SimpleDragStuff.swift

//
//  Created by Adam Whitcroft on 2023-01-23.
//

import SwiftUI

struct SimpleDragGesture: View {
    @State private var focussedItem: String = "middle"
    @State private var offset = CGSize.zero
    @State private var accumulatedOffset = CGSize.zero
    // constants
    let cardSize: CGFloat = 60
    let springAnimation: Animation = .interpolatingSpring(stiffness: 400, damping: 20)
    
    var body: some View {
        VStack {
            HStack(spacing: cardSize) {
                Rectangle()
                    .fill(focussedItem == "left" ? .red : .gray)
                    .frame(width: cardSize, height: cardSize)
                
                Rectangle()
                    .fill(focussedItem == "middle" ? .red : .gray)
                    .frame(width: cardSize, height: cardSize)
                
                Rectangle()
                    .fill(focussedItem == "right" ? .red : .gray)
                    .frame(width: cardSize, height: cardSize)
            }
            .offset(x: offset.width)
            .contentShape(Rectangle())
            .gesture (
                DragGesture()
                    .onChanged { gesture in
                        withAnimation(springAnimation) {
                            offset = CGSize(
                                width: gesture.translation.width + self.accumulatedOffset.width,
                                height: gesture.translation.height + self.accumulatedOffset.height
                            )
                            // clamp overdrag left
                            if offset.width > cardSize * 2.5 {
                                offset.width = cardSize * 2.5
                            }
                            // clamp overdrag right
                            if offset.width < -cardSize * 2.5 {
                                offset.width = -cardSize * 2.5
                            }
                        }
                    }
                    .onEnded { gesture in
                        // left item
                        if offset.width > cardSize {
                            focussedItem = "left"
                            withAnimation(springAnimation) {
                                offset.width = cardSize * 2
                            }
                            accumulatedOffset.width = offset.width
                        }
                        // right item
                        else if offset.width < -cardSize {
                            focussedItem = "right"
                            withAnimation(springAnimation) {
                                offset.width = -cardSize * 2
                            }
                            accumulatedOffset.width = offset.width
                        }
                        // middle
                        else {
                            focussedItem = "middle"
                            withAnimation(springAnimation) {
                                offset = .zero
                            }
                            accumulatedOffset.width = offset.width
                        }
                    }
            )
        }
    }
}

struct SimpleDragGesture_Previews: PreviewProvider {
    static var previews: some View {
        SimpleDragGesture()
    }
}