The State Of The Game
There are some great Flash and Flex application frameworks out there right now. Mate, Swiz and PureMVC (update: and Parsley!) stand out. The authors of these frameworks realized that the Flash Platform is different enough to the JVM to warrant a fresh approach to application design.
Someone mentioned recently that for such a young language (referring to Action Script 3), it’s quite surprising how many frameworks have sprung up around it. I’m not surprised at all: rich internet application development is complicated.. and it’s a pretty recent field in the grand scheme of things. It brings new (and often subtle) twists to traditional web and desktop software development. Writing elegant, maintainable code is hard enough as it is, but doing so on a rapidly evolving, heavily interactive platform.. well, people are still working on the best way to approach that (unless I missed the big news – in which case you can stop reading right here).
Hence all the groovy new architectural frameworks popping up. They share some common goals:
- Help developers write good, clean, understandable, testable, maintainable code
- Help them write that code faster
- Provide an easy-to-use mechanism for application/context/module subsystem inter-communication
- Provide an easy-to-use mechanism for wiring distinct parts of the application together whilst keeping those parts loosely coupled
And they succeed. To various degrees. I think they’re all pretty cool.
A Poke In The Back With A Sharp Stick
So why build another one?
- For fun – it’s how I learn stuff
- Because I find the other frameworks critically flawed
What kind of “critical” flaws?
- Singletons (big “S”)
- Central/Static Event Dispatchers
- Using the Display List as an Application Event Bus
- The Service/Model Locator Pattern
- The Presenter Pattern
- Injecting into View Components
And what’s wrong with those things?
They are bad ideas:
1. The Singleton – Bad Idea
Singletons bring bags of hurt to frameworks and applications.. suddenly, trying to build a modular application becomes an overly complicated affair, and unit testing becomes a nightmare. Singletons are viruses – they infect every class they touch.
And Multitons? If Singletons are evil, Multitons are diabolically evil. What makes the Multiton pattern incredible is that it manages to chump the Singleton as the worst Design Pattern Of All Time – no easy feat, and certainly the work of an evil genius with a wicked sense of humour. Not only do we have all the problems of the classic Singleton, we throw a poorly designed Model Locator into the mix – to pull out these horrendous Singletons we now need to use String keys.
Contextual singletons (as made possible by Dependency Injection frameworks) are a much nicer solution, they don’t require any extra (fluffy, nonsense, ill placed) code on the target classes themselves, and they allow identical applications/modules to exist side-by-side in the Virtual Machine without fighting and mass confusion. Which leads me to..
2. Central/Static Event Dispatchers – Bad Idea
As convenient as it is to use, a Central Dispatcher is a really bad idea. Another Singleton effectively. Pop two apps into the same SWF and watch, with excitement and glee, all the cross-talk that occurs – both apps responding to each other’s events. Oh noes!
Architectural frameworks generally provide you with an application event bus/channel/dispatcher to “chat” on, but this should be localised to the application itself (or some other context). Which leads me to…
3. Using the Display List as an Application Event Bus – Bad Idea
I find this one fairly disturbing too: dispatching system/application events along the display list. No!
A View is just that: “A view”, not “The view”. And don’t get me started on those innocent looking Event Maps! No, let’s move along…
4. Casting – Bad Idea
Besides wasting space and annoying programmers, casting distributes knowledge throughout a system – casting requires that you look elsewhere in your codebase for the “real” type.
And it weakens our lovely compile-time type checking (albeit temporarily), sending us back in time to our unhappy days building complex applications in the Flash IDE. And it’s boring. And it often occurs because of..
5. The Service/Model Locator Pattern – Bad Idea
Classes should not be responsible for “fetching” their dependencies – this is a separate responsibility that belongs somewhere else entirely – they should only contain code for doing their job. When classes reach outside to fetch things from the application/context they sneakily hide their dependencies away from the outside world.
Unit testers will despise you (which is a serious problem if you are writing your own tests) because it becomes incredibly hard to figure out how much set up needs to be done to write even the smallest of tests. You have to look inside the class for any calls to the Service Locator to find your direct dependencies, and then you have to look inside each of those classes to find theirs and so on. No no, that’s just cruel.
6. The Presenter Pattern – Bad Idea
Ok, it’s going to be tricky to back this one up. But think about it this way: the Presenter pattern requires that you modify your View component (to add a reference to a Presenter). Not a biggie, but once you’ve done that you’ve started coupling your View component to your application/context – a slippery slope.
I think the Mediator pattern is a much better fit for Flash and it’s Display List, and even more-so when it comes to Flex (and other UI frameworks with complex component life cycles).
It is far easier and cleaner to “wrap” around view components and poke their APIs than to have to modify (extend) them to make space for their Presenter references. Leave those components alone, leave them to the component developers, you’re supposed to be writing an application. While we’re on the Presenter pattern, let’s take a look at something dirty one might do in an attempt to salvage it’s reputation..
7. Injecting into View Components – Bad Idea
Besides being incredibly slow and wasteful (especially for Flex UIComponents), injecting directly into a View component couples it to your application/context.
View components, with any hope for reuse, should be as self-contained as possible. They should expose an API and dispatch Events. They shouldn’t be dependent on the application they are in or the framework it happens to be using. Give ’em ViewModels (M-V-VM) if you must, but don’t go and couple those ViewModels to your application, and certainly don’t inject them.
Wrapping Up This Micro-Post
Well that’s just your opinion, dude
Yeh, but I’m pretty proud of my little 7 point list there (cos I’m an OOP noob, and this stuff gets me amped). Using Dependency Injection I was able to avoid all those nasty things when building my own PureMVC-like framework.
But that just shifts the problem somewhere else
Yes, somewhere more appropriate.
Besides some convenient implementations (in the mvcs package), and some marker interfaces (in the core package) RobotLegs essentially provides two things:
- A Command Factory – for binding Commands to Events, and
- A Mediator Factory – for handling automatic Mediator creation and registration
Everything else is handled by a Dependency Injection framework and an Event Dispatcher.
Note: If you’ve been through all this before, I apologise, it’s new to me.. I’m a little slow.
Beware: the current default implementation of RobotLegs makes use of annotated setter injection. This breaks encapsulation, is open to misuse, and runs the risk of leaving objects in partially initialised states, but it’s very convenient! You can swap out SmartyPants-IOC for a Dependency Injection framework that performs constructor injection if you really want to play it safe.
I’ll admit straight-up (albeit as a footnote) that I never really gave Cairngorm much of a chance – preliminary research led me to demos, documentation and diagrams that screamed poor design and foreshadowed immense struggle. I’m sure there is a way to build a well designed Cairngorm application, but it looks like it’d take an awful amount of effort.. and code. And it probably wouldn’t be very much fun.
UPDATE: Removed some words that made me sound like a douche bag.