Maintain velocity when dismissing sheet

Sources/SheetController/SheetContainerDismissAnimationController.swift

@@ -32,15 +32,28 @@ class SheetContainerDismissAnimationController: NSObject, UIViewControllerAnimat
         container.addSubview(fromVC.view)
 
         let duration = transitionDuration(using: transitionContext)
-        UIView.animate(withDuration: duration, animations: {
+        let animator: UIViewPropertyAnimator
+        
+        if let initialVelocity = fromVC.dismissAnimationInitialVelocity {
+            let vector = CGVector(dx: initialVelocity, dy: 0)
+            let parameters = UISpringTimingParameters(dampingRatio: 1, initialVelocity: vector)
+            animator = UIViewPropertyAnimator(duration: duration, timingParameters: parameters)
+        } else {
+            animator = UIViewPropertyAnimator(duration: duration, curve: .easeOut)
+        }
+        
+        animator.addAnimations {
             dimmingView.frame = container.bounds
             dimmingView.alpha = 0
             fromVC.view.transform = CGAffineTransform(translationX: 0, y: fromVC.content.view.bounds.height)
-        }, completion: { (finished) in
+        }
+        animator.addCompletion { (position) in
             dimmingView.removeFromSuperview()
 
             transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
-        })
+        }
+        
+        animator.startAnimation()
     }
     
 }

Sources/SheetController/SheetContainerViewController.swift

@@ -59,6 +59,9 @@ open class SheetContainerViewController: UIViewController {
     var initialScrollViewContentOffset: CGPoint?
     var scrollViewIsMovingSheet = false
 
+    public var dismissAtBottomDetent = true
+    var dismissAnimationInitialVelocity: CGFloat?
+    
     public init(content: UIViewController) {
         self.content = content
         
@@ -204,10 +207,22 @@ open class SheetContainerViewController: UIViewController {
             return
         }
         
+        let springVelocity: CGFloat
+        let springDistance = abs(topConstraint.constant - springToDetent.1)
+        if springDistance != 0 {
+            springVelocity = velocity / springDistance
+        } else {
+            springVelocity = 0
+        }
+
+        if dismissAtBottomDetent, springToDetent.0 == .bottom {
+            dismissAnimationInitialVelocity = springVelocity
+            dismiss(animated: true)
+            return
+        }
+        
         if delegate?.sheetContainer(self, willSnapToDetent: springToDetent.0) ?? true {
-            let springDistance = abs(topConstraint.constant - springToDetent.1)
             self.topConstraint.constant = springToDetent.1
-            let springVelocity = velocity / springDistance
             
             UIView.animate(withDuration: 0.35, delay: 0, usingSpringWithDamping: 0.75, initialSpringVelocity: springVelocity, animations: {
                 self.view.layoutIfNeeded()