//
//  ContentView.swift
//  WonkyDragAnimationDemo
//
//  Created by Michelle Ellis on 2023-07-25.
//

import SwiftUI

struct ContentView: View {

    // 💰💰💰💰💰
    // The problem is the `.speed(2)` modifier on this animation.
    // If we omit the speed modifier, animations behave as expected.
    // With it, at the end of the animation, unpredictable things happen to the view's offset.

    // Good:
//        private static var defaultAnimation = { Animation.spring() }()

    // Bad:
    private static var defaultAnimation = { Animation.spring().speed(2) }()


    // Drag gesture stuff:

    // The offset from start of finger drag to current position.
    // Resets to (0,0) on end or cancel.
    @GestureState private var gestureDragOffset: CGSize = .zero

    // The *last* value received from onChanged.
    // We need this seperately from dragOffset because it has goodness like predictedEndLocation (which we use in our real app, not this demo)
    @State var lastDragUpdate: DragGesture.Value?

    @State var animation: Animation? = Self.defaultAnimation
    private static var defaultOffset: CGFloat = 100.0

    @State private var offset: CGFloat = Self.defaultOffset

    var body: some View {
        ZStack(alignment: .bottom) {
            VStack(spacing: 0) {
                Text("Hello, wonky drag animation!")
                Spacer()
            }
            .background(.purple)
            .offset(y: offset)
            .animation(animation, value: self.offset)
        }
        .gesture(
            DragGesture()
                .updating($gestureDragOffset) { value, state, transaction in
                    state = value.translation
                }
                .onChanged({ (value) in
                    lastDragUpdate = value
                })
        )
        .onChange(of: self.gestureDragOffset) { _, newOffset in
            if let lastDragUpdate {
                if self.gestureDragOffset == .zero {
                    self.lastDragUpdate = nil
                    self.dragEnded()
                }
                else {
                    self.dragChanged(lastDragUpdate)
                }
            } else {
                self.dragEnded()
            }
        }
    }

    // MARK: Drag Gesture
    func dragChanged(_ value: DragGesture.Value) {
        animation = nil
        offset = -1 * (value.startLocation.y - value.location.y - Self.defaultOffset)
    }

    func dragEnded() {
        offset = Self.defaultOffset
        animation = Self.defaultAnimation
    }
}