问iOS -如何在iOS上实现粒子动画

EN

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    // define emitter layer as centered w 80% of smallest dimension

    let image = emitterImage
    let side = min(view.bounds.width, view.bounds.height) * 0.8
    let origin = CGPoint(x: view.bounds.midX - side / 2, y: view.bounds.midY - side / 2)
    let center = CGPoint(x: view.bounds.midX, y: view.bounds.midY)
    let size = CGSize(width: side, height: side)
    let rect = CGRect(origin: origin, size: size)
    let emitterLayer = CAEmitterLayer()
    emitterLayer.emitterShape = kCAEmitterLayerRectangle
    emitterLayer.emitterSize = rect.size
    emitterLayer.emitterPosition = center

    // define cells

    let cell = CAEmitterCell()
    cell.birthRate = Float(size.width * size.height / 10)
    cell.lifetime = 1
    cell.velocity = 10
    cell.scale = 0.1
    cell.scaleSpeed = -0.1
    cell.emissionRange = .pi * 2
    cell.contents = image.cgImage
    emitterLayer.emitterCells = [cell]

    // add the layer

    view.layer.addSublayer(emitterLayer)

    // mask the layer

    let lineWidth = side / 8
    let mask = CAShapeLayer()
    mask.fillColor = UIColor.clear.cgColor
    mask.strokeColor = UIColor.white.cgColor
    mask.lineWidth = lineWidth
    mask.path = UIBezierPath(arcCenter: center, radius: (side - lineWidth) / 2, startAngle: .pi / 4, endAngle: .pi * 7 / 4, clockwise: true).cgPath
    emitterLayer.mask = mask
}

var emitterImage: UIImage {
    let rect = CGRect(origin: .zero, size: CGSize(width: 10, height: 10))
    UIGraphicsBeginImageContextWithOptions(rect.size, false, 0)
    #colorLiteral(red: 0.5725490451, green: 0, blue: 0.2313725501, alpha: 1).setFill()
    UIBezierPath(arcCenter: CGPoint(x: rect.midX, y: rect.midY), radius: rect.midX, startAngle: 0, endAngle: .pi * 2, clockwise: true).fill()
    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return image!
}