Particle emitters for apps with CAEmitterLayer on iOS

Particle emitters are encountered the most often in games. Apple’s SpriteKit framework contains SKEmitterNode for this exact purpose. Moreover, Xcode even has a live editor for configuring SKEmitterNode. As SKEmitterNode can only be added to SKScene, it is not a good fit for apps. But no worries, apps can take advantage of CAEmitterLayer which is part of the CoreAnimation framework. CAEmitterLayer is subclass of CALayer and therefore we can add it to anywhere in our UIKit based apps. Snowboarding and skiing related apps could add a cool snow effect with a little bit of code.

CAEmitterLayer and CAEmitterCell

Particles used by CAEmitterLayer are defined by CAEmitterCell. Every emitter layer can have multiple cells and every cell can have it’s own subcells as well. Meaning every particle has an ability of emitting other particles. CAEmitterCell itself is just a definition of the particle: how it moves, looks and lives.

	extension CAEmitterLayer {
	convenience init(image: CGImage) {
	self.init()
	let cell: CAEmitterCell = {
	let cell = CAEmitterCell()
	cell.birthRate = 40
	cell.contents = image
	cell.emissionLongitude = .pi / 2.0
	cell.emissionRange = CGFloat.pi / 4.0
	cell.lifetime = 16
	cell.scale = 0.3
	cell.scaleRange = 0.2
	cell.velocity = 80
	return cell
	}()
	emitterCells = [cell]
	}
	}

view raw CAEmitterLayer.swift hosted with ❤ by GitHub

This example presents a way of how to create a simple CAEmitterLayer what only has one cell with predefined image. Here we can see that cell defines birthrate: (particles per second), scale and its variance, lifetime in seconds, velocity, and emission direction. Emission longitude and range define a cone in which particles are emitted: if range is zero, all the particles have the same direction (emissionLongitude) and if range is 2 * pi, then particles are emitted in all the directions.

CAEmitterLayer also several different emitter shapes: circle, cuboid, line, point, rectangle, sphere (checkout the example project down below). It should be noted those shapes are defined by emitterPosition, emitterDepth, emitterSize, emitterZPosition and therefore, if you want to move emission location, update emitterPosition, not layer’s position.

Adding emitter layer

On iOS every UIView is backed by an instance of CALayer. As CAEmitterLayer is also a layer then adding it to view hierarchy is as simple as adding a sublayer to view’s backing layer.

	override func viewDidLoad() {
	super.viewDidLoad()
	emitterView.layer.addSublayer({
	guard let image = UIImage(named: "Spark")?.cgImage else { fatalError("Failed loading image.") }
	let emitterLayer = CAEmitterLayer(image: image)
	emitterLayer.emitterPosition = emitterView.bounds.center
	emitterLayer.name = "Emitter"
	return emitterLayer
	}())
	}
	extension CGRect {
	var center: CGPoint {
	return CGPoint(x: midX, y: midY)
	}
	}

view raw ViewController.swift hosted with ❤ by GitHub

Summary

CAEmitterLayer is an efficient way of adding particle systems to any app. Setting up emitter layer required to define particle(s) and then adding the CAEmitterLayer to the view hierarchy. With a little bit of code, we can add playful touch to any app. Only if I could design an awesome looking particle emitter. 🤔

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Example project

CAEmitterLayer (GitHub), Xcode 10.1, Swift 4.2

Resources

• CAEmitterLayer (Apple)
• CAEmitterCell (Apple)