A Completed Game: Belt Commander - Beginning iOS6 Games Development - page 285

for (Asteroid* asteroid in asteroids){
for (Bullet* bullet in bullets){
if ([bullet overlapsWith:asteroid]){
[bullet decrementDamage: self];
asteroids_destroyed++;
[asteroid doHit:self];
[self incrementScore: [asteroid level]*10];
break;
}
}
if ([asteroid overlapsWith:viper]){
[viper decrementHealth: [asteroid level]*2];
Shield* shield = [Shield shieldProtecting:viper From: asteroid];
[self addActor:shield];
asteroids_destroyed++;
[asteroid doHit:self];
}
}
In Listing 12-14, we see the first part of the task doCollisionDetection where we handle the
interaction concerning Asteroids. The first thing we do in this task is to get a reference to all of
the different Actors we are going to need to interact with. Once we have reference to all of the
Asteroids , Bullets , and the Viper , we can start testing to see if there are collisions. Inside the
outermost loop in Listing 12-14, we start by testing to see if each Asteroid overlaps with each
Bullet . If they do, we have the bullet decrement the Asteroid's damage, which usually removes
the Asteroid from the game, but more on that later. We also record that we have destroyed another
Asteroid before calling doHit; on the Asteroid . Lastly, we increment the score.
In Listing 12-14, after we have considered the relation between each Asteroid and Bullet , we check
to see if the Asteroid is overlapping the Viper . If it is, we decrement the health of the Viper and
add a Shield to the scene. The Shield is an Actor that is just decoration and has no game function
beyond that; it looks like a shield went up to protect the spaceship.
Let's look at the rest of the doCollisionDetection task and understand how the rest of the inter-
Actor relationships are handled. The task continues in Listing 12-15.
Listing 12-15. BeltCommanderController.m (doCollisionDetection, part 2)
for (Bullet* bullet in bullets){
if ([[bullet source] isKindOfClass:[Saucer class]]){
if ([viper overlapsWith: bullet]){
[viper decrementHealth: [bullet damage]];
Shield* shield = [Shield shieldProtecting:viper From: bullet];
[self addActor:shield];
[self removeActor:bullet];
break;
}