Adobe Flash ActionScript 3 has dramatically altered and improved the way in which visual elements are added to a project. In fact, in ActionScript 2, visual elements were not even recognized as all belonging to the same group: different syntax was required to handle each different type of visual element. ActionScript 3 rationalises the whole business of maniuplating visual assets by introducing a new, very flexible and very powerful concept: the display list.
Put simply, the display list is a built-in inventory of every visual element residing in an application and, like the DOM (document object model) found in JavaScript, it has a hierarchical structure. It contains a variety of disparate elements such as movie clips, sprites (simple movie clips), vector shapes, bitmaps and video.
One key distinction that should be made between the elements in the display list is the one between display objects and display object containers. Objects such as movie clips are referred to as display object containers since they are capable of containing other display objects. Objects such as bitmaps, video and vector shapes (which cannot contain other objects), are simply referred to as display objects.
ActionScript 3 has a rich body of syntax for manipulating the display list: display objects can be added and removed and there are a number of different ways of targeting objects within the display list hierarchy.
The Stage is the object which resides at the top of the display list hierarchy since it contains the entire Flash movie. Next, drilling down, we have the main timeline of the movie; and then we have all the various types of visual objects, divided into containers and objects. The main types of display object containers are movie clips, sprites and loaders.
Movie clips have been around in Flash for ages; they are a bit like clones of the main movie, each with its own timeline. Sprites are a new to ActionScript 3: they are effectively movie clips without a timeline into which other objects can be placed. Their relative lack of complexity makes them ideal candidates for ActionScript animation. The loader class is used specifically as a container into which external assets such as bitmaps and SWF files can be loaded.
As for objects which cannot act as containers, Shapes are vector objects usually created using Flash’s vector drawing tools. There is also a new class of object referred to as an Interactive Objects. This class includes simple buttons with an over, up and down state as well as text fields. There are two types of text field: dynamic text fields whose output can be manipulated with ActionScript and input text fields which can accept input from the user.
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