Introducing Mixing Loom – Runtime ActionScript Bytecode Modification

At this year’s 360|Flex conference in Denver, Mike Labriola and I unveiled a new project we’ve been working on called Mixing Loom. Our presentation was called “Planet of the AOPs” because Mixing Loom lays the foundation for true Aspect Oriented Programming (AOP) on the Flash Platform. Mixing Loom provides Flex and ActionScript applications the hooks they need to do bytecode modification either before runtime or at runtime. Through bytecode modification an application can apply a behavior across hierarchies of objects. There are a number of behaviors in a typical Flex application (such as logging, security, application configuration, accessibility, and styling) that could be represented as Aspects. Today these behaviors must either be included in every class that needs them or included way down the object hierarchy (i.e. UIComponent). With Mixing Loom a compiled SWF can be modified (applying necessary behaviors) after it’s been compiled or as it’s starting up.

If you are building Flex apps and want to take advantage of AOP then Mixing Loom is probably a bit lower level than what you need. Mixing Loom combined with AS3 Commons Bytecode provides the foundation for AOP systems to be built on top of. We hope that by providing developers the hooks to modify bytecode that frameworks will emerge that provide application developers higher level APIs based on AOP. As Mike says, “Mixing Loom kicks off the Summer of AOP.”

If you are one of those developers who likes getting dirty with bytecode modification then you might want to check out the slides from the “Planet of the AOPs” session:

If you are still following along and looking for more details on how to use Mixing Loom, then keep reading. Flex applications are broken into at least two pieces. The first piece is the thing that displays the loading / progress bar. That is located on the first “frame” of an application’s SWF file. The rest of the application is on the second frame of the main SWF and possibly in other SWF files like Modules and/or Runtime Shared Libraries (RSLs). Mixing Loom provides two ways to modify the bytecode of a running application. First, using a custom preloader an application can modify its second frame and/or any Flex Modules before they are loaded into the VM. The second way is to use LoomApplication and a custom preloader, which allows an application to modify its second frame, modules, and/or RSLs (even the signed Flex Framework RSLs). Let’s walk through a simple example of an application that uses a custom preloader to modify a string that exists in its second frame.

Let’s take a simple object Foo that has a getBar method, which returns a string “a bar” [...]

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