creating a vertical CAEmitterLayer

Question

I'm working on creating a CAEmitterLayer that's the height of the screen, and pushed off -X, so the CAEmitterCells travel from left (off the screen) to upper right.

There's an issue I am running into where the emitterSize height property of CAEmitterLayer is being ignored. This is causing all the cells to emit from a single point, and not what's set with emitterSize.

Here's the emitter:

emitter.emitterPosition = CGPoint(x: -50, y: (view.frame.height / 2))
emitter.emitterShape = kCAEmitterLayerLine
emitter.emitterSize = CGSize(width: 2, height: view.frame.height)

i mentioned emitterSize height is not working, because if i change the width of the above emitterSize, I can see the width changing properly! No matter what value I put for height... it gets ignored.

And the CAEmitterCells

cell.birthRate = 10
cell.lifetime = 10
cell.velocity = CGFloat(50)
cell.emissionLongitude = (45 * (.pi/180))

How can I set the emitterSize width to be 2 points wide and the height of the view?

rob mayoff rob mayoff · Accepted Answer · 2018-03-15T17:19:45

kCAEmitterLayerLine is always a horizontal line of zero thickness. Only emitterSize.width is used.

You can use the line shape vertically by setting your emitter layer's transform to a rotation. Playground example:

import UIKit
import PlaygroundSupport

class VerticalEmitterView: UIView {

    let emitter = CAEmitterLayer()

    override func layoutSubviews() {
        super.layoutSubviews()

        let bounds = self.bounds
        emitter.position = CGPoint(x: bounds.midX, y: bounds.midY)
        emitter.bounds = CGRect(x: 0, y: 0, width: bounds.size.height, height: bounds.size.width)
        emitter.emitterPosition = CGPoint(x: bounds.width / 2, y: -50)
        emitter.emitterSize = CGSize(width: emitter.bounds.size.width, height: 0)

        if emitter.superlayer != self.layer {
            emitter.setAffineTransform(CGAffineTransform(rotationAngle: -.pi / 2))
            emitter.emitterShape = kCAEmitterLayerLine

            let cell = CAEmitterCell()
            cell.birthRate = 100
            cell.lifetime = 10
            cell.velocity = 50
            cell.emissionLongitude = .pi
            cell.color = UIColor.red.cgColor
            let particleSize = CGSize(width: 5, height: 5)
            let image = UIGraphicsImageRenderer(size: particleSize).image(actions: { (rc) in
                UIColor.white.setFill()
                let gc = UIGraphicsGetCurrentContext()
                UIBezierPath(ovalIn: CGRect(origin: .zero, size: particleSize)).fill()
            })
            cell.contents = image.cgImage
            emitter.emitterCells = [cell]

            layer.addSublayer(emitter)
        }
    }

}

let rootView = UIView(frame: CGRect(x: 0, y: 0, width: 300, height: 300))
rootView.backgroundColor = .white
let emitterView = VerticalEmitterView(frame: rootView.bounds)
rootView.addSubview(emitterView)
PlaygroundPage.current.liveView = rootView

Result:

(The discontinuity is because this GIF is a one-second loop.)

creating a vertical CAEmitterLayer

2 Answers