Build Your Game: Vector Actors and Particles - Beginning iOS6 Games Development - page 174

Listing 7-22. Particle.m
@implementation Particle
@synthesize totalStepsAlive;
+(id)particleAt:(CGPoint)aCenter WithRep:(NSObject<Representation>*)rep Steps:(float)numStepsToLive{
Particle* particle = [[Particle alloc] initAt:aCenter WithRadius:32 AndRepresentation:rep];
[particle setTotalStepsAlive:numStepsToLive];
ExpireAfterTime* expire = [ExpireAfterTime expireAfter:numStepsToLive];
[expire setDelegate: particle];
[particle addBehavior:expire];
return particle;
}
-(void)stepsUpdated:(ExpireAfterTime*)expire In:(GameController*)controller{
self.alpha = [expire stepsRemaining]/totalStepsAlive;
}
@end
In Listing 7-22, we see that the constructor particleAt:WithRep:Steps: is pretty straightforward.
We create a new Particle object, passing in the provided Representation object. We also set the
property totalStepsAlive and create an ExpiresAfterTime Behavior . Notice that particle is set
as expire 's delegate; this causes the task stepsUpdated:In: to be called every time expire is
executed.
In the task stepsUpdated:In: we simply adjust the alpha value of Particle based on how far along
Particle's life cycle we are. In a more complex implementation of Particle , this fading behavior
would be configurable, not just on or off but also at which rate a Particle fades. In this simple
implementation, each particle simply fades in a linear way.
We have looked at the implementation of Asteroid and Particle, and see that it is pretty simple to
create a particle effect in our scene. In the next section, we will look at the Comet actor and see a
more visually striking example, though the implementation will be just as simple.
Creating Based Vector-Based Particles
In the previous example, we looked at Particles that were represented by a sequence of images.
Because we have a flexible way of rendering actors, we can just as easily create particles that are
programmatically drawn. In this example, we will be creating Particles that stay in one place, but
we will move the point where they are created. This will give our Comet actors a nice glowing tail, as
any comet should have. Figure 7-5 shows our new actor, the Comet .
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