Flash ActionScript 3 has drastically changed and improved the way in which visual elements are added to a project. In fact, in previous versions, visual elements were not treated as belonging to the same group: different coding techniques was required to handle different visual elements. ActionScript 3 rationalises the process of working with all visual assets by introducing a new, flexible and powerful concept called 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 important distinction that should be made between the elements in the display list is that between display objects and display object containers. Objects such as movie clips are known 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 known as display objects.
ActionScript 3 features a rich body of syntax for manipulating the display list: display objects can be easily added and removed and there are a number of different ways of targeting objects within the display list hierarchy.
The Stage is the object residing at the top of the display list hierarchy since it contains the entire Flash movie. Next, as we drill down through the hierarchy, 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 classes 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.
Turning to objects which cannot act as containers, Shapes are vector objects usually created using the drawing tools within Flash. There is also a new class of object referred to as an Interactive Object. This category includes simple buttons with an over, up and down state as well as text fields. As with previous versions of Flash, there are dynamic text fields whose output can be manipulated with ActionScript as well as input text fields, which can also accept keyboard input from the user.
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