CAEmitterLayer *snowEmitter = [CAEmitterLayer layer];
snowEmitter.emitterPosition = CGPointMake(self.view.bounds.size.width / 2.0, -30);
snowEmitter.emitterSize = CGSizeMake(self.view.bounds.size.width * 2.0, 0.0);;
// Spawn points for the flakes are within on the outline of the line
snowEmitter.emitterMode = kCAEmitterLayerOutline;
snowEmitter.emitterShape = kCAEmitterLayerLine;
// Configure the snowflake emitter cell
CAEmitterCell *snowflake = [CAEmitterCell emitterCell];
snowflake.birthRate = 2;//开始时间
snowflake.lifetime = 30.0;//持续时间
snowflake.velocity = -10; // falling down slowly
snowflake.velocityRange = 50;
snowflake.yAcceleration = 6;
snowflake.emissionRange = 0.5 * M_PI; // some variation in angle
snowflake.spinRange = 0.5 * M_PI; // slow spin
snowflake.contents = (id) [[UIImage imageNamed:@"DazFlake"] CGImage];
snowflake.color = [[UIColor colorWithRed:0.600 green:0.658 blue:0.743 alpha:1.000] CGColor];
// Make the flakes seem inset in the background
snowEmitter.shadowOpacity = 1.0;
snowEmitter.shadowRadius = 0.0;
snowEmitter.shadowOffset = CGSizeMake(0.0, 1.0);
snowEmitter.shadowColor = [[UIColor whiteColor] CGColor];
// Add everything to our backing layer below the UIContol defined in the storyboard
snowEmitter.emitterCells = [NSArray arrayWithObject:snowflake];
[scllView.layer insertSublayer:snowEmitter atIndex:0];