DirectionalPanGestureRecognizer.swift

enum PanAxis {
    case none
    case horizontal
    case vertical
}

/// Returns only translation in either horizontal or vertical direction determined at `UIGestureRecognizer.State.began`.
class DirectionalPanGestureRecognizer: UIPanGestureRecognizer {
    var axis = PanAxis.none
    var directionalTranslation: CGPoint {
        let translation = self.translation(in: view)
        switch axis {
        case .horizontal:
            return CGPoint(x: translation.x, y: 0)
        case .vertical:
            return CGPoint(x: 0, y: translation.y)
        case .none:
            return .zero
        }
    }
    var directionalVelocity: CGPoint {
        let velocity = self.velocity(in: view)
        switch axis {
        case .horizontal:
            return CGPoint(x: velocity.x, y: 0)
        case .vertical:
            return CGPoint(x: 0, y: velocity.y)
        case .none:
            return .zero
        }
    }

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesMoved(touches, with: event)
        guard let view = self.view else { return }
        if state == .began {
            let velocity = self.velocity(in: view)
            axis = abs(velocity.x) > abs(velocity.y) ? .horizontal : .vertical
        }
    }
}