粒子动画
效果：随机绘制一条路径，点击开始按钮，粒子动画
实现思路
1.搞个画板绘制路径，自定义view
2.给自定义view添加pan手势，和创建复制图层和圆形图层，只需要设置一次，在awakeFromNib方法中设置。
// 添加pan手势
UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)];
[self addGestureRecognizer:pan];
// 创建复制图层
CAReplicatorLayer *repLayer = [CAReplicatorLayer layer];
repLayer.frame = self.bounds;
[self.layer addSublayer:repLayer];
// 创建粒子图层
CALayer *layer = [CALayer layer];
layer.cornerRadius = 5;
layer.frame = CGRectMake(-100, 10, 10, 10);
layer.backgroundColor = [UIColor whiteColor].CGColor;
[repLayer addSublayer:layer];
_dotLayer = layer;
3.因为核心动画只能设置一个路径，因此只能创建一个路径，懒加载路径。
- (UIBezierPath *)path
{
if (_path == nil) {
_path = [UIBezierPath bezierPath];
}
return _path;
}
4.在一开始拖动的时候，保存路径起点，设置路径起点，拖动的时候每次添加线到某个点。
CGPoint curP = [pan locationInView:self];
if (pan.state == UIGestureRecognizerStateBegan) {
_startP = curP;
[self.path moveToPoint:_startP];
}
[self.path addLineToPoint:curP];
[self setNeedsDisplay];
5.路径绘制好了，点击开始按钮的时候，添加动画到图层
CAKeyframeAnimation *anim = [CAKeyframeAnimation animation];
anim.keyPath = @"position";
anim.duration = 4;
anim.path = self.path.CGPath;
anim.repeatCount = MAXFLOAT;
[_dotLayer addAnimation:anim forKey:@"anim"];
anim.delegate = self;
6.复制图层
repLayer.instanceCount = 20;
repLayer.instanceDelay = 4 / 20.0;
// 设置子层颜色
repLayer.instanceColor = [UIColor colorWithRed:0 green:1 blue:0 alpha:1].CGColor;
// 设置子层颜色绿色通道偏移量
repLayer.instanceGreenOffset = -0.03;
7.重绘
清空路径，重新绘制，移除图层动画。
_path = nil;
[_dotLayer removeAnimationForKey:@"anim"];
[self setNeedsDisplay];
