Actionscript Cookbook

12.8. Creating Subclasses

Problem

You want to create a subclass that inherits from a superclass.

Solution

Define a new class and set the new class’s prototype property to an instance of the superclass.

Discussion

A subclass is a class that has its own methods and properties but also uses the methods and properties of another class (its superclass). For example, all the built-in ActionScript classes, such as the Date and TextField classes, are subclasses of the top-level Object superclass. You can create a subclass that inherits all the properties and methods of a superclass by assigning a new instance of the superclass to the subclass’s prototype property. For example:

MySubClass.prototype = new MySuperClass(  );

Warning

Assigning a new instance of the superclass to the subclass’s prototype wipes out any existing methods or properties of that prototype. You must assign the value to prototype before you add any methods or properties to the subclass.

This is the correct order in which to perform the actions:

1. Define the constructor method for the subclass.

2. Assign inheritance (i.e., the superclass) to the prototype property.

3. Add methods and properties to the prototype property.

Here is a example of the correct order in which to perform the actions:

_global.MySubClass = function (  ) {};
MySubClass.prototype = new MySuperClass(  );
MySubClass.prototype.methodA = function (  ) {};
MySubClass.prototype.myProperty = "some property value";

The following is incorrect! Because the inheritance is defined ...