Organizing Game Objects - Building JavaScript Games for Phones, Tablets, and Desktop - page 143

The update method of the class contains a single instruction that updates the current position of the
game object:
ThreeColorGameObject.prototype.update = function (delta) {
this.position.addTo(this.velocity.multiply(delta));
};
Finally, you add a few convenient properties for getting and setting the color, and retrieving the
dimensions of the object. For example, this is the property for reading and writing the object's color:
Object.defineProperty(ThreeColorGameObject.prototype, "color",
{
get: function () {
if (this.currentColor === this.colorRed)
return Color.red;
else if (this.currentColor === this.colorGreen)
return Color.green;
else
return Color.blue;
},
set: function (value) {
if (value === Color.red)
this.currentColor = this.colorRed;
else if (value === Color.green)
this.currentColor = this.colorGreen;
else if (value === Color.blue)
this.currentColor = this.colorBlue;
}
});
As you can see, you use the colored sprite member variables here. Any class that inherits from
ThreeColorGameObject also now has this property. This saves you a lot of code copying! For the
complete ThreeColorGameObject class, see the Painter9 example belonging to this chapter.
Cannon as a Subclass of ThreeColorGameObject
Now that you've created a very basic class for colored game objects, you can reuse this basic
behavior for the actual game objects in your game by inheriting from this class. Let's first look at the
Cannon class. Because you've defined the basic ThreeColorGameObject class, you can create the
Cannon class as a subclass of that class, as follows:
function Cannon() {
// to do...
}
Cannon.prototype = Object.create(ThreeColorGameObject.prototype);
You create the Cannon.prototype object by making a copy of the ThreeColorGameObject.prototype
object. You still need to write the code in the constructor method, though.