BottomSheetTransition.swift

class BottomSheetTransition: UIPercentDrivenInteractiveTransition {
    private var dismissalAnimator: UIViewPropertyAnimator?
    
    private var presentationAnimator: UIViewPropertyAnimator?
        
    private let animationDuration = 0.5
    
    private let dampingRatio: Double = 0.9
    
    var dismissFractionComplete: CGFloat {
      dismissalAnimator?.fractionComplete ?? .zero
    }
  
    var presenting = true
}


extension BottomSheetTransition: UIViewControllerAnimatedTransitioning {
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        animationDuration
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        let animator = interruptibleAnimator(using: transitionContext)
        animator.startAnimation()
    }
    
    func interruptibleAnimator(using transitionContext: UIViewControllerContextTransitioning) -> UIViewImplicitlyAnimating {
        presenting ? presentationAnimator(using: transitionContext) : dismissAnimator(using: transitionContext)
    }
        
    private func presentationAnimator(
        using transitionContext: UIViewControllerContextTransitioning
    ) -> UIViewImplicitlyAnimating {
        guard let presentationAnimator else {
            guard
                let toViewController = transitionContext.viewController(forKey: .to),
                let toView = transitionContext.view(forKey: .to)
            else {
                return UIViewPropertyAnimator()
            }
            
            let animator = UIViewPropertyAnimator(
                duration: transitionDuration(using: transitionContext),
                dampingRatio: dampingRatio
            )
            
            presentationAnimator = animator
            
            toView.frame = transitionContext.finalFrame(for: toViewController)
            toView.frame.origin.y = transitionContext.containerView.frame.maxY
            transitionContext.containerView.addSubview(toView)
            
            animator.addAnimations {
                toView.frame = transitionContext.finalFrame(for: toViewController)
            }
            
            animator.addCompletion { [weak self] position in
                self?.presentationAnimator = nil
                
                guard case .end = position else {
                    transitionContext.completeTransition(false)
                    return
                }
                
                transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
            }
            
            return animator
        }
        
        return presentationAnimator
    }
    
    private func dismissAnimator(
        using transitionContext: UIViewControllerContextTransitioning
    ) -> UIViewImplicitlyAnimating {
        
        guard let dismissAnimator = self.dismissalAnimator else {
            guard let fromView = transitionContext.view(forKey: .from) else {
                return UIViewPropertyAnimator()
            }
            
            let animator = UIViewPropertyAnimator(
                duration: transitionDuration(using: transitionContext),
                dampingRatio: dampingRatio
            )
            
            dismissalAnimator = animator
            
            animator.addAnimations {
                fromView.frame.origin.y = fromView.frame.maxY
            }
            
            animator.addCompletion { [weak self] position in
                self?.dismissalAnimator = nil
                
                guard case .end = position else {
                    transitionContext.completeTransition(false)
                    return
                }
                
                fromView.removeFromSuperview()
                
                transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
            }
          
            return animator
        }
        
        return dismissAnimator
    }
}