20.1. Display API Overview

In ActionScript, all graphical content is created and manipulated using the classes in the display API. Even the interface widgets in the Flex framework and Flash authoring tool component sets use the display API as a graphical foundation. Many display API classes directly represent a specific type of on-screen graphical content. For example, the Bitmap class represents bitmap graphics, the Sprite class represents interactive graphics, and the TextField class represents formatted text. For the purposes of discussion, we'll refer to classes that directly represent on-screen content (and superclasses of such classes) as core display classes. The remaining classes in the display API define supplementary graphical information and functionality but do not, themselves, represent on-screen content. For example, the CapStyle and JointStyle classes define constants representing line-drawing preferences, while the Graphics and BitmapData classes define a variety of primitive drawing operations. We'll refer to these nondisplay classes as supporting display classes. Whether core or supporting, most of the display API classes reside in the package flash.display.

The core display classes, shown in Figure 20-1, are arranged in a class hierarchy that reflects three general tiers of functionality: display, user interactivity, and containment. Accordingly, the three central classes in the display API are: DisplayObject, InteractiveObject, and DisplayObjectContainer. Those three classes cannot be instantiated directly but rather provide abstract functionality that is applied by various concrete subclasses.