Card Flip Animation with a drag.

ContentView.swift

import SwiftUI

struct ViewContainer: View {
    
    var flipped: Bool
    var isReversed: Bool
    
    var body: some View {
        if flipped {
            ZStack {
                RoundedRectangle(cornerRadius: 20, style: .continuous)
                    .fill(.blue)
                
                Text("Number 1")
                    .font(.largeTitle)
            }
            .rotation3DEffect(.degrees(isReversed ? 180 : 0), axis: (x: 0, y: 1, z: 0))
        } else {
            ZStack {
                RoundedRectangle(cornerRadius: 20, style: .continuous)
                    .fill(.green)
                
                Text("Darn, it works just like that.")
            }
            .rotation3DEffect(.degrees(!flipped && !isReversed ? 180 : 0), axis: (x: 0, y: 1, z: 0))
        }
    }
}

struct ContentView: View {
    @State private var flipped = false
    @State private var startFlipped = false
    
    @State private var dragAmount: Double = 0
    let animationDuration = 0.4
    @State private var isReversed = true
    
    var body: some View {
        ViewContainer(flipped: flipped, isReversed: isReversed)
            .frame(width: 300, height: 300, alignment: .center)
            .rotation3DEffect(.degrees(dragAmount), axis: (x: 0, y: 1, z: 0))
            .gesture(
                DragGesture()
                    .onChanged({ dragValue in
                        if !flipped && !startFlipped {
                            isReversed = true
                        }
                        
                        dragAmount = dragValue.translation.width
                        if abs(dragAmount.truncatingRemainder(dividingBy: 270)) > 90 {
                            flipped = startFlipped ? false : true
                        } else {
                            flipped = startFlipped ? true : false
                        }
                    })
                    .onEnded({ dragValue in
                        withAnimation(.easeOut(duration: animationDuration)) {
                            if dragAmount > 270 {
                                dragAmount = 360
                                startFlipped = false
                            } else if dragAmount < -270 {
                                dragAmount = -360
                                startFlipped = false
                            } else if dragAmount > 90 {
                                dragAmount = 180
                                startFlipped = !startFlipped
                            } else if dragAmount < -90 {
                                dragAmount = -180
                                startFlipped = !startFlipped
                            } else {
                                dragAmount = 0
                            }
                        }
                        
                        // That's the hack. Putting it to 0 as quiet as leaving the room after the baby sleeps.
                        DispatchQueue.main.asyncAfter(deadline: .now() + animationDuration) {
                            dragAmount = 0
                            if startFlipped && flipped {
                                isReversed = false
                            } else if !flipped && !isReversed && !startFlipped {
                                isReversed = true
                            }
                        }
                    })
            )
    }
}

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