Introduction to ActionScript 3.0
ActionScript is the programming language for the Adobe® Flash® Player and Adobe® AIR™ run-time environments. It enables interactivity, data handling, and much more in Flash, Flex, and AIR content and applications.
ActionScript executes in the ActionScript Virtual Machine (AVM), which is part of Flash Player and AIR. ActionScript code is typically transformed into bytecode format by a compiler. (Bytecode is a type of programming language that’s written and understood by computers.) Examples of compilers include the one built in to Adobe® Flash® Professional and the one that is built in to Adobe® Flash® Builder™ and available in the Adobe® Flex™ SDK. The bytecode is embedded in SWF files, which Flash Player and AIR execute.
ActionScript 3.0 offers a robust programming model that is familiar to developers with a basic knowledge of object-oriented programming. Some of the key features of ActionScript 3.0 that improve over previous ActionScript versions include the following:
Advantages of ActionScript 3.0
ActionScript 3.0 goes beyond the scripting capabilities of previous versions of ActionScript. It is designed to facilitate the creation of highly complex applications with large data sets and object-oriented, reusable code bases. ActionScript 3.0 is not required for content that runs in Adobe Flash Player. However, it opens the door to performance improvements that are only available with the AVM2 (the ActionScript 3.0 virtual machine). ActionScript 3.0 code can execute up to ten times faster than legacy ActionScript code.
The previous version of ActionScript Virtual Machine, AVM1, executes ActionScript 1.0 and ActionScript 2.0 code. Flash Player 9 and 10 support AVM1 for backward compatibility.
What’s new in ActionScript 3.0
ActionScript 3.0 contains many classes and features that are similar to ActionScript 1.0 and 2.0. However, ActionScript 3.0 is architecturally and conceptually different from previous versions of ActionScript. The enhancements in ActionScript 3.0 include new features of the core language and an improved API that provides increased control of low-level objects.
Core language features
The core language defines the basic building blocks of the programming language, such as statements, expressions, conditions, loops, and types. ActionScript 3.0 contains many features that speed up the development process.
ActionScript 3.0 reports more error conditions than previous versions of ActionScript. Run-time exceptions are used for common error conditions, improving the debugging experience and enabling you to develop applications that handle errors robustly. Run-time errors can provide stack traces annotated with source file and line number information, helping you quickly pinpoint errors.
In ActionScript 3.0, type information is preserved at run time. This information is used to perform run-time type checking, improving the system’s type safety. Type information is also used to represent variables in native machine representations, which improves performance and reduces memory usage. By way of comparison, in ActionScript 2.0 type annotations are primarily a developer aid and all values are dynamically typed at run time.
ActionScript 3.0 includes the concept of sealed classes. A sealed class possesses only the fixed set of properties and methods that are defined at compile time; additional properties and methods cannot be added. The inability of changing a class at run time enables stricter compile-time checking, resulting in more robust programs. It also improves memory usage by not requiring an internal hash table for each object instance. Dynamic classes are also possible using the dynamic keyword. All classes in ActionScript 3.0 are sealed by default, but can be declared to be dynamic with the dynamic keyword.
ActionScript 3.0 enables a method closure to automatically remember its original object instance. This feature is useful for event handling. In ActionScript 2.0, method closures do not remember what object instance they were extracted from, leading to unexpected behavior when the method closure is called.
ECMAScript for XML (E4X)
ActionScript 3.0 implements ECMAScript for XML (E4X), recently standardized as ECMA-357. E4X offers a natural, fluent set of language constructs for manipulating XML. In contrast to traditional XML-parsing APIs, XML with E4X performs like a native data type of the language. E4X streamlines the development of applications that manipulate XML by drastically reducing the amount of code needed.
To view the ECMA E4X specification, go to www.ecma-international.org.
ActionScript 3.0 includes native support for regular expressions so that you can quickly search for and manipulate strings. ActionScript 3.0 implements support for regular expressions as they are defined in the ECMAScript (ECMA-262) edition 3 language specification.
Namespaces are similar to the traditional access specifiers used to control visibility of declarations (public, private, protected). They work as custom access specifiers, which can have names of your choice. Namespaces are outfitted with a Universal Resource Identifier (URI) to avoid collisions, and are also used to represent XML namespaces when you work with E4X.
New primitive types
ActionScript 3.0 contains three numeric types: Number, int, and uint. Number represents a double-precision, floating-point number. The int type is a 32-bit signed integer that lets ActionScript code take advantage of the fast integer math capabilities of the CPU. The int type is useful for loop counters and variables where integers are used. The uint type is an unsigned, 32-bit integer type that is useful for RGB color values, byte counts, and more. In contrast, ActionScript 2.0 only has a single numeric type, Number.
The APIs in ActionScript 3.0 contain many classes that allow you to control objects at a low level. The architecture of the language is designed to be more intuitive than previous versions. While there are too many classes to cover in detail, some significant differences are worth noting.
DOM3 event model
Document Object Model Level 3 event model (DOM3) provides a standard way of generating and handling event messages. This event model is designed to allow objects within applications to interact and communicate, maintain their state, and respond to change. The ActionScript 3.0 event model is patterned after the World Wide Web Consortium DOM Level 3 Events Specification. This model provides a clearer and more efficient mechanism than the event systems available in previous versions of ActionScript.
Events and error events are located in the flash.events package. The Flash Professional components and Flex framework use the same event model, so the event system is unified across the Flash Platform.
Display list API
The API for accessing the display list—the tree that contains any visual elements in the application—consists of classes for working with visual primitives.
The Sprite class is a lightweight building block, designed to be a base class for visual elements such as user interface components. The Shape class represents raw vector shapes. These classes can be instantiated naturally with the new operator and can be dynamically reparented at any time.
Depth management is automatic. Methods are provided for specifying and managing the stacking order of objects.
Handling dynamic data and content
ActionScript 3.0 contains mechanisms for loading and handling assets and data in your application that are intuitive and consistent across the API. The Loader class provides a single mechanism for loading SWF files and image assets and provides a way to access detailed information about loaded content. The URLLoaderclass provides a separate mechanism for loading text and binary data in data-driven applications. The Socket class provides a means to read and write binary data to server sockets in any format.
Low-level data access
Various APIs provide low-level access to data. For data that is being downloaded, the URLStream class provides access to data as raw binary data while it is being downloaded. The ByteArray class lets you optimize reading, writing, and working with binary data. The sound API provides detailed control of sound through the SoundChannel and SoundMixer classes. Security APIs provide information about the security privileges of a SWF file or loaded content, enabling you to handle security errors.
Working with text
ActionScript 3.0 contains a flash.text package for all text-related APIs. The TextLineMetrics class provides detailed metrics for a line of text within a text field; it replaces the TextFormat.getTextExtent() method in ActionScript 2.0. The TextField class contains low-level methods that provide specific information about a line of text or a single character in a text field. For example, the getCharBoundaries() method returns a rectangle representing the bounding box of a character. The getCharIndexAtPoint() method returns the index of the character at a specified point. The getFirstCharInParagraph() method returns the index of the first character in a paragraph. Line-level methods include getLineLength(), which returns the number of characters in a specified line of text, and getLineText(), which returns the text of the specified line. The Font class provides a means to manage embedded fonts in SWF files.
For even lower-level control over text, the classes in the flash.text.engine package make up the Flash Text Engine. This set of classes provide low-level control over text and are designed for creating text frameworks and components.