SliderView.swift

//
//  CustomSlider.swift
//  Custom SliderView using SwiftUI
//
//  Created by Costantino Pistagna on 02/07/2020.
//

import SwiftUI

struct ContentView: View {
    @State var sliderValue: Float = 0.0
    var body: some View {
        ZStack {
            Color.init(white: 0.97).edgesIgnoringSafeArea(.all)
            VStack {
                Text(String(format: "%.2f", sliderValue)).fontWeight(.semibold).font(.title2).foregroundColor(.blue)
                    .shadow(color: Color.init(white: 1), radius: 4, x: -2, y: -2)
                    .shadow(color: Color.init(white: 0.9), radius: 4, x: 2, y: 2)
                AnyView(SliderView(value: $sliderValue)
                    .shadow(color: Color.init(white: 1), radius: 5, x: -5, y: -5)
                    .shadow(color: Color.init(white: 0.9), radius: 5, x: 5, y: 5))
                    .padding(.all, 30)
                Slider(value: $sliderValue).padding(.all, 30)

            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

struct SliderView: UIViewRepresentable {
    var value: Binding<Float>
    var minimumTrackTintColor = Color(white: 0.93)
    var maximumTrackTintColor = Color(white: 0.96)
    var thumbImage: UIImage?
    var barHeight: Float = 10

    func makeUIView(context: Context) -> some UIView {
        let aView = SASlider() { (newValue) in
            value.wrappedValue = newValue
        }
        aView.minimumTrackTintColor = UIColor(minimumTrackTintColor)
        aView.maximumTrackTintColor = UIColor(maximumTrackTintColor)
        if var thumbImage = thumbImage, thumbImage.size.width == thumbImage.size.height {
            if thumbImage.size.width > 30 {
                let newSize = CGSize(width: 30.0, height: 30.0)
                UIGraphicsBeginImageContextWithOptions(newSize, false, 0)
                defer { UIGraphicsEndImageContext() }
                thumbImage.draw(in: CGRect(origin: .zero, size: newSize))
                thumbImage = UIGraphicsGetImageFromCurrentImageContext() ?? thumbImage
            }
            for state: UIControl.State in [.normal, .selected, .application, .reserved] {
                aView.setThumbImage(thumbImage, for: state)
            }
        }
        return aView
    }
    
    func updateUIView(_ uiView: UIViewType, context: Context) {
        if let slider = uiView as? UISlider {
            slider.value = value.wrappedValue
        }
    }
    
    private class SASlider:UISlider {
        var barHeight:CGFloat = 10.0
        private var completionHandler: ((Float)->Void)?
        
        convenience init(customHeight: Float? = nil, _ completion:((Float)->Void)? = nil) {
            self.init()
            barHeight = CGFloat(customHeight ?? 10.0)
            self.completionHandler = completion
            self.addTarget(self, action: #selector(sliderDidChangeValue(sender:)), for: .valueChanged)
        }
        @objc func sliderDidChangeValue(sender: UISlider) {
            completionHandler?(sender.value)
        }
        override open func trackRect(forBounds bounds: CGRect) -> CGRect {
            let defaultBounds = super.trackRect(forBounds: bounds)
            return CGRect(
                x: defaultBounds.origin.x,
                y: defaultBounds.origin.y + defaultBounds.size.height/2 - barHeight/2,
                width: defaultBounds.size.width,
                height: barHeight
            )
        }
    }

}