SwiftUI Flip transition

AnyTransition+flip.swift

extension AnyTransition {
    static var flip: AnyTransition {
        .modifier(
            active: FlipEffect(flip: .active, animatableData: 1),
            identity: FlipEffect(flip: .identity, animatableData: 0)
        )
    }
}

private struct FlipEffect: GeometryEffect {

    enum Flip {
        case active, identity
    }

    let flip: Flip
    var animatableData: Double

    func effectValue(size: CGSize) -> ProjectionTransform {
        var rotate = CATransform3DIdentity
        rotate.m34 = -1 / max(size.width, size.height)
        rotate = CATransform3DRotate(rotate,  Angle(degrees: 180).radians * rotation, 1, 0, 0)
        rotate = CATransform3DTranslate(rotate, size.width / -2, size.height / -2, 0)

        let translate = CGAffineTransform(
            translationX: size.width / 2,
            y: size.height / 2
        )

        return .init(rotate).concatenating(.init(translate))
    }

    private var rotation: Double {
        switch flip {
        case .active: return animatableData <= 0.5 ? animatableData : 0.5
        case .identity: return animatableData < 0.5 ? -animatableData : 0.5
        }
    }
}