RainingParticleView.swift

  //
//  RainingParticleView.swift
//  Provider
//
//  Created by Bryan Irace on 1/26/17.
//
//

import ServiceKit
import UIKit
import QuartzCore

struct ParticleDescriptor {
    let image: UIImage
    let color: UIColor?
}

public enum ParticleType {
    case confetti
    case money
    
    func descriptors() -> [ParticleDescriptor] {
        switch self {
        case .confetti:
            return [
                .confettiPurple,
                .confettiRed,
                .confettiBlue,
                .confettiOrange,
            ].map({ colorName in
                ParticleDescriptor(image: UIImage(asset: .confetti), color: UIColor(named: colorName))
            })

        case .money:
            return [ParticleDescriptor(image: UIImage(asset: .cash), color: nil)]
        }
    }
}

/// A view that makes it look like money is raining down from the sky.
final class RainingParticleView: UIView {
    private enum Constants {
        static let emitterHeight: CGFloat = 1
        
        static let birthRate: Float = 120
        static let lifetime: Float = 10.5
        static let velocity: CGFloat = 262.5
        static let velocityRange: CGFloat = 60
        static let emissionLongitude = CGFloat(Double.pi)
        static let emissionRange = CGFloat(Double.pi/4)
        static let spin: CGFloat = 2.625
        static let spinRange: CGFloat = 3
        static let yAcceleration: CGFloat = 200
    }
    
    // MARK: - Mutable state
    
    private var rain = PerformOnce()
    
    // MARK: - Sublayers
    
    private let emitter: CAEmitterLayer
    
    init(frame: CGRect, type: ParticleType) {
        emitter = CAEmitterLayer().then {
            $0.beginTime = CACurrentMediaTime()
            $0.emitterPosition = CGPoint(x: frame.midX, y: 0)
            $0.emitterSize = CGSize(width: frame.size.width, height: Constants.emitterHeight)
            $0.emitterShape = kCAEmitterLayerLine
            $0.emitterCells = RainingParticleView.cells(from: type.descriptors())
        }
        
        super.init(frame: frame)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    // MARK: - Private
    
    private static func cells(from particleDescriptors: [ParticleDescriptor]) -> [CAEmitterCell] {
        return particleDescriptors.map({ descriptor in
            CAEmitterCell().then {
                $0.contents = descriptor.image.cgImage
                $0.birthRate = Constants.birthRate
                $0.lifetime = Constants.lifetime
                $0.velocity = Constants.velocity
                $0.velocityRange = Constants.velocityRange
                $0.emissionLongitude = Constants.emissionLongitude
                $0.emissionRange = Constants.emissionRange
                $0.spin = Constants.spin
                $0.spinRange = Constants.spinRange
                $0.yAcceleration = Constants.yAcceleration
                
                if let color = descriptor.color {
                    $0.color = color.cgColor
                }
            }
        })
    }
    
    // MARK: - Public

    /// Start raining down money. Won’t do anything if you call it more than once.
    func start() {
        rain.performOnce({
            layer.addSublayer(emitter)
        })
    }
    
    /// Stop producing new money.
    func stop() {
        emitter.birthRate = 0
    }
}