下雪动画 CAEmitterLayer CAEmitterCel

参考：https://ios.devdon.com/archives/1046

    _snowEmitter = [CAEmitterLayer layer];
    
    CGRect rect = CGRectMake(0.0, 0.0, windowSize.width, 40.0);
    
    _snowEmitter.emitterPosition    = CGPointMake(rect.size.width / 2, -rect.size.height / 2);
    _snowEmitter.emitterSize        = rect.size;
    
    _snowEmitter.emitterMode        = kCAEmitterLayerOutline;
    _snowEmitter.emitterShape       = kCAEmitterLayerLine;
    _snowEmitter.renderMode         = kCAEmitterLayerOldestLast;
    
    CAEmitterCell *confetti = [CAEmitterCell emitterCell];
    
    confetti.contents = (__bridge id)[[UIImage imageNamed:@"snow.png"] CGImage];
    confetti.name = @"confetti";
    confetti.birthRate = 5;
    confetti.lifetime = 20.0;
    
    confetti.velocity = 30;
    confetti.velocityRange = 100;
    
    confetti.emissionLongitude = (CGFloat) M_PI;
    
    confetti.yAcceleration = 25;
    
    confetti.alphaRange = 1.0;
    confetti.alphaSpeed = -0.1;
    
    confetti.spinRange = 10.0;
    confetti.scale = 0.6;
    confetti.scaleRange = 0.6;
    
    _snowEmitter.emitterCells = [NSArray arrayWithObject:confetti];
    
    [superLayer insertSublayer:_snowEmitter atIndex:0];