title: Create a circle of particles using CAEmitterLayer
body: <p>I'm trying to use the UIKit CAEmitterLayer to give feedback to the user when he touches the screen. 
  What I'd like to do is a circle of particles, that grows overtime. </p>
  
  <p>I was thinking that if I setup the particles with the same speed (no velocity range), the particles would all move from the center point at the same speed, and therefore, draw a circle.</p>
  
  <p>However, it looks like the speed is still random, in a range between 0 and the speed I've chosen as velocity. </p>
  
  <p>Is there a way to force the speed to be an exact value ?</p>
  
  <pre><code>    dragEmitter.emitterPosition = CGPointMake(50, 50);
  dragEmitter.emitterSize = CGSizeMake(1, 1);
  
  CAEmitterCell* fire = [CAEmitterCell emitterCell];
  fire.birthRate = 10;
  fire.lifetime = 0.7;
  fire.lifetimeRange = 0;
  fire.color = [[UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:1.0] CGColor];
  fire.contents = (id)[[UIImage imageNamed:@"Particles_fire.png"] CGImage];
  [fire setName:@"fire"];
  
  fire.velocity = 50;
  fire.velocityRange = 0;
  fire.emissionRange = 2*M_PI;
  fire.emissionLatitude = 0;
  fire.emissionLongitude = 0;
  </code></pre>
  
  <p>Thanks for you help !</p>