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This change is position is necessary to prevent the same ladybug from registering a collision with the same spider during the next iteration of thegame loop. The forward movement of the spiders and the ladybugs each time they move is less than the dimensions of the intersecting rectangles. Therefore,without a purposeful shift in position of either the ladybug or the spider, the pair would continue to register collisions until the rectangles finally separatefrom one another.
As you read in the Preview section, if a black widow spider collides with a ladybug, it simply moves 128 pixels tothe right and changes into a green spider. If a green spider collides with a ladybug, it simply moves 128 pixels to the right and changes into a brownspider. Finally, if a brown spider collides with a ladybug, it is eaten and removed from the population of spiders. This is accomplished by the code in Listing 12 .
Listing 12 . Spiders don't die easily.
//If the collision was with a black widow
// spider, cause it to reincarnate into a// green spider.
if(target.Image.Name == "blackWidowSpider") {target.SetImage("greenspider",Content);
}//If the collision was with a green spider,
// cause it to reincarnate into a brown// spider.
else if(target.Image.Name == "greenspider") {target.SetImage("brownSpider",Content);
}//If the collision was with a brown spider,
// it gets eaten. Remove it from the list of// spiders.
else if(target.Image.Name == "brownSpider") {spiders.Remove(target);
}// end else-if}//end if
}//end for loopbase.Update(gameTime);
}//end Update method The code in Listing 12 is relatively straightforward.
The first statement in Listing 12 shows why I needed to set an identifiable string value into the Name property of the Texture2D object referred to by image in Listing 2 and Listing 3 . I needed a way to determine the type of spider involved in each collision so that I could take the appropriate action when acollision was detected.
The call to the spiders.Remove method in Listing 12 removes the target Sprite object's reference from the list of spiders. Since this is the only reference to the Sprite object, this action should make the memory occupied by the Sprite object eligible for garbage collection.
However, I'm not certain that garbage collection is sufficient to free up all of the resources owned by the now defunct object and its image. It might also beadvisable to use a Dispose Method . At the time I am writing this, I simply don't know.
Listing 12 also signals the end of the overridden Update method.
The overridden Game.Draw method is shown in its entirety in Listing 13 .
Listing 13 . The overridden Game.Draw method.
protected override void Draw(GameTime gameTime) {
spriteBatch.Begin();spiderWeb.Draw(spriteBatch);//draw background
//Draw all spiders.for(int cnt = 0;cnt<spiders.Count;cnt++) {
spiders[cnt].Draw(spriteBatch);
}//end for loop//Draw all ladybugs.
for(int cnt = 0;cnt<ladybugs.Count;cnt++) {
ladybugs[cnt].Draw(spriteBatch);
}//end for loopspriteBatch.End();
base.Draw(gameTime);}//end Draw method
//-------------------------------------------------//}//end class
}//end namespace 
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