General background information
You learned how to design, create, and use a simple Sprite class in an earlier module. You also learned to use a generic List object to store references to objects of the Sprite class.
Preview
I will explain improvements made to the Sprite class and will show you how to write a Game1 class that takes advantage of those improvements.
The screen output
Figure 1 shows a reduced screen shot of the program while it is running.
Figure 1 . Screen shot of the running program.
Lots and lots of sprites
When this screen shot was taken, the program had 24 space rocks, 12 red power pills and six UFOs all navigating in the game window.
Discussion and sample code
As usual, I will discuss and explain the program code in fragments. A complete listing of the Sprite class is provided in Listing 19 and a complete listing of the Game1 class is provided in Listing 20 .
The Sprite class
The Sprite class begins in Listing 1 with the declaration of several instance variables.
Listing 1 . Beginning of the Sprite class.
namespace XNA0128Proj {
class Sprite {private Texture2D image;
private Vector2 position = new Vector2(0,0);private Vector2 direction = new Vector2(0,0);
private Point windowSize;private Random random;
double elapsedTime;//in milliseconds//The following value is the inverse of speed in
// moves/msec expressed in msec/move.double elapsedTimeTarget; The purpose of these instance variables will become clear later in the discussion.
Properties
This class has the following properties:
- Position - position of the sprite in the game window relative to the upper left corner of the game window. The X and Y values aresaved as type Vector2 in the instance variable named position .
- WindowSize - The dimensions of the game window. The width and height values are saved as type Point in the instance variable named windowSize .
- Direction - Direction of motion of the sprite expressed as a 2D vector with an X and a Y component. The length of this vector is thedistance that the sprite moves each time it moves. The values are saved as type Vector2 in the instance variable named direction .
- Speed - Speed of the sprite expressed in moves per millisecond. The actual speed in pixels per millisecond is the product ofthis value and the length of the direction vector.
Other instance variables
In addition, the class has the following instance variables that are set either by the constructor or by a method:
- image - the image that is drawn to represent the sprite in the game window. The image can be modified as the game progresses throughcalls to the SetImage method.
- random - a reference to a random number generator.
- elapsedTime - used to keep track of the amount of elapsed time in milliseconds since the last time that the sprite wasactually moved.
- elapsedTimeTarget - will be explained later.
The first three property accessor methods
The first three property accessor methods are shown in Listing 2 .
Listing 2 . The first three property accessor methods.
//Position property accessor
public Vector2 Position {get {
return position;}
set {position = value;
}//end set}//end Position property accessor
//-------------------------------------------------////WindowSize property accessor
public Point WindowSize {set {
windowSize = value;}//end set
}//end WindowSize property accessor//-------------------------------------------------//
//Direction property accessorpublic Vector2 Direction {
get {return direction;
}set {
direction = value;}//end set
}//end Direction property accessor