CAEmitterLayer 没有用 CACurrentMediaTime() 正确计时，有时根本不显示

Question

我目前正在使用 CAEmitterLayer 制作粒子发射器，但在启动它时遇到了层预加载动画的问题，因此当我显示它时粒子到处都是。

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()。

见：

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，这个解决方案不起作用，当在设备上运行它时，运行 iOS 12.1 的 iPad Air，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

这里是主要代码，我有 7 个随机选择的不同粒子图像和一个按钮，用于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]
    }


}

这是一段 20 秒的短视频，展示了在设备上运行的应用程序，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

如果有人可以查看他们是否可以重现此问题或阐明这种奇怪的行为，将不胜感激。

Answer 1

我在 Core Animation 方面拥有丰富的经验，尽管我不得不承认 CAEmitterLayer 方面的经验并不多。一切看起来都是正确的，对于一个非常了解 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，发现它不起作用。对我来说，在单元格上设置 beginTime 达到了我所期望的效果。
意义

//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]
    }


}