swift 實現漂亮的粒子效果CAEmitterLayer

我們經常會在一些游戲或者應用中看到一些炫酷的粒子效果，我們在iOS中也能很輕松的搞一些粒子效果

我們本次做得是一個下雪的效果，看下效果圖

源碼地址：  https://github.com/smalldu/IOS-Animations 中的AnimationDemo10

iOS中的粒子效果是用CAEmitterLayer和CAEmitterCell 實現的 ， 對cell的粒子數 ，存活時間。。。各種東西進行配置就可以了

常用的產生粒子模式 三種kCAEmitterLayerPoint 點 kCAEmitterLayerLine線 kCAEmitterLayerRectangle 矩形，大家可以自己試試 。我就不一一展示了。

下面是所有代碼 ,代碼量很少 。而且都有注釋

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
  super.viewDidLoad()
  let rect = CGRect(x: 0.0, y: -70.0, width: view.bounds.width,
      height: 50.0)
  let emitter = CAEmitterLayer()
  emitter.frame = rect
  view.layer.addSublayer(emitter)
  emitter.emitterShape = kCAEmitterLayerRectangle
  //kCAEmitterLayerPoint
  //kCAEmitterLayerLine
  //kCAEmitterLayerRectangle
  emitter.emitterPosition = CGPointMake(rect.width/2, rect.height/2)
  emitter.emitterSize = rect.size
  let emitterCell = CAEmitterCell()
  emitterCell.contents = UIImage(named: "xh")!.scaleImageToWidth(30).CGImage
  emitterCell.birthRate = 120  //每秒產生120個粒子
  emitterCell.lifetime = 3    //存活1秒
  emitterCell.lifetimeRange = 3.0
  emitter.emitterCells = [emitterCell]  //這里可以設置多種粒子 我們以一種為粒子
  emitterCell.yAcceleration = 70.0  //給Y方向一個加速度
  emitterCell.xAcceleration = 20.0 //x方向一個加速度
  emitterCell.velocity = 20.0 //初始速度
  emitterCell.emissionLongitude = CGFloat(-M_PI) //向左
  emitterCell.velocityRange = 200.0   //隨機速度 -200+20 --- 200+20
  emitterCell.emissionRange = CGFloat(M_PI_2) //隨機方向 -pi/2 --- pi/2
       //emitterCell.color = UIColor(red: 0.9, green: 1.0, blue: 1.0,
   //   alpha: 1.0).CGColor //指定顏色
  emitterCell.redRange = 0.3
  emitterCell.greenRange = 0.3
  emitterCell.blueRange = 0.3  //三個隨機顏色
  emitterCell.scale = 0.8
  emitterCell.scaleRange = 0.8  //0 - 1.6 
  emitterCell.scaleSpeed = -0.15  //逐漸變小
  emitterCell.alphaRange = 0.75   //隨機透明度
  emitterCell.alphaSpeed = -0.15  //逐漸消失
  // Do any additional setup after loading the view, typically from a nib.
    }
    override func didReceiveMemoryWarning() {
  super.didReceiveMemoryWarning()
  // Dispose of any resources that can be recreated.
    }
}