Fix detent snapping

SheetImagePicker/SheetContainerViewController.swift

@@ -14,13 +14,17 @@ public class SheetContainerViewController: UIViewController {
     
     public var detents: [Detent] = [.bottom, .middle, .top] {
         didSet {
-            
+            sortDetents()
         }
     }
+    var sortedDetentOffsets: [CGFloat] = []
 
     var topConstraint: NSLayoutConstraint!
     lazy var initialConstant: CGFloat = view.bounds.height / 2
     
+    public var minimumDetentJumpVelocity: CGFloat = 500
+    public var maximumStretchDistance: CGFloat = 15
+    
     public init(content: UIViewController) {
         self.content = content
         
@@ -49,6 +53,18 @@ public class SheetContainerViewController: UIViewController {
         content.view.addGestureRecognizer(panGesture)
     }
     
+    public override func viewDidLayoutSubviews() {
+        super.viewDidLayoutSubviews()
+        
+        sortDetents()
+    }
+    
+    private func sortDetents() {
+        sortedDetentOffsets = detents.map {
+            $0.offset(in: view)
+        }.sorted()
+    }
+    
     @objc func panGestureRecognized(_ recognizer: UIPanGestureRecognizer) {
         switch recognizer.state {
         case .began:
@@ -57,31 +73,62 @@ public class SheetContainerViewController: UIViewController {
         case .changed:
             let translation = recognizer.translation(in: content.view)
             var realOffset = initialConstant + translation.y
-            if realOffset < view.safeAreaInsets.top {
-                print(realOffset)
-                realOffset = view.safeAreaInsets.top - realOffset / pow(CGFloat(M_E), realOffset / view.safeAreaInsets.top)
+            if realOffset < sortedDetentOffsets.first! {
+                func clamp(_ value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
+                    if value < from {
+                        return from
+                    } else if value > to {
+                        return to
+                    } else {
+                        return value
+                    }
+                }
+                func smoothstep(value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
+                    let x = clamp((value - from) / (to - from), from: 0, to: 1)
+                    // 3x^2 - 2x^3
+                    return 3 * pow(x, 2) - 2 * pow(x, 3)
+                }
+                let topOffset = sortedDetentOffsets.first!
+                let smoothed = smoothstep(value: realOffset, from: topOffset, to: 0)
+                realOffset = topOffset - smoothed * maximumStretchDistance
+                
             }
             topConstraint.constant = realOffset
             
         case .ended:
-            if let offset = nearestDetentOffset(offset: topConstraint.constant) {
-                let distance = abs(topConstraint.constant - offset)
-                self.topConstraint.constant = offset
             let velocity = recognizer.velocity(in: view)
-                let springVelocity = velocity.y / distance
-                UIView.animate(withDuration: 0.5, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: springVelocity, animations: {
-                    self.view.layoutIfNeeded()
-                })
+
+            let springToDetent: CGFloat
+            if abs(velocity.y) > minimumDetentJumpVelocity,
+                let offsetInVelocityDirection = nearestDetentOffset(currentOffset: topConstraint.constant, direction: velocity.y) {
+                springToDetent = offsetInVelocityDirection
+            } else if let nearestOffset = nearestDetentOffset(currentOffset: topConstraint.constant) {
+                springToDetent = nearestOffset
+            } else {
+                return
             }
             
+            let springDistance = abs(topConstraint.constant - springToDetent)
+            self.topConstraint.constant = springToDetent
+            let springVelocity = velocity.y / springDistance
+            
+            UIView.animate(withDuration: 0.35, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: springVelocity, animations: {
+                self.view.layoutIfNeeded()
+            })
         default:
             return
         }
     }
     
-    func nearestDetentOffset(offset: CGFloat) -> CGFloat? {
-        return detents.map { $0.offset(in: view) }.min { (a, b) -> Bool in
-            return abs(offset - a) < abs(offset - b)
+    func nearestDetentOffset(currentOffset: CGFloat) -> CGFloat? {
+        return sortedDetentOffsets.min(by: { abs($0 - currentOffset) < abs($1 - currentOffset) })
+    }
+    
+    func nearestDetentOffset(currentOffset: CGFloat, direction: CGFloat) -> CGFloat? {
+        if direction < 0 {
+            return sortedDetentOffsets.last(where: { $0 < currentOffset })
+        } else {
+            return sortedDetentOffsets.first(where: { $0 > currentOffset })
         }
     }