Organizing Game Objects - Learning C# by Programming Games - page 149

public void Update(GameTime gameTime)
{

∗ position += velocity ( float )gameTime.ElapsedGameTime.TotalSeconds;

}

Finally, we can add a few convenient properties for getting and setting the color,
position or velocity. For the complete ThreeColorGameObject class, see the example
project Painter9 belonging to this chapter.

10.4 Creating Special Versions of Game Objects

Now that we have created a very basic game object class, we can reuse this basic
behavior for the actual game objects in our game by inheriting from this class, just
like we inherited from the generic Game class.

10.4.1 The Cannon Class

Let us first have a look at the cannon game object. Since we now have the basic
ThreeColorGameObject class, we can create the Cannon class, and inherit from the
ThreeColorGameObject class:

class Cannon : ThreeColorGameObject
{

...

}

Now we need to add a constructor method to this class. Before we start writing the
code for this constructor, let us first think about what it means when we create an
instance of Cannon . In the previous version of the Painter game, the Cannon instance
can be depicted as in Fig. 10.1 .
In the new version of the Cannon class, it is no longer a class that stands on its
own, but it inherits from the ThreeColorGameObject class. This means that Cannon
is basically a special version of a ThreeColorGameObject instance. From the point of
view of the compiler, this means that when a Cannon instance is created, this instance
consists of a ThreeColorGameObject instance and the extensions that make it a Cannon
object. Next to the member variables that are already in the ThreeColorGameObject
class, a Cannon object also has a barrel that is drawn at a certain angle. As a result,
we need two additional member variables:

Texture2D cannonBarrel;
float angle;