我目前正在使用 CAEmitterLayer 制作粒子发射器，并且在我启动动画时遇到了层预加载动画的问题，因此当我显示它时，粒子遍布整个地方。
许多答案都说罪魁祸首是 CAEmitterLayer 被预加载，我们只需在发射器上将其 beginTime 设置为 CACurrentMediaTime() 即可。
看:
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，这个解决方案没有奏效，在运行 iOS 12.1 的 iPad Air 设备上运行时，发射器经常不显示，有时显示延迟很大。
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，我有 7 个不同的粒子图像，随机选择的粒子和一个按钮在按下时显示发射器。
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]
}
}
这是在设备上运行的应用程序的 20 秒短视频，iPad Air 运行 iOS 12.1，而不是通过 xcode 运行。 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，我们将不胜感激。
最佳答案
我对 Core Animation 有很多经验，尽管我不得不承认 CAEmitterLayer 并不多。一切看起来都不错，对于非常了解 CALayer 的人来说，使用 CACurrentMediaTime() 设置 beginTime 是有意义的。但是，我运行了您的项目并发现它不起作用。对我来说，在单元格上设置 beginTime 产生了我期望的效果。
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]
}
}