Source for part 3 of large sticky toggles CodeSlicing episode

StickyTogglePhase3.swift

//
//  StickyTogglePhase3.swift
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
//  of the Software, and to permit persons to whom the Software is furnished to do so,
//  subject to the following conditions:
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
//  INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
//  PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
//  COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
//  AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
//  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//
//  Created by Adam Fordyce on 21/12/2020.
//  Copyright © 2020 Adam Fordyce. All rights reserved.
//

import SwiftUI
import PureSwiftUI

private let toggleLayoutConfig = LayoutGuideConfig.grid(columns: 10, rows: 10)

private struct StickyToggle: View {
    
    let size: CGFloat
    let stickyThreshold: CGFloat
    
    @State private var stuck = true
    @State private var atTop = true
    @State private var potentiallyAtTop = true
    @State private var yTranslation = CGFloat.zero
    @State private var halfViewHeight = CGFloat.zero
    @State private var offsetAtBottom = CGFloat.zero
    
    private var validTranslation: Bool {
        (atTop && yTranslation > 0) || (!atTop && yTranslation < 0)
    }
    
    private var absTranslation: CGFloat {
        abs(yTranslation)
    }
    
    private var offsetForTranslation: CGFloat {
        guard validTranslation else {
            return 0
        }
        
        if stuck {
            return yTranslation / 2
        } else {
            return yTranslation
        }
    }
    
    private var scaleForTranslation: CGFloat {
        guard stuck && validTranslation else {
            return 1
        }
        
        return 1 + absTranslation / size
    }
    
    private var stretchFactor: CGFloat {
        guard stuck && validTranslation else {
            return 0
        }
        
        let distanceToThreshold = stickyThreshold - absTranslation
        let normalisedDistanceToThreshold = distanceToThreshold / stickyThreshold
        
        return 1 - normalisedDistanceToThreshold
    }
    
    var body: some View {
        let designing = false
        let debug = false
        GeometryReader { (geo: GeometryProxy) in
            ZStack {
                VStack {
                    StretchableSquare(stretchFactor: stretchFactor, atTop: atTop, designing: designing)
                        .toggleStyle(color: potentiallyAtTop ? .green : .red, designing: designing)
                        .frame(size)
                        .yScale(scaleForTranslation)
                        .yOffset(offsetForTranslation)
                        .yOffsetIfNot(atTop, offsetAtBottom)
                        .shadowIfNot(designing, radius: 10)
                        .layoutGuide(toggleLayoutConfig)
                        .showLayoutGuides(designing)
                        .gesture(DragGesture(minimumDistance: 0)
                                    .onChanged(onChanged)
                                    .onEnded(onEnded))
                        Spacer()
                }
                .onAppear {
                    halfViewHeight = geo.heightScaled(0.5)
                    offsetAtBottom = geo.height - size
                }
                if debug && !designing {
                    VStack {
                        TitleText("atTop: \(atTop)")
                        TitleText("potentiallyAtTop: \(potentiallyAtTop)")
                        TitleText("stuck: \(stuck)")
                    }
                    .animation(nil)
                }
            }
            .greedyFrame()
        }
    }
    
    private func inTopHalf(_ gesture: DragGesture.Value) -> Bool {
        gesture.location.y <= halfViewHeight
    }
    
    private func onChanged(_ gesture: DragGesture.Value) {
        yTranslation = gesture.translation.y
        if stuck {
            stuck = absTranslation < stickyThreshold
        }
        potentiallyAtTop = inTopHalf(gesture)
    }
    
    private func onEnded(_ gesture: DragGesture.Value) {
        yTranslation = .zero
        stuck = true
        atTop = inTopHalf(gesture)
    }
}

private extension Shape {
    @ViewBuilder func toggleStyle(color: Color, designing: Bool = false) -> some View {
        if designing {
            stroke(Color.black, style: .init(lineWidth: 2, lineCap: .round, lineJoin: .round))
        } else {
            fill(color)
        }
    }
}

private struct StretchableSquare: Shape {
    
    private let atTop: Bool
    private let designing: Bool
    var animatableData: CGFloat
    
    init(stretchFactor: CGFloat, atTop: Bool, designing: Bool = false) {
        animatableData = stretchFactor
        self.atTop = atTop
        self.designing = designing
    }
    
    func path(in rect: CGRect) -> Path {
        var path = Path()
        
        let g = toggleLayoutConfig.layout(in: rect)
            .yScaled(-1, factor: atTop || designing ? 0 : 1)

        let p1 = g.topLeading
        let p2 = g.topTrailing
        let cpToP3 = g[10, 1].to(g[6, 1], animatableData)
        
        let p3 = g.bottomTrailing.to(g[6, 10], animatableData)
        let p4 = g.bottomLeading.to(g[4, 10], animatableData)
        let cpToP1 = g[0, 1].to(g[4, 1], animatableData)
        
        path.move(p1)
        path.line(p2)
        
        path.quadCurve(p3, cp: cpToP3, showControlPoints: designing)
        
        path.line(p4)
        
        path.quadCurve(p1, cp: cpToP1, showControlPoints: designing)
        
        return path
    }
}

struct StickyTogglePhase3_Previews: PreviewProvider {
    struct StickyTogglePhase3_Harness: View {

        var body: some View {
            GeometryReader { (geo: GeometryProxy) in
                StickyToggle(size: geo.widthScaled(0.3), stickyThreshold: geo.heightScaled(0.3))
            }
            .ignoresSafeArea()
        }
    }

    static var previews: some View {
        StickyTogglePhase3_Harness()
            .previewDevice(.iPhone_8_Plus)
    }
}