A view for your iOS app to draw some falling snow on top of your content. :)

Snow.swift

//
//  Snow.swift
//
//  Created by Mathijs Bernson on 05/12/2021.
//

import UIKit
import SwiftUI

/// A transparent view that displays falling snow on top of itself.
struct Snow: UIViewRepresentable {
  func makeUIView(context: Context) -> UIView {
    SnowView(frame: .zero)
  }

  func updateUIView(_ view: UIView, context: Context) { }
}

/// A transparent view that displays falling snow on top of itself.
class SnowView: UIView {
  private let emitterLayer = CAEmitterLayer()

  private var snowflake: CGImage? {
    let size = CGSize(width: 40, height: 40)
    let renderer = UIGraphicsImageRenderer(size: size)
    let img = renderer.image { ctx in
      let emoji = NSAttributedString(string: "❄️", attributes: [
        .font: UIFont.systemFont(ofSize: 40)
      ])
      emoji.draw(in: CGRect(origin: .zero, size: size))
    }
    return img.cgImage
  }

  override init(frame: CGRect) {
    super.init(frame: frame)

    isUserInteractionEnabled = false

    let cell = CAEmitterCell()
    cell.birthRate = 30
    cell.lifetime = 20
    cell.velocity = 50
    cell.scale = 0.1
    cell.emissionLongitude = CGFloat.pi
    cell.emissionRange = CGFloat.pi / 2.0
    cell.contents = snowflake
    cell.spinRange = 5

    emitterLayer.emitterCells = [cell]
    emitterLayer.emitterShape = .line

    layer.addSublayer(emitterLayer)
    updateEmitterPosition()
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  private func updateEmitterPosition() {
    emitterLayer.frame = frame
    emitterLayer.emitterSize = CGSize(width: frame.width * 2, height: 0)
    emitterLayer.emitterPosition = CGPoint(x: frame.width / 2, y: frame.minY)
  }

  override func layoutSubviews() {
    super.layoutSubviews()
    updateEmitterPosition()
  }

  override func hitTest(_ point: CGPoint, with event: UIEvent?) -> UIView? {
    let view = super.hitTest(point, with: event)
    return view == self ? nil : view
  }
}

struct Snow_Previews: PreviewProvider {
  static var previews: some View {
    Snow()
      .ignoresSafeArea()
  }
}