Learning ActionScript 2.0 in Adobe Flash
Object-Oriented Programming with ActionScript 1.0 > Creating inheritance in ActionScript 1.0

Creating inheritance in ActionScript 1.0

Inheritance is a means of organizing, extending, and reusing functionality. Subclasses inherit properties and methods from superclasses, and add their own specialized properties and methods. For example, reflecting the real world, Bike would be a superclass and MountainBike and Tricycle would be subclasses of the superclass. Both subclasses contain, or inherit, the methods and properties of the superclass (for example, wheels). Each subclass also has its own properties and methods that extend the superclass (for example, the MountainBike subclass would have a gears property). You can use the elements prototype and __proto__ to create inheritance in ActionScript.

All constructor functions have a prototype property that is created automatically when the function is defined. The prototype property indicates the default property values for objects created with that function. You can use the prototype property to assign properties and methods to a class. (For more information, see Assigning methods to a custom object in ActionScript 1.0.)

All instances of a class have a __proto__ property that tells you the object from which they inherit. When you use a constructor function to create an object, the __proto__ property is set to refer to the prototype property of its constructor function.

Inheritance proceeds according to a definite hierarchy. When you call an object's property or method, ActionScript looks at the object to see if such an element exists. If it doesn't exist, ActionScript looks at the object's __proto__ property for the information (myObject.__proto__). If the property is not a property of the object's __proto__ object, ActionScript looks at myObject.__proto__.__proto__, and so on.

The following example defines the constructor function Bike():

function Bike(length, color) {
    this.length = length;
    this.color = color;
    this.pos = 0;
}

The following code adds the roll() method to the Bike class:

Bike.prototype.roll = function() {return this.pos += 20;};

Then, you can trace the position of the bike with the following code:

var myBike = new Bike(55, "blue");
trace(myBike.roll());   // traces 20.
trace(myBike.roll());   // traces 40.

Instead of adding roll() to the MountainBike class and the Tricycle class, you can create the MountainBike class with Bike as its superclass, as shown in the following example:

MountainBike.prototype = new Bike();

Now you can call the roll() method of MountainBike, as shown in the following example:

var myKona = new MountainBike(20, "teal");
trace(myKona.roll());   // traces 20

Movie clips do not inherit from each other. To create inheritance with movie clips, you can use Object.registerClass() to assign a class other than the MovieClip class to movie clips.

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