800 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 class
MovieClip, and all buttons are instances of the built-in class Button. 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.
emma = new Biker(30, 5);
hamish = new Biker(40, 5);
In object-oriented scripting, classes can receive properties and methods from each other according
to a specific order; this 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 = 000609;
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.
Creating a custom object in ActionScript 1
To create a custom object, you define a constructor function. A constructor function is always
given the same name as the type of object it creates. You can use the keyword
this inside the
body of the constructor function to refer to the object that the constructor creates; when you call
a constructor function, Flash passes it
this as a hidden parameter. For example, the following is a
constructor function that creates a circle with the property
radius:
function Circle(radius) {
this.radius = radius;
}
To Next Page
Loading...