Create a circle of particles using CAEmitterLayer

问题

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.

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.

However, it looks like the speed is still random, in a range between 0 and the speed I've chosen as velocity.

Is there a way to force the speed to be an exact value ?

    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;

Thanks for you help !

回答1:

Frankly, it's hard to imagine 100% the same picture of the particle system you want but i think it gets close to that what needed a few months ago. Please try the following set-up for the emitter cell or try experimenting with the values based on the set-up below:

    .
    .
    emitter.emitterPosition = CGPointMake(...);
    emitter.renderMode = kCAEmitterLayerBackToFront;

    CAEmitterCell* star = [CAEmitterCell emitterCell]
    star.emissionLongitude = M_PI*2;
    star.emissionRange = M_PI*2;
    star.lifetime = 2.5;
    star.birthRate = 20;
    star.velocity = 120;
    star.velocityRange = 20;
    star.color = [UIColor colorWithRed:49.0/255.0 green:168.0/255.0 blue:181.0/255.0 alpha:0.6].CGColor;
    star.contents = (id)image.CGImage;
    star.redRange = 0.1;
    star.greenRange = 0.1;
    star.blueRange = 0.1;
    .
    .

来源：https://stackoverflow.com/questions/12972658/create-a-circle-of-particles-using-caemitterlayer