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The logic in Listing 8 is fairly complex due mainly to the need to adjust the frame rate from fast to slow or from slow to fast when transitioning between thetwo rows of sprites in Listing 1 . Rather than to try to explain this logic, I am going to leave it as an exercise for the student to analyze the code andto determine where the frame rate transitions occur.
Although I haven't mentioned it before, the SpriteBatch.Draw method (not the Game.Draw method) that will be used to draw the sprites in the game window has a scaling parameter that can be used to scale theimages before drawing them.
Listing 1 declares a variable named scale and sets its value to 4. This will be used as a scale factor when the sprite images are drawn.
Listing 1 also declares a variable named slide and sets its value to 8. This variable is used to control how far the sprite moves each timeit is drawn.
That distance, along with the new sprite position, is computed in Listing 8 as the product of the width of the sprite image and the scale factor divided bythe value of slide . This is a distance that I determined experimentally to cause the animation to look like I wanted it to look.
It is worth noting that the sign of the variable named slide determines whether the incremental distance is positive or negative.
It is also worth noting that the change in spritePosition.X only occurs when sprites from the top row in Figure 1 are being drawn. When sprites from the bottom row in Figure 1 are being drawn, the sprite is animated in place.
The code in Listing 9 causes the sprite image to move back and forth across the game window always facing in the correct direction.
Listing 9 . Move the sprite image back and forth across the game window.
if(spritePosition.X>winWidth - frameWidth * scale) {slide *= -1;
spriteEffect = noEffect;}//end if
if(spritePosition.X<0){
slide *= -1;spriteEffect = flipEffect;
}//end if}//end if//-----------------------------------------------//
//New code ends here.base.Update(gameTime);
}//end Update Listing 9 tests for a collision between the sprite and the right edge or the left edge of the game window. If the sprite collides with the right edge, thesign on the variable named slide is changed to cause future incremental distance movements to be negative (from right to left).
If the sprite collides with the left edge, the sign on the variable named slide is changed to cause future incremental distance movements to be positive (from left to right).
In addition, when the sprite collides with one edge or the other, the value of the spriteEffect variable is set such that the dog will be facing the correct direction as it moves toward the other edge of the game window.
That concludes the explanation of the overridden Update method.
The overridden Draw method selects the correct sprite image by extracting a rectangular area from the sprite sheet and draws the rectanglecontaining the sprite image at a specified location in the game window.
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