Don't use UIPageViewController for SegmentedPageViewController

Tusker/Screens/Utilities/SegmentedPageViewController.swift

@@ -12,8 +12,8 @@ protocol SegmentedPageViewControllerPage: Hashable {
     var segmentedControlTitle: String { get }
 }
 
-class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIPageViewController, UIPageViewControllerDelegate, TabbedPageViewController {
-
+class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIViewController, UIPageViewControllerDelegate, TabbedPageViewController {
+    
     private(set) var pages: [Page]!
     private let pageProvider: (Page) -> UIViewController
     private var pageControllers = [Page: UIViewController]()
@@ -23,12 +23,10 @@ class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIPage
     var currentIndex: Int! {
         pages.firstIndex(of: currentPage)
     }
-    var currentViewController: UIViewController! {
-        viewControllers?.first
-    }
+    var currentViewController: UIViewController!
     
     let segmentedControl = ScrollingSegmentedControl<Page>()
-
+    
     init(pages: [Page], pageProvider: @escaping (Page) -> UIViewController) {
         precondition(!pages.isEmpty)
         
@@ -37,7 +35,7 @@ class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIPage
         initialPage = pages.first!
         currentPage = pages.first!
         
-        super.init(transitionStyle: .scroll, navigationOrientation: .horizontal, options: nil)
+        super.init(nibName: nil, bundle: nil)
         
         setPages(pages, animated: false)
         
@@ -104,12 +102,12 @@ class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIPage
             initialPage = page
             return
         }
-        let direction: UIPageViewController.NavigationDirection
+        let direction: AnimationMode
         if let prevIndex = currentIndex {
             let index = pages.firstIndex(of: page)!
             direction = index - prevIndex > 0 ? .forward : .reverse
         } else {
-            direction = .forward
+            direction = .none
         }
         
         currentPage = page
@@ -121,12 +119,40 @@ class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIPage
             pageControllers[page] = newController
         }
         
-        setViewControllers([newController], direction: direction, animated: animated)
+        setViewController(newController, animated: animated ? direction : .none)
         navigationItem.title = newController.title
         
         segmentedControl.setSelectedOption(page, animated: animated)
     }
     
+    private func setViewController(_ newViewController: UIViewController, animated: AnimationMode) {
+        guard let currentViewController,
+              animated != .none else {
+            currentViewController?.removeViewAndController()
+            newViewController.view.translatesAutoresizingMaskIntoConstraints = false
+            embedChild(newViewController)
+            self.currentViewController = newViewController
+            return
+        }
+        guard currentViewController !== newViewController else {
+            return
+        }
+        self.currentViewController = newViewController
+        newViewController.view.translatesAutoresizingMaskIntoConstraints = false
+        embedChild(newViewController)
+        let direction: CGFloat = animated == .forward ? 1 : -1
+        newViewController.view.transform = CGAffineTransform(translationX: direction * view.bounds.width, y: 0)
+        let animator = UIViewPropertyAnimator(duration: 0.5, timingParameters: UISpringTimingParameters(dampingRatio: 1, initialVelocity: .zero))
+        animator.addAnimations {
+            newViewController.view.transform = .identity
+            currentViewController.view.transform = CGAffineTransform(translationX: -1 * direction * self.view.bounds.width, y: 0)
+        }
+        animator.addCompletion { _ in
+            currentViewController.removeViewAndController()
+        }
+        animator.startAnimation()
+    }
+    
     // MARK: TabbedPageViewController
     
     func selectNextPage() {
@@ -138,7 +164,14 @@ class SegmentedPageViewController<Page: SegmentedPageViewControllerPage>: UIPage
         guard currentIndex > 0 else { return }
         selectPage(pages[currentIndex - 1], animated: true)
     }
-
+}
+    
+extension SegmentedPageViewController {
+    enum AnimationMode: Equatable {
+        case none
+        case forward
+        case reverse
+    }
 }
 
 extension SegmentedPageViewController: TabBarScrollableViewController {