Fix issues with SwiftUI views being reset when updating

The current implementation calls reloadData on each update, which
re-creates the entire content view and resets any state (like
scrolling). To prevent this, we update the existing
UIHostingController if the current page is the same.

This requires us to have stable identifiers, to know if the current
page is the same as before or a new one. For title-based pages, we
just use the title as the identifier. For pages with custom SwiftUI
headers, we default to using the index of the view as the
identifier. This is the same behaviour that we have today, although it
will only work when having static pages. In order to support dynamic
pages with custom SwiftUI headers, we introduce a new initializer that
allows specifying the identifier.

ExampleSwiftUI/ExampleApp.swift

@@ -17,6 +17,7 @@ struct ExampleApp: App {
                         NavigationLink("Change items", destination: ChangeItemsView())
                         NavigationLink("Dynamic items", destination: DynamicItemsView())
                         NavigationLink("Custom indicator", destination: CustomIndicatorView())
+                        NavigationLink("Scrolling Views", destination: ScrollingView())
                     }
                 }
                 .navigationBarTitleDisplayMode(.inline)

ExampleSwiftUI/ScrollingView.swift

@@ -0,0 +1,31 @@
+import Parchment
+import SwiftUI
+import UIKit
+
+struct ScrollingView: View {
+    var body: some View {
+        PageView {
+            Page("First") {
+                ScrollingContentView()
+            }
+            Page("Second") {
+                ScrollingContentView()
+            }
+            Page("Third") {
+                ScrollingContentView()
+            }
+        }
+    }
+}
+
+struct ScrollingContentView: View {
+    var body: some View {
+        List {
+            ForEach(0...50 , id: \.self) { item in
+                NavigationLink(destination: Text("\(item)")) {
+                    Text("\(item)")
+                }
+            }
+        }
+    }
+}

Parchment.xcodeproj/project.pbxproj

@@ -47,6 +47,7 @@
 		952D802F1E37CC09003DCB18 /* PagingTransition.swift in Sources */ = {isa = PBXBuildFile; fileRef = 952D802E1E37CC09003DCB18 /* PagingTransition.swift */; };
 		9530E25329DEC2E5004FC88C /* PageContentConfiguration.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9530E25229DEC2E5004FC88C /* PageContentConfiguration.swift */; };
 		953B8D352416C3DC0047BBA1 /* SelfSizingViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 953B8D342416C3DC0047BBA1 /* SelfSizingViewController.swift */; };
+		95428A562B80F6EA00D61143 /* ScrollingView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 95428A552B80F6EA00D61143 /* ScrollingView.swift */; };
 		9546B2AB2A1D2F06000390C6 /* PagingHostingIndicatorView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9546B2AA2A1D2F06000390C6 /* PagingHostingIndicatorView.swift */; };
 		9546B2AD2A1D44EF000390C6 /* CustomIndicatorView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9546B2AC2A1D44EF000390C6 /* CustomIndicatorView.swift */; };
 		9546B2AF2A1D4767000390C6 /* PagingIndicatorStyle.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9546B2AE2A1D4767000390C6 /* PagingIndicatorStyle.swift */; };
@@ -269,6 +270,7 @@
 		952D802E1E37CC09003DCB18 /* PagingTransition.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = PagingTransition.swift; sourceTree = "<group>"; };
 		9530E25229DEC2E5004FC88C /* PageContentConfiguration.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = PageContentConfiguration.swift; sourceTree = "<group>"; };
 		953B8D342416C3DC0047BBA1 /* SelfSizingViewController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SelfSizingViewController.swift; sourceTree = "<group>"; };
+		95428A552B80F6EA00D61143 /* ScrollingView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ScrollingView.swift; sourceTree = "<group>"; };
 		9546B2AA2A1D2F06000390C6 /* PagingHostingIndicatorView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = PagingHostingIndicatorView.swift; sourceTree = "<group>"; };
 		9546B2AC2A1D44EF000390C6 /* CustomIndicatorView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CustomIndicatorView.swift; sourceTree = "<group>"; };
 		9546B2AE2A1D4767000390C6 /* PagingIndicatorStyle.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = PagingIndicatorStyle.swift; sourceTree = "<group>"; };
@@ -776,6 +778,7 @@
 				95D2AE51242BCC9500AC3D46 /* ExampleApp.swift */,
 				95D2AE55242BCC9500AC3D46 /* DefaultView.swift */,
 				956FFFCB29BE1FD100477E94 /* ChangeItemsView.swift */,
+				95428A552B80F6EA00D61143 /* ScrollingView.swift */,
 				956FFFD129BE273B00477E94 /* CustomizedView.swift */,
 				95F83D822623804F003B728F /* DynamicItemsView.swift */,
 				956F000929CCFC6C00477E94 /* InterpolatedView.swift */,
@@ -1194,6 +1197,7 @@
 				956FFFD229BE273B00477E94 /* CustomizedView.swift in Sources */,
 				95D2AE52242BCC9500AC3D46 /* ExampleApp.swift in Sources */,
 				956F000A29CCFC6C00477E94 /* InterpolatedView.swift in Sources */,
+				95428A562B80F6EA00D61143 /* ScrollingView.swift in Sources */,
 				956FFFCC29BE1FD100477E94 /* ChangeItemsView.swift in Sources */,
 				95F83D6426237D2B003B728F /* SelectedIndexView.swift in Sources */,
 				95D2AE56242BCC9500AC3D46 /* DefaultView.swift in Sources */,

Parchment/Classes/PageViewCoordinator.swift

@@ -2,7 +2,16 @@ import UIKit
 
 @available(iOS 14.0, *)
 final class PageViewCoordinator: PagingViewControllerDataSource, PagingViewControllerDelegate {
+    final class WeakReference<T: AnyObject> {
+        weak var value: T?
+
+        init(value: T) {
+            self.value = value
+        }
+    }
+
     var parent: PagingControllerRepresentableView
+    var controllers: [Int: WeakReference<UIViewController>] = [:]
 
     init(_ pagingController: PagingControllerRepresentableView) {
         parent = pagingController
@@ -17,14 +26,14 @@ final class PageViewCoordinator: PagingViewControllerDataSource, PagingViewContr
         viewControllerAt index: Int
     ) -> UIViewController {
         let item = parent.items[index]
-        var hostingViewController: UIViewController
+        let hostingViewController: UIViewController
 
-        if let item = item as? PageItem {
-            hostingViewController = item.page.content()
-        } else if let content = parent.content {
-            hostingViewController = content(item)
+        if let controller = controllers[item.identifier]?.value {
+            hostingViewController = controller
         } else {
-            hostingViewController = UIViewController()
+            let controller = hostingController(for: item)
+            controllers[item.identifier] = WeakReference(value: controller)
+            hostingViewController = controller
         }
 
         let backgroundColor = parent.options.pagingContentBackgroundColor
@@ -52,7 +61,6 @@ final class PageViewCoordinator: PagingViewControllerDataSource, PagingViewContr
         }
 
         parent.onDidScroll?(pagingItem)
-
     }
 
     func pagingViewController(
@@ -70,4 +78,18 @@ final class PageViewCoordinator: PagingViewControllerDataSource, PagingViewContr
     ) {
         parent.onDidSelect?(pagingItem)
     }
+
+    private func hostingController(for pagingItem: PagingItem) -> UIViewController {
+        var hostingViewController: UIViewController
+        if let item = pagingItem as? PageItem {
+            hostingViewController = item.page.content()
+        } else {
+            assertionFailure("""
+            PageItem is required when using the SwiftUI wrappers.
+            Please report if you somehow ended up here.
+            """)
+            hostingViewController = UIViewController()
+        }
+        return hostingViewController
+    }
 }

Parchment/Structs/Page.swift

@@ -27,8 +27,10 @@ import SwiftUI
 @available(iOS 14.0, *)
 public struct Page {
     let reuseIdentifier: String
+    let pageIdentifier: String?
     let header: (PagingOptions, PageState) -> UIContentConfiguration
     let content: () -> UIViewController
+    let update: (UIViewController) -> Void
 
     /// Creates a new page with the given header and content views.
     ///
@@ -49,6 +51,61 @@ public struct Page {
         let content = content()
 
         self.reuseIdentifier = "CellIdentifier-\(String(describing: Header.self))"
+        self.pageIdentifier = nil
+
+        self.header = { options, state in
+            if #available(iOS 16.0, *) {
+                return UIHostingConfiguration {
+                    PageCustomView(
+                        content: header(state),
+                        options: options,
+                        state: state
+                    )
+                }
+                .margins(.all, 0)
+            } else {
+                return PageContentConfiguration {
+                    PageCustomView(
+                        content: header(state),
+                        options: options,
+                        state: state
+                    )
+                }
+                .margins(.all, 0)
+            }
+        }
+        self.content = {
+            UIHostingController(rootView: content)
+        }
+        self.update = { viewController in
+            let hostingController = viewController as! UIHostingController<Content>
+            hostingController.rootView = content
+        }
+    }
+
+    /// Creates a new page with the given header and content views.
+    ///
+    /// - Parameters:   
+    ///   - id: A unique identifier for this page.
+    ///   - header: A closure that takes a `PageState` instance as
+    ///     input and returns a `View` that represents the header view
+    ///     for the page. The `PageState` instance will be updated as
+    ///     the page is scrolled, allowing the header view to adjust
+    ///     its appearance accordingly.
+    ///   - content: A closure that returns a `View` that represents
+    ///     the content view for the page.
+    ///
+    ///    - Returns: A new `Page` instance with the given header and content views.
+    public init<Header: View, Content: View, Id: LosslessStringConvertible>(
+        id: Id,
+        @ViewBuilder header: @escaping (PageState) -> Header,
+        @ViewBuilder content: () -> Content
+    ) {
+        let content = content()
+
+        self.reuseIdentifier = "CellIdentifier-\(String(describing: Header.self))"
+        self.pageIdentifier = id.description
+
         self.header = { options, state in
             if #available(iOS 16.0, *) {
                 return UIHostingConfiguration {
@@ -73,6 +130,10 @@ public struct Page {
         self.content = {
             UIHostingController(rootView: content)
         }
+        self.update = { viewController in
+            let hostingController = viewController as! UIHostingController<Content>
+            hostingController.rootView = content
+        }
     }
 
     /// Creates a new page with the given localized title and content views.
@@ -92,6 +153,8 @@ public struct Page {
         let content = content()
 
         self.reuseIdentifier = "CellIdentifier-PageTitleView"
+        self.pageIdentifier = "PageIdentifier-\(titleKey)"
+
         self.header = { options, state in
             if #available(iOS 16.0, *) {
                 return UIHostingConfiguration {
@@ -118,6 +181,10 @@ public struct Page {
         self.content = {
             UIHostingController(rootView: content)
         }
+        self.update = { viewController in
+            let hostingController = viewController as! UIHostingController<Content>
+            hostingController.rootView = content
+        }
     }
 
     /// Creates a new page with the given title and content views.
@@ -137,6 +204,8 @@ public struct Page {
         let content = content()
 
         self.reuseIdentifier = "CellIdentifier-PageTitleView"
+        self.pageIdentifier = "PageIdentifier-\(title)"
+
         self.header = { options, state in
             if #available(iOS 16.0, *) {
                 return UIHostingConfiguration {
@@ -163,6 +232,10 @@ public struct Page {
         self.content = {
             UIHostingController(rootView: content)
         }
+        self.update = { viewController in
+            let hostingController = viewController as! UIHostingController<Content>
+            hostingController.rootView = content
+        }
     }
 }
 

Parchment/Structs/PageView.swift

@@ -72,9 +72,8 @@ public struct PageView: View {
         self.items = content()
             .enumerated()
             .map { (index, page) in
-                // TODO: What should we use as the identifier?
                 PageItem(
-                    identifier: index,
+                    identifier: page.pageIdentifier?.hashValue ?? index,
                     index: index,
                     page: page
                 )

Parchment/Structs/PagingControllerRepresentableView.swift

@@ -41,13 +41,37 @@ struct PagingControllerRepresentableView: UIViewControllerRepresentable {
         _ pagingViewController: PagingViewController,
         context: UIViewControllerRepresentableContext<PagingControllerRepresentableView>
     ) {
+        var oldItems: [Int: PagingItem] = [:]
+
+        for oldItem in context.coordinator.parent.items {
+            if let oldItem = oldItem as? PageItem {
+                oldItems[oldItem.identifier] = oldItem
+            }
+        }
+
         context.coordinator.parent = self
 
         if pagingViewController.dataSource == nil {
             pagingViewController.dataSource = context.coordinator
         }
 
-        pagingViewController.reloadData()
+        // We only want to reload the content views when the items have actually
+        // changed. For items that are added, a new view controller instance will
+        // be created by the PageViewCoordinator.
+        if let currentItem = pagingViewController.state.currentPagingItem,
+           let pageItem = currentItem as? PageItem,
+            let oldItem = oldItems[pageItem.identifier] {
+            pagingViewController.reloadMenu()
+
+            if !oldItem.isEqual(to: currentItem) {
+                if let pageItem = currentItem as? PageItem,
+                   let viewController = context.coordinator.controllers[currentItem.identifier]?.value {
+                    pageItem.page.update(viewController)
+                }
+            }
+        } else {
+            pagingViewController.reloadData()
+        }
 
         // HACK: If the user don't pass a selectedIndex binding, the
         // default parameter is set to .constant(Int.max) which allows