如何实现iOS动画中的雨花与樱花特效

@interface XMWeatherView ()

@property(nonatomic,strong) CAEmitterLayer *sunshineEmitterLayer;

@property(nonatomic,strong) CAEmitterLayer *rainDropEmitterLayer;

@property(nonatomic,strong) UIImageView *backgroundView;

@end

#pragma mark - 下雨特效
-(void)addRainningEffect{

  self.backgroundView.image=[UIImage imageNamed:@"rainning.jpeg"];

  //粒子发射器图层
  self.rainDropEmitterLayer=[CAEmitterLayer layer];

  //粒子发射器位置
  _rainDropEmitterLayer.emitterPosition=CGPointMake(100, -30);

  //粒子发射器的范围
  _rainDropEmitterLayer.emitterSize=CGSizeMake(self.bounds.size.width*4, 0);

  //发射模式
  _rainDropEmitterLayer.emitterMode=kCAEmitterLayerOutline;

  //粒子模式
  _rainDropEmitterLayer.emitterShape=kCAEmitterLayerLine;

  //创建粒子
  CAEmitterCell *emitterCell=[CAEmitterCell emitterCell];

  //设置粒子内容
  emitterCell.contents=(__bridge id)([UIImage imageNamed:@"42-Raindrop"].CGImage);

  //设置粒子缩放比例
  emitterCell.scale=0.9;

  //缩放范围
  emitterCell.scaleRange=0.5;

  //每秒粒子产生数量
  emitterCell.birthRate=130;

  //粒子生命周期
  emitterCell.lifetime=5;

  //粒子透明速度
  emitterCell.alphaSpeed=-0.1;

  //粒子速度
  emitterCell.velocity=280;
  emitterCell.velocityRange=100;

  //设置发射角度
  emitterCell.emissionLongitude=-M_PI;
//  emitterCell.emissionRange=M_PI;

  //设置粒子旋转角速度
//  emitterCell.spin=M_PI_4;

  //设置layer阴影
  _rainDropEmitterLayer.shadowOpacity=1.0;

  //设置圆角
  _rainDropEmitterLayer.shadowRadius=2;

  //设置偏移
  _rainDropEmitterLayer.shadowOffset=CGSizeMake(1, 1);

  //设置颜色
  _rainDropEmitterLayer.shadowColor=[UIColor whiteColor].CGColor
  ;

  //设置layer的粒子
  _rainDropEmitterLayer.emitterCells=@[emitterCell];

  _rainDropEmitterLayer.transform=CATransform3DMakeRotation(-M_PI/4, 0, 0, 1);

  [self.layer addSublayer:_rainDropEmitterLayer];
}