Fast Intro to Flash

There is plenty of documentation, tutorials and explanation of Flash on the internet, blogs and books. However, it’s hard for me to find a good, concise article that covers Flash well. I’ve been exposing Flash 9 to a friend of mine recently and I’ve decided that a concise explanation is something I should write. This post will be brief, high level and will cover Flash as a platform and not the Flash IDE. This article is also intended for those with previous exposure to technical concepts such as virtual machines and compilers.

Overview
Flash is a platform for building and deploying interactive multimedia on the internet. The term Flash is ambiguous because it can be used to describe the runtime environment (Flash Player) or one of two IDEs targeting the player (Adobe Flash and Adobe Flex Builder). Flash currently uses an ECMA-based scripting language known as Actionscript for authoring. This same language is used across all of Adobe’s platform -which includes Adobe AIR, Adobe Flex, and Adobe Flash. Actionscript uses a strong, static, safe type system which makes it’s syntax more similar to Java than Javascript.

Runtime Environment

SWF Files
Swf files are compressed, multipurpose packages designed to contain the following data: raster and vector graphics, video, audio, text, fonts, animation, and actionscript code. Swfs can be loaded at runtime whenever necessary and can use both static and dynamic linking based upon the compiler settings. The runtime environment uses security sandboxes for loaded swf files. The security settings primarily limit the extent of filesystem access and cross-domain access that swf files can use. Adobe has provided the swf file format spec and has recently provided the FLV video format spec and has declared that they will provide the RTMP spec as well.

Flash Player & AVM2
The Flash Player runtime is a very light-weight, virtual machine and handles system calls. The Flash Player includes both AVM1 (AS1, AS2, for Flash versions 1-8) and AVM2 (AS3 for Flash 9,10+) which processes compiled Adobe Bytecode (ABC). The term AVM stands for “ActionScript Virtual Machine”*. The Flash Player uses a Deferred Reference Counting (DRC) mechanism combined with a conservative mark/sweep garbage collector. The Adobe VM uses both JIT compiling and code interpretation and may benefit from compile-time bytecode optimizations based upon compiler input parameters. AS3 computational performance is generally closer to Java than Javascript [source]. The player also maintains a variable framerate that reflects the time consumed for each frame update [source]. Each frame update consists of multiple stages that include internal player processing, rendering, and author script processing. The program framerate is the principle metric reflecting application performance.

Development Environment

Actionscript API
The Actionscript API is divided up amongst the Adobe AIR environment, the Flex environment, and the Flash environment and the Flash Player. The source for all the packages are divided up amongst swc files. Swc files are pre-compiled catalogs of classes and assets. They are similar to swf files but they are used for compile-time dependency resolution. Furthermore, the native swc’s predominantly contain code and not visual assets. An overview of the Actionscript packages is kept up to date in the Adobe Live Docs.

The Flash IDE
The Flash IDE was designed for designers, artists and animators. So, it is equiped with a layer system which reflects the stage display list and a hierarchical timeline which reflects the temporal nature of the runtime. The objects placed on the stage are represented by the timeline and can be “tweened” (using runtime interpolated 2D geometric transformations or custom shape morphs). The same timeline is also used for object creation and destruction. All stage instances are confined to a small set of native data types including: MovieClips, Sprites, Shapes, TextFields, Videos, and Bitmaps. The IDE also has a library that contains source objects for all stage instances and even instances that aren’t used on the stage. Flash projects often have several build targets (compiled swf files) that use parts of the project codebase. After Macromedia was purchased by Adobe, the Flash authoring environment has been updated to provide much better interoperability with the Adobe Creative Suite.

Adobe Flex Builder
Flex Builder is an Eclipse-based Actionscript IDE for writing and compiling swf files. Flex Builder is primarily designed to build “Flex” projects which can be written in an interface tagging language known as MXML. MXML is a declarative XML langauge used to define interface elements in a project. MXML files provide the “view” in the MVC software design pattern. Flex Builder also provides plenty of useful features like code completion, syntax formatting, active debugging, memory profiling, and project building that expose errors and warnings during development time. Because the Flex framework is not bundled with the Flash Player, Flex projects must statically link to all swc assets specific to Flex which amounts to greater than 500kb. This is very unnatractive for most users, thus Adobe has provided means by which the download can be significantly decreased. Flex Builder is also used to deploy applications for the Adobe AIR platform. Adobe AIR is a desktop runtime that combines webkit with the AVM2 portion of the Flash Player. AIR can be likened to a closed source Java SE runtime but provides a smaller learning curve and greater accessibility than what comes with the Java ecosystem.

Compiler
Since Adobe has launched the Flex platform they have provided free access to their Flex and Actionscript compiler and SDK. They have also provided open source projects under the Mozilla public license. The compiler (mxmlc) provides an impressive amount of flexibility and customization for swf compiling. It can be configured with command line options or a configuration XML file.

* correction 09-12-2009, I had previously stated that the term AVM meant “Adobe Virtual Machine”, this is a mixup. It actually means “ActionScript Virtual Machine”. Thanks dbam.