Exposes UIScrollViewDelegate to clients (#133)

*Have you read the [Contributing
Guidelines](https://github.com/jessesquires/.github/blob/master/CONTRIBUTING.md)?*
🖤

Closes #131 

## Describe your changes

- Allows clients to set a `weak var scrollViewDelegate:
UIScrollViewDelegate?` on `CollectionViewDriver`
- Implement the `UIScrollViewDelegate` methods in `CollectionViewDriver`
and forwards them to this delegate
- Keeps the method signatures in the same order and groupings as
`UIScrollViewDelegate`

### Example
- Adds example to `ListViewController`, demonstrating an override of
`scrollViewShouldScrollToTop(scrollView:)`

| Before | After |
| ------ | ----- |
|
![rf_131_before](https://github.com/user-attachments/assets/7d0dfd8b-1be2-4eef-8899-65503b783ed0)
|
![rf_131_after](https://github.com/user-attachments/assets/3ec172c8-ad56-47ce-8e23-89cf0828ab66)
|

## Todo

- Add unit tests.

---------

Co-authored-by: Jesse Squires <jessesquires@users.noreply.github.com>

CHANGELOG.md

@@ -14,6 +14,7 @@ NEXT
 - Reverted back to Swift 5 language mode because of issues in UIKit. ([@jessesquires](https://github.com/jessesquires), [#116](https://github.com/jessesquires/ReactiveCollectionsKit/pull/116))
 - Applying a snapshot using `reloadData` now always occurs on the main thread. ([@jessesquires](https://github.com/jessesquires), [#116](https://github.com/jessesquires/ReactiveCollectionsKit/pull/116))
 - Implemented additional selection APIs for `CellViewModel`: `shouldSelect`, `shouldDeselect`, `didDeselect()`. ([@nuomi1](https://github.com/nuomi1), [#127](https://github.com/jessesquires/ReactiveCollectionsKit/pull/127))
+- Allow setting a `UIScrollViewDelegate` object to receive scroll view events from the collection view. ([@ruddfawcett](https://github.com/ruddfawcett), [#131](https://github.com/jessesquires/ReactiveCollectionsKit/pull/131), [#133](https://github.com/jessesquires/ReactiveCollectionsKit/pull/133))
 
 0.1.6
 -----

Example/Sources/List/ListViewController.swift

@@ -17,12 +17,19 @@ import UIKit
 
 final class ListViewController: ExampleViewController, CellEventCoordinator {
 
-    lazy var driver = CollectionViewDriver(
-        view: self.collectionView,
-        options: .init(diffOnBackgroundQueue: true),
-        emptyViewProvider: sharedEmptyViewProvider,
-        cellEventCoordinator: self
-    )
+    lazy var driver: CollectionViewDriver = {
+        let driver = CollectionViewDriver(
+            view: self.collectionView,
+            options: .init(diffOnBackgroundQueue: true),
+            emptyViewProvider: sharedEmptyViewProvider,
+            cellEventCoordinator: self
+        )
+
+        // Access `UIScrollViewDelegate` and handle protocol
+        driver.scrollViewDelegate = self
+
+        return driver
+    }()
 
     override var model: Model {
         didSet {
@@ -131,3 +138,11 @@ final class ListViewController: ExampleViewController, CellEventCoordinator {
         return CollectionViewModel(id: "list_view", sections: [peopleSection, colorSection])
     }
 }
+
+extension ListViewController: UIScrollViewDelegate {
+
+    // Demonstrate delegate override; tapping status bar does not scroll to top
+    func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool {
+        false
+    }
+}

Sources/CollectionViewDriver.swift

@@ -31,6 +31,9 @@ public final class CollectionViewDriver: NSObject {
 
     /// The collection view model.
     @Published public private(set) var viewModel: CollectionViewModel
+
+    /// The scroll view delegate to forward.
+    public weak var scrollViewDelegate: UIScrollViewDelegate?
 
     private let _emptyViewProvider: EmptyViewProvider?
 
@@ -373,3 +376,96 @@ extension CollectionViewDriver: UICollectionViewDelegate {
         self.viewModel._safeSupplementaryViewModel(ofKind: elementKind, at: indexPath)?.didEndDisplaying()
     }
 }
+
+// MARK: UIScrollViewDelegate
+
+extension CollectionViewDriver: UIScrollViewDelegate {
+    // MARK: Managing offset and zoom scale changes
+
+    /// :nodoc:
+    public func scrollViewDidScroll(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewDidScroll?(scrollView)
+    }
+
+    /// :nodoc:
+    public func scrollViewDidZoom(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewDidZoom?(scrollView)
+    }
+
+    // MARK: Tracking dragging
+
+    /// :nodoc:
+    public func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewWillBeginDragging?(scrollView)
+    }
+
+    /// :nodoc:
+    public func scrollViewWillEndDragging(_ scrollView: UIScrollView,
+                                          withVelocity velocity: CGPoint,
+                                          targetContentOffset: UnsafeMutablePointer<CGPoint>) {
+        self.scrollViewDelegate?.scrollViewWillEndDragging?(scrollView, withVelocity: velocity, targetContentOffset: targetContentOffset)
+    }
+
+    /// :nodoc:
+    public func scrollViewDidEndDragging(_ scrollView: UIScrollView,
+                                         willDecelerate decelerate: Bool) {
+        self.scrollViewDelegate?.scrollViewDidEndDragging?(scrollView, willDecelerate: decelerate)
+    }
+
+    // MARK: Tracking deceleration and scrolling animation
+
+    /// :nodoc:
+    public func scrollViewWillBeginDecelerating(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewWillBeginDragging?(scrollView)
+    }
+
+    /// :nodoc:
+    public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewDidEndDecelerating?(scrollView)
+    }
+
+    /// :nodoc:
+    public func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewDidEndScrollingAnimation?(scrollView)
+    }
+
+    // MARK: Managing and tracking zooming more granularly
+
+    /// :nodoc:
+    public func viewForZooming(in scrollView: UIScrollView) -> UIView? {
+        self.scrollViewDelegate?.viewForZooming?(in: scrollView)
+    }
+
+    /// :nodoc:
+    public func scrollViewWillBeginZooming(_ scrollView: UIScrollView,
+                                           with view: UIView?) {
+        self.scrollViewDelegate?.scrollViewWillBeginZooming?(scrollView, with: view)
+    }
+
+    /// :nodoc:
+    public func scrollViewDidEndZooming(_ scrollView: UIScrollView,
+                                        with view: UIView?,
+                                        atScale scale: CGFloat) {
+        self.scrollViewDelegate?.scrollViewDidEndZooming?(scrollView, with: view, atScale: scale)
+    }
+
+    // MARK: Managing if should scroll to top and tracking if done so
+
+    /// :nodoc:
+    public func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool {
+        self.scrollViewDelegate?.scrollViewShouldScrollToTop?(scrollView) ?? true
+    }
+
+    /// :nodoc:
+    public func scrollViewDidScrollToTop(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewDidScrollToTop?(scrollView)
+    }
+
+
+    // MARK: Tracking adjusted content insets on scroll view
+
+    /// :nodoc:
+    public func scrollViewDidChangeAdjustedContentInset(_ scrollView: UIScrollView) {
+        self.scrollViewDelegate?.scrollViewDidChangeAdjustedContentInset?(scrollView)
+    }
+}