Ch 5 Applying Some Object-Oriented (OO) Design
CS 583 08Feb2013

5.1 Designing Your Classes
5.2 Creating a Sprite Class
5.3 Creating a User-Controlled Sprite Class
5.4 Creating an Automated Sprite Class
5.5 Game Components
5.6 Coding the SpriteManager
5.7 Cleaning Up
5.8 Making Them Move
5.9 What You Just Did
5.10 Summary
5.11 Test Your Knowledge: Quiz
5.12 Test Your Knowledge: Exercise

Chapter 4 allowed us to place animated sprites and have them interact. Chapter 3 let us code a) Basics Sprite b) Moving Sprite and c) Animated Sprite. But they all felt like specific coding for each example. We wish avoid this pain through design in this chapter.

5.1 Designing Your Classes

First things first, let�s look at your objects. The basic visual object in a 2D game is a sprite. Based on the code you put together in previous chapters, you have two different types of objects: objects that are controlled by the player, and objects that are not. Aside from that, all other functionality of the animated sprites you�ve worked on thus far is exactly the same.

Define the base Sprite class by its functionality and then let specialized sprite inherit.

5.2 Creating a Sprite Class

What properties do all sprites have? These are the members of the base sprite class.

What activites must all sprites accomplish? These are the methods of the base sprite class.

This code will build on the code from Chapter 4. We add a new Class name Sprite.cs. We want this to be an abstract class to that when we instantiate obejcts we used a derived class.

Add two XNA namespaces to your new class, which you�ll need to enable the use of XNA objects:
using Microsoft.Xna.Framework; using Microsoft.Xna.Framework.Graphics;

Based on Table 5-1, we have variables for each member of the class
Make sure you mark the protected variables appropriately, or the subclasses you build later will not work properly:

Texture2D textureImage;
protected Point frameSize;
Point currentFrame;
Point sheetSize;
int collisionOffset;
int timeSinceLastFrame = 0;
int millisecondsPerFrame;
const int defaultMillisecondsPerFrame = 16;
protected Vector2 speed;
protected Vector2 position;

SpriteManager.cs
NOTE: difference between the two constructors - second one requires a millisecondsPerFrame variable, used to calculate the animation speed.
the first constructor calls the second constructor (using the this keyword) and pass to that constructor all of its parameters

In addition to the Update and Draw methods, you�re going to want to add a property to this class that represents the direction in which this sprite is moving. The direction will always be a Vector2, indicating movement in the X and Y directions, but it will also always be defined by the subclasses (i.e., automated sprites will move differently than user-controlled sprites). So, this property needs to exist in the base class, but it should be abstract, meaning that in the base class it has no body and therefore must be defined in all subclasses.
Add the abstract direction property to the base class as follows: public abstract Vector2 direction

{
    get;
}

There�s one more thing to add to your Sprite base class: a property that returns a rectangle that can be used for collision detection. Add the following property to your Sprite class:

public Rectangle collisionRect
{
get
    {
        return new Rectangle(
            (int)position.X + collisionOffset,
            (int)position.Y + collisionOffset,
            frameSize.X - (collisionOffset * 2),
            frameSize.Y - (collisionOffset * 2));
    }
}

5.3 Creating a User-Controlled Sprite Class

Add a new class to your project by right-clicking on the project in Solution Explorer and selecting Add?Class. Name the new class file UserControlledSprite.cs. Once the file is ready, mark this class as a subclass of the Sprite class:

class UserControlledSprite: Sprite

Next, you�ll need to add some XNA using statements. Use the same ones as you did in the Sprite base class, plus an additional one (Microsoft.Xna.Framework.Input) that will allow you to read data from input devices:

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

Next, add the constructors for the UserControlledSprite class. These will be basically the same as the constructors for the Sprite class and will just pass the parameters on to the base class:

public UserControlledSprite(Texture2D textureImage, Vector2 position,
    Point frameSize, int collisionOffset, Point currentFrame, Point sheetSize,
    Vector2 speed)
    : base(textureImage, position, frameSize, collisionOffset, currentFrame,
    sheetSize, speed)
{
}
public UserControlledSprite(Texture2D textureImage, Vector2 position,
    Point frameSize, int collisionOffset, Point currentFrame, Point sheetSize,
    Vector2 speed, int millisecondsPerFrame)
    : base(textureImage, position, frameSize, collisionOffset, currentFrame,
    sheetSize, speed, millisecondsPerFrame)
{
}

UserControlledSprite.cs

5.4 Creating an Automated Sprite Class

Now that you have a class that allows the user to control a sprite, it�fs time to add a class that will generate an animated sprite that moves on its own. Add a new class to your project by right-clicking on the project in Solution Explorer and selecting Add��Class. Name the class file AutomatedSprite.cs. Once the file is ready, mark the new class as a subclass of your Sprite class:

class AutomatedSprite: Sprite

Add the same XNA namespaces as before, but without the input namespace because you won�t be gathering input from any devices in this class:

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

Next, add two constructors for the AutomatedSprite class. These constructors will be identical to the ones used for the UserControlledSprite class:

public AutomatedSprite(Texture2D textureImage, Vector2 position, Point frameSize,
int collisionOffset, Point currentFrame, Point sheetSize, Vector2 speed)
: base(textureImage, position, frameSize, collisionOffset, currentFrame,
sheetSize, speed)
{}
public AutomatedSprite(Texture2D textureImage, Vector2 position, Point frameSize,
int collisionOffset, Point currentFrame, Point sheetSize, Vector2 speed,
int millisecondsPerFrame)
: base(textureImage, position, frameSize, collisionOffset, currentFrame,
sheetSize, speed, millisecondsPerFrame)
{}

Your automated sprite will use the speed member of the base class to move around the screen. This will be done through an overridden direction property because that property is abstract in the base class and therefore must be defined in this class. Create the override for the direction property as follows:

public override Vector2 direction
{
get { return speed; }
}

AutomatedSprite.cs
Artificial Intelligence?
So, this sprite is moving by itself�but is that what we call artificial intelligence? Well, what is artificial intelligence? That�s a tough question. Artificial intelligence refers to the science of making a computer act as if it is intelligent. The problem with that definition is that there really is no clear definition of intelligence. Therefore, it�s extremely difficult to define artificial intelligence. The class you�ve just coded allows your sprite to move on its own, so you could argue that you�ve just created an artificial intelligence algorithm�a very weak and unimpressive one, but an artificial intelligence algorithm nonetheless. On the other hand, you could argue that your sprite is no more intelligent than a bullet flying through the air, because all it does is move in a constant direction with a constant velocity. And not many people would argue that bullets are intelligent! In later chapters, we look at different ways to make your automated sprites and 3D objects act more and more intelligent, but in the meantime, intelligent or not, you�ve made a good start.

5.5 Game Components

5.6 Coding the SpriteManager

5.7 Cleaning up

5.8 Making Them Move

5.9 What You Just Did

5.10 Summary

How do share your XNA code? When you turn your project in, you will be required to create a zip-file of the directory you developed in. I have created such a structure for the codes that we have covered in lecture, below:
Ch5AnimatedSprites.zip
Ch4AnimatedSprites.zip; Ch3BasicSprite.zip, Ch3MovingSprites.zip, and Ch3AnimatedSprites.zip