我目前正在使用CAEmitterLayer制作粒子发射器，并在启动动画时遇到了预加载动画的层的问题，因此在显示动画时会在整个位置产生粒子。
许多答案都表明，罪魁祸首是预加载的CAEmitterLayer，我们只需要在发射器上将其beginTime设置为CACurrentMediaTime（）。
请参阅：
CAEmitterLayer emits random unwanted particles on touch events
Initial particles from CAEmitterLayer don't start at emitterPosition
iOS 7 CAEmitterLayer spawning particles inappropriately
对我来说，此解决方案不起作用，当在运行iOS 12.1的iPad Air设备上运行该解决方案时，发射器通常不会显示，有时显示时间会很长。
为了说明这个问题，我在github上做了一个项目： https://github.com/roodoodey/CAEmitterLayer/tree/master/CAEmitterLayerApp
这是主要代码，我为随机选择的粒子有7个不同的图像，并在按下按钮时显示了发射器。
import UIKit
class ViewController: UIViewController {
var particleImages = [UIImage]()
var emitter: CAEmitterLayer?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
view.backgroundColor = UIColor.black
// Populate the random images array for the particles
for index in 1..<8 {
if let image = UIImage(named: "StarParticle00\(index)") {
particleImages.append(image)
}
}
// Button pressed to make the emitter emit the particles
let button = UIButton(frame: CGRect(x: view.frame.width * 0.5 - 60, y: view.frame.height * 0.5 - 40, width: 120, height: 80))
button.setTitle("Emit!", for: .normal)
button.setTitleColor(UIColor.white, for: .normal)
button.backgroundColor = UIColor.blue
button.addTarget(self, action: #selector(changeButton(sender:)), for: .touchDown)
button.addTarget(self, action: #selector(addEmitter(sender:)), for: .touchUpInside)
view.addSubview(button)
}
@objc func changeButton(sender: UIButton) {
sender.alpha = 0.5
}
@objc func addEmitter(sender: UIButton) {
sender.alpha = 1.0
// IF an emitter already exists remove it.
if emitter?.superlayer != nil {
emitter?.removeFromSuperlayer()
}
emitter = CAEmitterLayer()
emitter?.emitterShape = CAEmitterLayerEmitterShape.point
emitter?.position = CGPoint(x: self.view.frame.width * 0.5, y: self.view.frame.height * 0.5)
// So that the emitter starts now, and is not preloaded.
emitter?.beginTime = CACurrentMediaTime()
var cells = [CAEmitterCell]()
for _ in 0..<40 {
let cell = CAEmitterCell()
cell.birthRate = 1
cell.lifetime = 3
cell.lifetimeRange = 0.5
cell.velocity = 500
cell.velocityRange = 100
cell.emissionRange = 2 * CGFloat(Double.pi)
cell.contents = getRandomImage().cgImage
cell.scale = 1
cell.scaleRange = 0.5
cells.append(cell)
}
emitter?.emitterCells = cells
view.layer.addSublayer( emitter! )
}
func getRandomImage() -> UIImage {
let upperBound = UInt32(particleImages.count)
let randomIndex = Int(arc4random_uniform( upperBound ))
return particleImages[randomIndex]
}
}
这是一则短短20秒的视频，显示了在设备上运行的应用程序（运行iOS 12.1的iPad Air）而不是通过xcode运行的视频。 https://www.dropbox.com/s/f9uol3yot67drm8/ScreenRecording_11-25-2018%2013-19-29.MP4?dl=0
如果有人可以看到他们是否可以重现此问题或对这种奇怪的行为有所了解，将不胜感激。
答案 0 :(得分：1)
尽管我不得不承认对CAEmitterLayer的了解不多，但我对Core Animation拥有大量的经验。一切看起来都很正确，对于一个非常了解CALayer的人来说，使用CACurrentMediaTime（）设置beginTime很有意义。但是，我运行了您的项目，发现它不起作用。对我来说，在单元格上设置beginTime会产生我期望的效果。
含义
//delay for 5.0 seconds
cell.beginTime = CACurrentMediaTime() + 5.0
cell.beginTime = CACurrentMediaTime() //immediate
cell.beginTime = CACurrentMediaTime() - 5.0 //5 seconds ago
整个文件
import UIKit
class ViewController: UIViewController {
var particleImages = [UIImage]()
var emitter: CAEmitterLayer?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
view.backgroundColor = UIColor.black
// Populate the random images array for the particles
for index in 1..<8 {
if let image = UIImage(named: "StarParticle00\(index)") {
particleImages.append(image)
}
}
// Button pressed to make the emitter emit the particles
let button = UIButton(frame: CGRect(x: view.frame.width * 0.5 - 60, y: view.frame.height * 0.5 - 40, width: 120, height: 80))
button.setTitle("Emit!", for: .normal)
button.setTitleColor(UIColor.white, for: .normal)
button.backgroundColor = UIColor.blue
button.addTarget(self, action: #selector(changeButton(sender:)), for: .touchDown)
button.addTarget(self, action: #selector(addEmitter(sender:)), for: .touchUpInside)
view.addSubview(button)
}
@objc func changeButton(sender: UIButton) {
sender.alpha = 0.5
}
@objc func addEmitter(sender: UIButton) {
sender.alpha = 1.0
// IF an emitter already exists remove it.
if emitter?.superlayer != nil {
emitter?.removeFromSuperlayer()
}
emitter = CAEmitterLayer()
emitter?.emitterShape = CAEmitterLayerEmitterShape.point
emitter?.position = CGPoint(x: self.view.frame.width * 0.5, y: self.view.frame.height * 0.5)
// So that the emitter starts now, and is not preloaded.
var cells = [CAEmitterCell]()
for _ in 0..<40 {
let cell = CAEmitterCell()
cell.birthRate = 1
cell.lifetime = 3
cell.lifetimeRange = 0.5
cell.velocity = 500
cell.velocityRange = 100
cell.emissionRange = 2 * CGFloat(Double.pi)
cell.contents = getRandomImage().cgImage
cell.scale = 1
cell.scaleRange = 0.5
cell.beginTime = CACurrentMediaTime()
cells.append(cell)
}
emitter?.emitterCells = cells
view.layer.addSublayer( emitter! )
}
func getRandomImage() -> UIImage {
let upperBound = UInt32(particleImages.count)
let randomIndex = Int(arc4random_uniform( upperBound ))
return particleImages[randomIndex]
}
}