. This is what guides are for, people.
- One mistake Mike's made that I see quite a lot is that he hasn't put his Main.as document class file in his package structure, it's just in the root of src with the fla. The Main class is just as much a part of the project as anything else, and should be in the package with it's friends.
- the game is a pretty basic asteroid-type shoot 'em up.
- the home-made visual and audio style is acceptable in the Flash game world, although Mike could have chosen a more hand-written looking font, rather than Helvetica. There are lots of free handwriting fonts out there, and there's always Comic Sans!
- the controls are optimised for a touchscreen mobile device, with a little virtual joystick in the bottom corner, so it doesn't control well on the desktop.
- the joystick doesn't allow you to set speed, only direction. A simulated analogue stick would have been nicer.
- There is some commented-out code for making the ship follow the mouse for browser play, but it is incomplete. Mike would have done better to have a boolean for isMouseControlled which could be switched when targeting the browser, rather than completely commenting out the code. I rarely delete or comment out working code, I'm much more likely to keep it in my class as a switched-off option.
- At 480x800 it's too tall for a browser game, as you have to consider users with a minimum 1024x768 display.
- The movement of the ship is not very graceful - it doesn't seem to accelerate or decelerate.
- The speed of the bullets is way too slow.
- The collision detection between the bullets and enemies is inconsistent, and bullets often sail under the wings of the UFOs without scoring a hit.
- There are no roll-overs or even finger cursors on any buttons - again this is probably an artefact of originally being a touch-screen game.
- There aren't any transitions between screens.
- The enemies bounce around the screen like asteroids, but they are UFOs, which doesn't make any sense, as you would expect UFOs to know you are there and either attack or make defensive manoeuvres.
- Mike's library is beautifully arranged into folders with all the items named properly, and no Symbol 1's lying around. I rarely organise my libraries into folders any more - I just rely on the invaluable CS4/5 library search box.
- Mike has linked library items directly to classes. This is what I often do as it is fast and makes it easy to understand what code relates to what graphics. It can have it's draw-backs though, for example if you want to runtime load assets from multiple swfs without having to republish them with every code change, and I'm coming round to the idea that it is often better to treat movieclips as "skins", with no code of there own, and have another class that controls them. It would be nice if at runtime you could tell Flash - take this movieclip and make it an instance of this class. That would be cool right, and totally feasible, Adobe engineers?
- All Mike's code is beautifully organised, with well-named full-word function and variable names like isOnStage, displayTitleGraphic and onGameOver. This is painfully rare to see, so I salute you Mike!
- Mike has commented to the point of obsession, which is great, but as his code is so well named and self documenting, most are unnecessary, such as:
//start the game
//added to enemies vector
- Throughout his code Mike has used // double slash comments on his public vars and functions, where he should have used /* slash star */ JavaDoc style comments. This would have allowed code editors like FlashDevelop to give the comment as a hint / tooltip. Really useful when working with an unfamiliar API.
- There are some slightly strange things with splitting single lines of code over multiple carriage returns, with orphans tabbed weirdly across the page. This is either a Mac/PC formatting issue, or a sign of madness.
- Mike wrote in his blog that he thought he had over-engineered the game and he's not wrong. I ran a little app called cloc (count lines of code) on the folder and it told me that not including white space and comments there are 2420 lines of ActionScript. That's quite a lot when you consider that I managed to get a similar game running in 25 lines of code. Not all the code in a framework needs to be used on every project though, so it's not as bad as it sounds! If you ignore the framework it's only 1644 :) I have a feeling cloc might be over reporting because of block comments, but I might be wrong. So where is all that code going?
- The GameArea class contains most the game logic, and is a fairly typical well organised game class. I has more white space and comments than it does code though, which I think makes it less readable. Overall this class is very sane though and doesn't include anything particularly wacky.
- SoundManager doesn't do much - it doesn't even play sounds! Instead it returns and instance of a sound which you can then call play() on. It also contains constants for each sound class name in his library. This might sound like a good idea, but it isn't. Either use the class reference directly or just put the string in your function call. Not every string you ever type has to be put in a constant.
- For entities there is an inheritance chain of MovieClip > GameObject > PewPewGameObject > Enemy > ChaserEnemy. I've used this kind of approach on many games, and it works up to a point, but it doesn't make your code very reusable, and you often have to hunt around different inheritance levels to find code. These days I use a very simple component system (nothing to do with native Flash components by the way) where functionality is broken down into modules which are owned by objects. So you would have a health component, a weapon component, a position/movement component, a sprite component etc. In Flash we lurve inheritance because it makes us feel like proper programmers, but composition is often a much better approach. This has taken me literally years to come to terms with, so I won't be surprised if lots of people disagree with me. Mike would also be in trouble if he wanted to adapt his game to use Away3D for rendering for example, as he has extended MovieClip. That said, you should always focus on the game you are making rather than worry about "what if" scenarios that will never happen.
- Mike is using standard Flash events in his game. Again, I've done this many times myself, but since AS3Signals comes out I just use that - but TurboSignals is supposed to be even faster and so probably more appropriate for games. If you want real speed don't use eventing at all - give each entity a reference to your game class and call methods directly on that.
- Mike has a reusable GameObjectPool class for object pooling, which is nice. However, when you ask it for e.g. a ChaserEnemy, it returns an instance of GameObject which must then be cast back to ChaserEnemy anyway, so he might as well of made an object pool that supports any class type, not just GameObjects.
- Mike also has the start of some nice util classes for useful Maths functions, but they're not particularly extensive.
Overall PewPew has the hallmarks of a disciplined programmer with good style. Unfortunately it also has many of the limitations that 90% of Flash games suffer, which is a lack of some basic features that you would have seen in games from 20 years ago:
- A health bar rather than instant death.
- A scrolling world.
- A solid world with walls, rooms, corridors etc, rather than just an open void.
- Basic physics like momentum
- Ability to pause the game
- Particle effects for trails and explosions
- Animated sprites
Good on Mike for releasing his code for public scrutiny - I worry that this is the extent of Adobe's knowledge of game development though. Their gamedev hub http://www.adobe.com/devnet/games/ seems to have been mostly contributed by 3rd parties, and it even includes some advice on such topics as making games with Cairngorm, which is never going to end well. Flash is badly in need of an official gaming API/framework on the scale of Flex. I don't see that happening any time soon, so until then we're all just going to keep re-solving solved problems and repeating the mistakes of the past.