About ActionScript 1 801
After you define the constructor function you must create an instance of the object. Use the new
operator before the name of the constructor function and assign the new instance a variable name.
For example, the following code uses the
new operator to create a Circle object with a radius of 5,
and assigns it to the variable
myCircle:
myCircle = new Circle(5);
Note: An object has the same scope as the variable to which it is assigned.
Assigning methods to a custom object in ActionScript 1
You can define the methods of an object inside the object’s constructor function. However, this
technique is not recommended because it defines the method every time you use the constructor
function, as in the following example, which creates the methods
area() and diameter():
function Circle(radius) {
this.radius = radius;
this.area = Math.PI * radius * radius;
this.diameter = function() {return 2 * this.radius;}
}
Each constructor function has a prototype property that is created automatically when you
define the function. The
prototype property indicates the default property values for objects
created with that function. Each new instance of an object has a
__proto__ property that refers
to the
prototype property of the constructor function that created it. Therefore, if you assign
methods to an object’s
prototype property, they are available to any newly created instance of
that object. It’s best to assign a method to the
prototype property of the constructor function
because it exists in one place and is referenced by new instances of the object (or class). You can
use the
prototype and __proto__ properties to extend objects so that you can reuse code in an
object-oriented manner. (For more information, see “Creating inheritance in ActionScript 1”
on page 803.)
The following procedure shows how to assign an
area() method to a custom Circle object.
To assign a method to a custom object:
1 Define the constructor function Circle(), as follows.
function Circle(radius) {
this.radius = radius;
}
2 Define the area() method of the Circle object. The area() method calculates the area of the
circle. You can use a function literal to define the
area() method and assign the area property
to the circle’s prototype object, as follows:
Circle.prototype.area = function () {
return Math.PI * this.radius * this.radius;
};
3 Create an instance of the Circle object, as follows:
var myCircle = new Circle(4);
4 Call the area() method of the new myCircle object, as follows:
var myCircleArea = myCircle.area();
ActionScript searches the myCircle object for the area() method. Since the object doesn’t
have an area() method, its prototype object Circle.prototype is searched for area().
ActionScript finds it and calls it.
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