Don't use UIPageViewController for profiles

Closes #228

Tusker/Screens/Profile/ProfileStatusesViewController.swift

@@ -31,7 +31,7 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
     private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
     private(set) var headerCell: ProfileHeaderCollectionViewCell?
     
-    private var state: State = .unloaded
+    private(set) var state: State = .unloaded
     
     init(accountID: String?, kind: Kind, owner: ProfileViewController) {
         self.accountID = accountID
@@ -99,8 +99,6 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
                     Task {
                         await load()
                     }
-                case .loading:
-                    break
                 case .loaded, .setupInitialSnapshot:
                     var snapshot = dataSource.snapshot()
                     snapshot.reconfigureItems([.header(id)])
@@ -108,6 +106,13 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
                 }
             }
             .store(in: &cancellables)
+        
+        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
+        snapshot.appendSections([.header, .pinned, .statuses])
+        snapshot.appendItems([.header(accountID)], toSection: .header)
+        dataSource.apply(snapshot, animatingDifferences: false)
+        
+        state = .setupInitialSnapshot
     }
     
     private func createDataSource() -> UICollectionViewDiffableDataSource<Section, Item> {
@@ -133,6 +138,8 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
                         view.updateUI(for: id)
                         view.pagesSegmentedControl.selectedSegmentIndex = self.owner?.currentIndex ?? 0
                         cell.addHeader(view)
+                    case .useExistingView(let view):
+                        cell.addHeader(view)
                     case .placeholder(height: let height):
                         _ = cell.addConstraint(height: height)
                     }
@@ -174,24 +181,18 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
     private func load() async {
         guard isViewLoaded,
               let accountID,
-              case .unloaded = state,
+              state == .setupInitialSnapshot,
               mastodonController.persistentContainer.account(for: accountID) != nil else {
             return
         }
         
-        state = .loading
-        
-        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
-        snapshot.appendSections([.header, .pinned, .statuses])
-        snapshot.appendItems([.header(accountID)], toSection: .header)
-        await apply(snapshot, animatingDifferences: false)
-        
-        state = .setupInitialSnapshot
-        
         await controller.loadInitial()
         await tryLoadPinned()
         
         state = .loaded
+        
+        // remove any content inset that was added when switching pages to this VC
+        collectionView.contentInset = .zero
     }
     
     private func tryLoadPinned() async {
@@ -260,7 +261,6 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
 extension ProfileStatusesViewController {
     enum State {
         case unloaded
-        case loading
         case setupInitialSnapshot
         case loaded
     }
@@ -271,7 +271,7 @@ extension ProfileStatusesViewController {
         case statuses, withReplies, onlyMedia
     }
     enum HeaderMode {
-        case createView, placeholder(height: CGFloat)
+        case createView, useExistingView(ProfileHeaderView), placeholder(height: CGFloat)
     }
 }
 

Tusker/Screens/Profile/ProfileViewController.swift

@@ -10,7 +10,7 @@ import UIKit
 import Pachyderm
 import Combine
 
-class ProfileViewController: UIPageViewController {
+class ProfileViewController: UIViewController {
     
     weak var mastodonController: MastodonController!
     
@@ -42,7 +42,7 @@ class ProfileViewController: UIPageViewController {
         self.accountID = accountID
         self.mastodonController = mastodonController
         
-        super.init(transitionStyle: .scroll, navigationOrientation: .horizontal, options: nil)
+        super.init(nibName: nil, bundle: nil)
         
         self.pageControllers = [
             .init(accountID: accountID, kind: .statuses, owner: self),
@@ -146,26 +146,31 @@ class ProfileViewController: UIPageViewController {
         
         state = .animating
         
-        let direction: UIPageViewController.NavigationDirection
-        if currentIndex == nil || index - currentIndex > 0 {
-            direction = .forward
-        } else {
-            direction = .reverse
-        }
-        
-        guard let old = viewControllers?.first as? ProfileStatusesViewController else {
-            // if old doesn't exist, we're selecting the initial view controller, so moving the header around isn't necessary
-            pageControllers[index].initialHeaderMode = .createView
-            setViewControllers([pageControllers[index]], direction: direction, animated: animated) { finished in
-                self.state = .idle
-                completion?(finished)
-            }
-            currentIndex = index
-            return
-        }
         let new = pageControllers[index]
         
-        currentIndex = index
+        guard let currentIndex else {
+            assert(!animated)
+            // if old doesn't exist, we're selecting the initial view controller, so moving the header around isn't necessary
+            new.initialHeaderMode = .createView
+            embedChild(new)
+            self.currentIndex = index
+            state = .idle
+            completion?(true)
+            return
+        }
+        
+        let direction: CGFloat
+        if index - currentIndex > 0 {
+            direction = 1 // forward
+        } else {
+            direction = -1 // reverse
+        }
+        
+        let old = pageControllers[currentIndex]
+        
+        new.loadViewIfNeeded()
+        
+        self.currentIndex = index
         
         // TODO: old.headerCell could be nil if scrolled down and key command used
         let oldHeaderCell = old.headerCell!
@@ -173,8 +178,8 @@ class ProfileViewController: UIPageViewController {
         // old header cell must have the header view
         let headerView = oldHeaderCell.addConstraint(height: oldHeaderCell.bounds.height)!
         
-        if new.isViewLoaded {
+        if let newHeaderCell = new.headerCell {
-            _ = new.headerCell!.addConstraint(height: oldHeaderCell.bounds.height)
+            _ = newHeaderCell.addConstraint(height: oldHeaderCell.bounds.height)
         } else {
             new.initialHeaderMode = .placeholder(height: oldHeaderCell.bounds.height)
         }
@@ -195,60 +200,64 @@ class ProfileViewController: UIPageViewController {
         // hide scroll indicators during the transition because otherwise the show through the
         // profile header, even though it has an opaque background
         old.collectionView.showsVerticalScrollIndicator = false
-        if new.isViewLoaded {
+        new.collectionView.showsVerticalScrollIndicator = false
-            new.collectionView.showsVerticalScrollIndicator = false
+        
+        let origOldContentOffset = old.collectionView.contentOffset
+        // we can't just change the content offset during the animation, otherwise the new collection view doesn't size the cells at the top
+        // and new's offset doesn't physically match old's, even though they're numerically the same
+        let needsMatchContentOffsetWithTransform = new.state != .loaded
+        let yTranslationToMatchOldContentOffset: CGFloat
+        if needsMatchContentOffsetWithTransform {
+            yTranslationToMatchOldContentOffset = -origOldContentOffset.y - view.safeAreaInsets.top
+        } else {
+            new.collectionView.contentOffset = origOldContentOffset
+            yTranslationToMatchOldContentOffset = 0
         }
         
-        // if the new view isn't loaded or it isn't tall enough to match content offsets, animate scrolling old back to top to match new
+        if animated {
-        if animated,
+            // if the new view isn't tall enough to match content offsets
-           !new.isViewLoaded || new.collectionView.contentSize.height - new.collectionView.bounds.height < old.collectionView.contentOffset.y {
+            if new.collectionView.contentSize.height - new.collectionView.bounds.height < old.collectionView.contentOffset.y {
-            // We need to display a snapshot over the old view because setting the content offset to the top w/o animating
+                let additionalHeightNeededToMatchContentOffset = old.collectionView.contentOffset.y + old.collectionView.bounds.height - new.collectionView.contentSize.height
-            // results in the collection view immediately removing cells that will be offscreen.
+                new.collectionView.contentInset = UIEdgeInsets(top: 0, left: 0, bottom: additionalHeightNeededToMatchContentOffset, right: 0)
-            // And we can't just call setContentOffset(_:animated:) because its animation curve does not match ours/the page views
-            // So, we capture a snapshot before the content offset is changed, so those cells can be shown during the animation,
-            // rather than a gap appearing during it.
-            let snapshot = old.collectionView.snapshotView(afterScreenUpdates: true)!
-            let origOldContentOffset = old.collectionView.contentOffset
-            old.collectionView.contentOffset = CGPoint(x: 0, y: view.safeAreaInsets.top)
-            
-            snapshot.frame = old.collectionView.bounds
-            snapshot.frame.origin.y = 0
-            snapshot.layer.zPosition = 99
-            view.addSubview(snapshot)
-            
-            // empirically, 0.3s seems to match the UIPageViewController animation
-            UIView.animate(withDuration: 0.3, delay: 0, options: .curveEaseInOut) {
-                // animate the snapshot offscreen in the same direction as the old view
-                snapshot.frame.origin.x = direction == .forward ? -self.view.bounds.width : self.view.bounds.width
-                // animate the snapshot to be "scrolled" to top
-                snapshot.frame.origin.y = self.view.safeAreaInsets.top + origOldContentOffset.y
-                // if scrolling because the new collection view's content isn't tall enough, make sure to scroll it to top as well
-                if new.isViewLoaded {
-                    new.collectionView.contentOffset = CGPoint(x: 0, y: -self.view.safeAreaInsets.top)
-                }
-                headerView.transform = CGAffineTransform(translationX: 0, y: -headerTopOffset)
-            } completion: { _ in
-                snapshot.removeFromSuperview()
             }
-        } else if new.isViewLoaded {
-            new.collectionView.contentOffset = old.collectionView.contentOffset
-        }
             
-        setViewControllers([pageControllers[index]], direction: direction, animated: animated) { finished in
+            embedChild(new)
-            // reenable scroll indicators after the switching animation is done
+            new.view.transform = CGAffineTransform(translationX: direction * view.bounds.width, y: yTranslationToMatchOldContentOffset)
-            old.collectionView.showsVerticalScrollIndicator = true
-            new.collectionView.showsVerticalScrollIndicator = true
             
-            headerView.isUserInteractionEnabled = true
+            let animator = UIViewPropertyAnimator(duration: 0.5, timingParameters: UISpringTimingParameters(dampingRatio: 1, initialVelocity: .zero))
+            animator.addAnimations {
+                new.view.transform = CGAffineTransform(translationX: 0, y: yTranslationToMatchOldContentOffset)
+                old.view.transform = CGAffineTransform(translationX: -direction * self.view.bounds.width, y: 0)
+            }
+            animator.addCompletion { _ in
+                old.removeViewAndController()
+                old.collectionView.transform = .identity
                 
-            headerView.transform = .identity
+                new.collectionView.transform = .identity
-            headerView.layer.zPosition = 0
+                new.collectionView.contentOffset = origOldContentOffset
-            // move the header view into the new page controller's cell
-            // new's headerCell should always be non-nil, because the account must be loaded (in order to have triggered this switch), and so new should add the cell immediately on load
-            new.headerCell!.addHeader(headerView)
                 
-            self.state = .idle
+                // reenable scroll indicators after the switching animation is done
-            completion?(finished)
+                old.collectionView.showsVerticalScrollIndicator = true
+                new.collectionView.showsVerticalScrollIndicator = true
+                
+                headerView.isUserInteractionEnabled = true
+                headerView.transform = .identity
+                headerView.layer.zPosition = 0
+                // move the header view into the new page controller's cell
+                if let newHeaderCell = new.headerCell {
+                    newHeaderCell.addHeader(headerView)
+                } else {
+                    new.initialHeaderMode = .useExistingView(headerView)
+                }
+                
+                self.state = .idle
+                completion?(true)
+            }
+            animator.startAnimation()
+        } else {
+            old.removeViewAndController()
+            embedChild(new)
+            completion?(true)
         }
     }