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Listing 10 . Compute components of a new direction vector.
//Compute a new X component as a random portion of
// the vector length.direction.X =
(float)(length * random.NextDouble());//Compute a corresponding Y component that will
// keep the same vector length.direction.Y = (float)Math.Sqrt(length*length -
direction.X*direction.X);//Set the signs on the X and Y components to match
// the signs from the original direction vector.if(xNegative)
direction.X = -direction.X;if(yNegative)
direction.Y = -direction.Y;}//end NewDirection The first statement in Listing 10 computes a new value for the X component of the current direction vector, which is a randomportion of the length of the current direction vector ranging from 0 to the full length of the vector.
Then the second statement in Listing 10 uses the Sqrt method along with the Pythagorean Theorem to compute a new value for the Y component, which when combined with the new Xcomponent will produce a direction vector having the same length as before.
Finally, the last two statements in Listing 10 use the information gleaned earlier to cause the signs of the new X and Ycomponents to match the signs of the original components.
By modifying the lengths of the X and Y components, the code in Listing 10 causes the direction pointed to by the new vector to be different from thedirection pointed to by the original direction vector.
By causing the X and Y components to have the same signs, the code in Listing 10 causes the new direction vector to point into the same quadrant as before.
The Sprite.Draw method is shown in its entirety in Listing 11 .
Listing 11 . The Sprite.Draw method.
public void Draw(SpriteBatch spriteBatch) {
//Call the simplest available version of// SpriteBatch.Draw
spriteBatch.Draw(image,position,Color.White);}//end Draw method
//-------------------------------------------------//}//end Sprite class
}//end namespace This Draw method is essentially the same as the Draw method that I explained in an earlier module so it shouldn't require further explanation.
To avoid becoming confused, however, you should keep in mind that this program deals with the following three methods having the name Draw :
Listing 11 also signals the end of the Sprite class.
The Game1 class begins in Listing 12 .
Listing 12 . Beginning of the Game1 class.
namespace XNA0128Proj {
public class Game1 : Microsoft.Xna.Framework.Game {GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;//Use the following values to set the size of the
// client area of the game window. The actual window// with its frame is somewhat larger depending on
// the OS display options. On my machine with its// current display options, these dimensions
// produce a 1024x768 game window.int windowWidth = 1017;
int windowHeight = 738;//This is the length of the greatest distance in
// pixels that any sprite will move in a single// frame of the game loop.
double maxVectorLength = 5.0;//References to the space rocks are stored in this
// List object.List<Sprite>rocks = new List<Sprite>();
int numRocks = 24;//Number of rocks.//The following value should never exceed 60 moves
// per second unless the default frame rate is also// increased to more than 60 frames per second.
double maxRockSpeed = 50;//moves per second//References to the power pills are stored in
// this List.List<Sprite>pills = new List<Sprite>();
int numPills = 12;//Number of pills.double maxPillSpeed = 40;//moves per second
//References to the UFOs are stored in this List.List<Sprite>ufos = new List<Sprite>();
int numUfos = 6;//Max number of ufosdouble maxUfoSpeed = 30;
//Random number generator. It is best to use a single// object of the Random class to avoid the
// possibility of using different streams that// produce the same sequence of values.
//Note that the random.NextDouble() method produces// a pseudo-random value where the sequence of values
// is uniformly distributed between 0.0 and 1.0.Random random = new Random(); 
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