Basics of display programming
Introduction to display programming
Each application built with ActionScript
3.0 has a hierarchy of displayed objects known as the display list,
illustrated below. The display list contains all the visible elements
in the application.
As the illustration shows, display elements fall into one or
more of the following groups:
Stage is the base container of display objects. Each application
has one Stage object, which contains all on-screen display objects.
The Stage is the top-level container and is at the top of the display
Each SWF file has an associated ActionScript
class, known as the main class of the SWF file. When a SWF
file opens in Flash Player or Adobe AIR, Flash Player or AIR calls
the constructor function for that class and the instance that is created
(which is always a type of display object) is added as a child of
the Stage object. The main class of a SWF file always extends the Sprite class (for more information,
see Advantages of the display list approach).
You can access the
Stage through the stage property of any DisplayObject instance.
For more information, see Setting Stage properties.
3.0, all elements that appear on screen in an application are types
of display objects. The flash.display package includes a DisplayObject class, which is a base
class extended by a number of other classes. These different classes
represent different types of display objects, such as vector shapes,
movie clips, and text fields, to name a few. For an overview of
these classes, see Advantages of the display list approach.
Display object containers
object containers are special types of display objects that, in
addition to having their own visual representation, can also contain
child objects that are also display objects.
The DisplayObjectContainer class is a
subclass of the DisplayObject class. A DisplayObjectContainer object
can contain multiple display objects in its childlist.
For example, the following illustration shows a type of DisplayObjectContainer
object known as a Sprite that contains various display objects:
- A SimpleButton object. This type of display object
has different “up,” “down,” and “over” states.
- A Bitmap
object. In this case, the Bitmap object was loaded from an external
JPEG through a Loader object.
- A Shape
object. The “picture frame” contains a rounded rectangle that is
drawn in ActionScript. This Shape object has a Drop Shadow filter
applied to it.
- A TextField object.
the context of discussing display objects, DisplayObjectContainer
objects are also known as display object containers or simply containers.
As noted earlier, the Stage is a display object container.
all visible display objects inherit from the DisplayObject class,
the type of each is of a specific subclass of DisplayObject class.
For example, there is a constructor function for the Shape class
or the Video class, but there is no constructor function for the
Common display programming tasks
Since so much of ActionScript
programming involves creating and manipulating visual elements,
there are numerous tasks that are related to display programming.
This chapter describes common tasks that apply to all display objects,
Working with the display list and display object containers
Adding display objects to the display list
Removing objects from the display list
Moving objects among display containers
Moving objects in front of or behind other objects
Working with the Stage
Handling display object events
Positioning display objects, including creating drag-and-drop
Resizing, scaling, and rotating display objects
Applying blending modes, color transformations, and transparency
to display objects
Masking display objects
Animating display objects
Loading external display content (such as SWF files or images)
Later chapters in this manual describe additional tasks for working
with display objects. These tasks include both tasks that apply
to any display object and tasks associated with specific types of
Important concepts and terms
following reference list contains important terms that you will
encounter in this chapter:
Alpha: The color value representing the amount of transparency
(or more correctly, the amount of opacity) in a color. For example,
a color with an alpha channel value of 60% only shows 60% of its
full strength, and is 40% transparent.
Bitmap graphic: A graphic that is defined in the computer
as a grid (rows and columns) of colored pixels. Commonly bitmap
graphics include digital photos and similar images.
Blending mode: A specification of how the contents of two
overlapping images should interact. Commonly an opaque image on
top of another image simply blocks the image underneath so that
it isn’t visible at all; however, different blending modes cause
the colors of the images to blend together in different ways so
the resulting content is some combination of the two images.
Display list: The hierarchy of display objects that will
be rendered as visible screen content by Flash Player and AIR. The
Stage is the root of the display list, and all the display objects
that are attached to the Stage or one of its children form the display
list (even if the object isn’t actually rendered, for example if
it’s outside the boundaries of the Stage).
Display object: An object which represents some type of visual
content in Flash Player or AIR. Only display objects can be included
in the display list, and all display object classes are subclasses
of the DisplayObject class.
Display object container: A special type of display object
which can contain child display objects in addition to (generally)
having its own visual representation.
Main class of the SWF file: The class that defines the behavior
for the outermost display object in a SWF file, which conceptually
is the class for the SWF file itself. For instance, a SWF created
in Flash authoring has a “main timeline” which contains all other
timelines; the main class of the SWF file is the class of which
the main timeline is an instance.
Masking: A technique of hiding from view certain parts of
an image (or conversely, only allowing certain parts of an image
to display). The portions of the mask image become transparent,
so content underneath shows through. The term is related to painter’s
masking tape that is used to prevent paint from being applied to
Stage: The visual container that is the base or background
of all visual content in a SWF.
Transformation: An adjustment to a visual characteristic
of a graphic, such as rotating the object, altering its scale, skewing
or distorting its shape, or altering its color.
Vector graphic: A graphic that is defined in the computer
as lines and shapes drawn with particular characteristics (such
as thickness, length, size, angle, and position).