User Guide

324 Appendix E: Object-Oriented Programming with ActionScript 1
Objects in ActionScript can be pure containers for data, or they can be graphically represented on
the Stage as movie clips, buttons, or text fields. All movie clips are instances of the built-in
MovieClip class, and all buttons are instances of the built-in Button class. Each movie clip
instance contains all the properties (for example,
_height, _rotation, _totalframes) and all
the methods (for example,
gotoAndPlay(), loadMovie(), startDrag()) of the MovieClip class.
To define a class, you create a special function called a constructor function. (Built-in classes have
built-in constructor functions.) For example, if you want information about a bicycle rider in
your application, you could create a constructor function,
Biker(), with the properties time and
distance and the method getSpeed(), which tells you how fast the biker is traveling:
function Biker(t, d) {
this.time = t;
this.distance = d;
this.getSpeed = function() {return this.time / this.distance;};
}
In this example, you create a function that needs two pieces of information, or parameters, to do
its job:
t and d. When you call the function to create new instances of the object, you pass it the
parameters. The following code creates instances of the object Biker called
emma and hamish, and
it traces the speed of the
emma instance, using the getSpeed() method from the previous
ActionScript:
emma = new Biker(30, 5);
hamish = new Biker(40, 5);
trace (emma.getSpeed()); //traces 6
In object-oriented scripting, classes can receive properties and methods from each other according
to a specific order, which is called inheritance. You can use inheritance to extend or redefine the
properties and methods of a class. A class that inherits from another class is called a subclass. A
class that passes properties and methods to another class is called a superclass. A class can be both a
subclass and a superclass.
An object is a complex data type containing zero or more properties and methods. Each property,
like a variable, has a name and a value. Properties are attached to the object and contain values
that can be changed and retrieved. These values can be of any data type: String, Number,
Boolean, Object, MovieClip, or undefined. The following properties are of various data types:
customer.name = "Jane Doe";
customer.age = 30;
customer.member = true;
customer.account.currentRecord = 609;
customer.mcInstanceName._visible = true;
The property of an object can also be an object. In line 4 of the previous example, account is a
property of the object
customer, and currentRecord is a property of the object account. The
data type of the
currentRecord property is Number.