Allow scroll view to bounce while scrolled all the way down

Sources/SheetController/SheetContainerViewController.swift

@@ -8,7 +8,7 @@
 
 import UIKit
 
-public protocol SheetContainerViewControllerDelegate {
+public protocol SheetContainerViewControllerDelegate: class {
     func sheetContainer(_ sheetContainer: SheetContainerViewController, willSnapToDetent detent: Detent) -> Bool
     func sheetContainer(_ sheetContainer: SheetContainerViewController, didSnapToDetent detent: Detent)
     func sheetContainerContentScrollView(_ sheetContainer: SheetContainerViewController) -> UIScrollView?
@@ -29,9 +29,9 @@ public extension SheetContainerViewControllerDelegate {
     }
 }
 
-public class SheetContainerViewController: UIViewController {
+open class SheetContainerViewController: UIViewController {
 
-    public var delegate: SheetContainerViewControllerDelegate?
+    public weak var delegate: SheetContainerViewControllerDelegate?
     
     public let content: UIViewController
     
@@ -72,7 +72,7 @@ public class SheetContainerViewController: UIViewController {
         fatalError("init(coder:) has not been implemented")
     }
 
-    override public func viewDidLoad() {
+    override open func viewDidLoad() {
         super.viewDidLoad()
         
         dimmingView = UIView()
@@ -136,16 +136,16 @@ public class SheetContainerViewController: UIViewController {
         let velocity = recognizer.velocity(in: scrollView)
 
         let topContentOffset: CGFloat = delegate?.sheetContainer(self, topContentOffsetForScrollView: scrollView) ?? 0
-        let shouldMoveSheetDown = scrollView.contentOffset.y <= topContentOffset && velocity.y > 0 // scrolled to top and dragging down
+        let shouldMoveSheetDown = scrollView.contentOffset.y <= -topContentOffset && velocity.y > 0 // scrolled to top and dragging down
         let shouldMoveSheetUp = topConstraint.constant > topDetent.offset && velocity.y < 0 // not fully expanded and dragging up
 
         let shouldMoveSheet = shouldMoveSheetDown || shouldMoveSheetUp
 
         switch recognizer.state {
         case .began:
-            scrollView.bounces = false
             scrollViewIsMovingSheet = shouldMoveSheet
             if shouldMoveSheet {
+                scrollView.bounces = false
                 initialScrollViewContentOffset = scrollView.contentOffset
             }
             
@@ -154,6 +154,10 @@ public class SheetContainerViewController: UIViewController {
             if scrollViewIsMovingSheet {
                 if initialScrollViewContentOffset == nil {
                     initialScrollViewContentOffset = scrollView.contentOffset
+                    
+                    // if the sheet begins moving part way through the gesture, we need to ignore any translation
+                    // that's already taken place, otherwise the sheet will jump when it starts moving
+                    recognizer.setTranslation(.zero, in: scrollView)
                 }
                 scrollView.setContentOffset(initialScrollViewContentOffset!, animated: false)
                 
@@ -163,8 +167,8 @@ public class SheetContainerViewController: UIViewController {
             }
 
         case .ended:
-            scrollView.bounces = true
             if scrollViewIsMovingSheet {
+                scrollView.bounces = true
                 scrollView.setContentOffset(initialScrollViewContentOffset!, animated: false)
                 springToNearestDetent(verticalVelocity: velocity.y)
             }