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1import SwiftUI
2
3struct Particle: Identifiable {
4    let id = UUID()
5    var position: CGPoint
6    var velocity: CGVector
7    var hue: Double
8}
9
10@Observable
11final class ParticleField {
12    private(set) var particles: [Particle] = []
13    var size: CGSize = .zero
14
15    func populate(_ count: Int) {
16        particles = (0..<count).map { _ in
17            Particle(
18                position: .random(in: size),
19                velocity: .init(dx: .random(in: -50...50),
20                                dy: .random(in: -50...50)),
21                hue: .random(in: 0...1)
22            )
23        }
24    }
25
26    func advance(by dt: Double) {
27        for i in particles.indices {
28            particles[i].position.x += particles[i].velocity.dx * dt
29            particles[i].position.y += particles[i].velocity.dy * dt
30            bounce(&particles[i])
31        }
32    }
33
34    private func bounce(_ p: inout Particle) {
35        if p.position.x < 0 || p.position.x > size.width {
36            p.velocity.dx *= -1
37        }
38        if p.position.y < 0 || p.position.y > size.height {
39            p.velocity.dy *= -1
40        }
41    }
42}
43
44struct AuroraView: View {
45    @State private var field = ParticleField()
46
47    var body: some View {
48        TimelineView(.animation) { timeline in
49            Canvas { ctx, size in
50                field.size = size
51                for p in field.particles {
52                    let dot = Path(ellipseIn: .init(
53                        origin: p.position,
54                        size: .init(width: 6, height: 6)))
55                    ctx.fill(dot, with: .color(
56                        .init(hue: p.hue, saturation: 0.7, brightness: 1)))
57                }
58            }
59            .onChange(of: timeline.date) { _, _ in
60                field.advance(by: 1.0 / 60.0)
61            }
62        }
63        .task { field.populate(140) }
64        .background(.black)
65    }
66}