Build Your Game: Vector Actors and Particles - Beginning iOS6 Games Development - page 158

The last thing the updateScene task does is check to see if any Bullet has collided with the Saucer.
This is done by calling actorsOfType: and getting all Bullet objects in the scene stored in the
NSMutableSet * bullets . We simply iterate over each Bullet and Saucer and check if there is a hit by
calling overlapsWith: . If there is a hit, we call the doHit:with: task of the class Saucer .
Let's take a closer look at them implementation of the Actor classes used in this example.
The Actor Classes
In this section, we learn about the Saucer actor and the HealthBar actor classes. The HealthBar is
different from other actors we have looked at up until this point. It is different because its location
in the scene is dependent on the location of another Actor— in this case, a Saucer . Once we have
reviewed the classes Saucer and HealthBar , we will look at the Behavior class FollowActor and see
how we implement this feature.
Instantiating the Saucer Class
We will start by looking at the constructor for the class Saucer . We want to start with the Saucer 's
constructor because this is where we create the HealthBar . In this way, we don't have to worry about
adding a HealthBar when we create a new Saucer because one will be created automatically every
time we create a Saucer . The constructor for Saucer is shown in Listing 7-3.
Listing 7-3. Saucer.m (saucer:)
+(id)saucer:(GameController*)controller{
CGSize gameAreaSize = [controller gameAreaSize];
CGPoint gameCenter = CGPointMake(gameAreaSize.width/2.0, gameAreaSize.height/2.0);
ImageRepresentation* rep = [ImageRepresentation imageRep];
[rep setBackwards:arc4random()%2 == 0];
[rep setStepsPerFrame:3];
Saucer* saucer = [[Saucer alloc] initAt:gameCenter WithRadius:32 AndRepresentation:rep];
[rep setDelegate:saucer];
[saucer setVariant:arc4random()%VARIATION_COUNT];
[saucer setMaxHealth:100];
[saucer setCurrentHealth:100];
HealthBar* healthBar = [HealthBar healthBar:saucer];
[healthBar setPercent:1];
[saucer setHealthBar:healthBar];
[controller addActor:healthBar];
return saucer;
}
In Listing 7-3, we start by finding the center of the game area and storing that value in the CGPoint
gameCenter . Next, we create an ImageRepresentation called rep . We indicate that rep should
spin the Saucer backward half the time at a rate of 3 steps per frame of animation. The CGRect