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Listing 4 tests to determine if the upper-right corner of the sprite has encountered the right edge of the game window or if the upper-left corner of thesprite has encountered the left edge of the game window. If so, the sign of stepsX is changed to cause the direction of motion of the sprite to be reversed. You will see how the change in sign causes a reversal inthe direction of motion shortly.
The code in Listing 5 tests to determine if the sprite has encountered the bottom or top edges of the game window. If so, the sign is changed on stepsY to cause the sprite to reverse its vertical direction of motion.
Listing 5 . Test for sprite out of bounds vertically.
if(((spritePosition.Y + myTexture.Height)>Window.ClientBounds.Height) ||(spritePosition.Y<0)) {
stepsY *= -1;//Out of bounds, reverse direction}//end if The logic in Listing 5 is essentially the same as the logic in Listing 4 with the difference being that Listing 5 deals with vertical motion and the top and bottom edges of the game window instead of horizontal motion and the left andright edges of the game window.
Listing 6 changes the current position of the sprite by adding the incremental distances, stepX and stepY , to the current horizontal and vertical coordinates of the sprite.
Listing 6 . Change the current position of the sprite.
spritePosition.X += stepsX;//move horizontal
spritePosition.Y += stepsY;//move vertical The incremental distances were initialized with positive values in Listing 2 . Adding positive incremental distance values to the current coordinate valuescauses the position of the sprite to move to the right and down.
The logic in Listing 4 and Listing 5 reverses the sign on the incremental distance values when the sprite encounters a left, right, top, or bottom edge ofthe game window.
Adding a negative incremental distance value to the current horizontal coordinate of the sprite causes it to be moved to the left. Adding a negativeincremental distance value to the current vertical coordinate of the sprite causes it to be moved up the screen.
Simply changing the current horizontal and vertical coordinates of the sprite does not produce a visible change in position. The visible change in positionhappens later in the Draw method when the game window is cleared to a constant color and the sprite is redrawn in the game window in itsnew position.
The Draw method in this program was not modified relative to the version in the earlier module titled Xna0118-The XNA Framework and the Game Class . You can view the code in the Draw method in Listing 9 near the end of the module.
The remainder of the overridden Update method is shown in Listing 7 .
Listing 7 . The remainder of the overridden Update method.
//The following statement is always required.
base.Update(gameTime);}//end Update 
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