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The iOS 5 Developer's Cookbook: Combining Emitter Layers with User Touches

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Erica Sadun, author of The iOS 5 Developer's Cookbook, shows how you can add some sparkle to your apps by displaying clouds, fireworks, or other particle effects at a user's touch.
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Looking for a little sparkle in your apps? The CAEmitterLayer class may be exactly what you're looking for. Emitters render particles in real time, creating clouds, fireworks, or other particle effects on demand. They're built using a suite of custom properties that you adjust.

You can add visual interest to your interfaces by using emitters in tandem with user touches. The following class demonstrates how to follow a touch over its lifetime, adding a little sparkle to wherever the user touches on-screen.

The class begins as soon as the user touches the screen, creating an emitter layer and a single emitter cell. The cell defines the particles — their color, their birth rate, their lifetime, velocity, and so forth.

As the user's touch progresses, this class updates the emitter's location, removing the emitter once the touch is removed from the screen. Although this example is written for single touch interaction, you can easily update the code to add an array of emitters (rather than a single instance) for multi-touch interaction.

Emitters are easily added to your projects and efficient to run. While too much animation is never a good design idea, a little sparkle used judiciously can add life and movement.

@interface SparkleTouchView : UIView
    CAEmitterLayer *emitter;

@implementation SparkleTouchView
- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    float multiplier = 0.25f;
    CGPoint pt = [[touches anyObject] locationInView:self];
    //Create the emitter layer
    emitter = [CAEmitterLayer layer];
    emitter.emitterPosition = pt;
    emitter.emitterMode = kCAEmitterLayerOutline;
    emitter.emitterShape = kCAEmitterLayerCircle;
    emitter.renderMode = kCAEmitterLayerAdditive;
    emitter.emitterSize = CGSizeMake(100 * multiplier, 0);
    //Create the emitter cell
    CAEmitterCell* particle = [CAEmitterCell emitterCell];
    particle.emissionLongitude = M_PI;
    particle.birthRate = multiplier * 1000.0;
    particle.lifetime = multiplier;
    particle.lifetimeRange = multiplier * 0.35;
    particle.velocity = 180;
    particle.velocityRange = 130;
    particle.emissionRange = 1.1;
    particle.scaleSpeed = 1.0; // was 0.3
    particle.color = [[COOKBOOK_PURPLE_COLOR colorWithAlphaComponent:0.5f] CGColor];
    particle.contents = (__bridge id)([UIImage imageNamed:@"spark.png"].CGImage);
    particle.name = @"particle";

    emitter.emitterCells = [NSArray arrayWithObject:particle];
    [self.layer addSublayer:emitter];

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    CGPoint pt = [[touches anyObject] locationInView:self];
    // Disable implicit animations
    [CATransaction begin];
    [CATransaction setValue:(id)kCFBooleanTrue forKey:kCATransactionDisableActions];
    emitter.emitterPosition = pt;    
    [CATransaction commit];    

- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    [emitter removeFromSuperlayer];
    emitter = nil;

- (void) touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    [self touchesEnded:touches withEvent:event]; } @end
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