Improve reliability of EPUB pagination view (#550)

Sources/Navigator/EPUB/EPUBNavigatorViewController.swift

@@ -346,9 +346,9 @@ open class EPUBNavigatorViewController: UIViewController,
             hasPositions: !positionsByReadingOrder.isEmpty
         )
 
-        paginationView.frame = view.bounds
-        paginationView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
-        view.addSubview(paginationView)
+        paginationView!.frame = view.bounds
+        paginationView!.autoresizingMask = [.flexibleHeight, .flexibleWidth]
+        view.addSubview(paginationView!)
 
         applySettings()
 
@@ -481,7 +481,10 @@ open class EPUBNavigatorViewController: UIViewController,
 
     /// Goes to the next or previous page in the given scroll direction.
     private func go(to direction: EPUBSpreadView.Direction, options: NavigatorGoOptions) async -> Bool {
-        guard on(.move(direction)) else {
+        guard
+            let paginationView = paginationView,
+            on(.move(direction))
+        else {
             return false
         }
 
@@ -515,7 +518,7 @@ open class EPUBNavigatorViewController: UIViewController,
 
     // MARK: - Pagination and spreads
 
-    private var paginationView: PaginationView!
+    private var paginationView: PaginationView?
 
     private func makePaginationView(hasPositions: Bool) -> PaginationView {
         let view = PaginationView(
@@ -529,11 +532,20 @@ open class EPUBNavigatorViewController: UIViewController,
         return view
     }
 
+    private func invalidatePaginationView() async {
+        guard let paginationView = paginationView else {
+            return
+        }
+
+        paginationView.isScrollEnabled = isPaginationViewScrollingEnabled
+        await reloadSpreads(force: true)
+    }
+
     private var spreads: [EPUBSpread] = []
 
     /// Index of the currently visible spread.
     private var currentSpreadIndex: Int {
-        paginationView.currentIndex
+        paginationView?.currentIndex ?? 0
     }
 
     // Reading order index of the left-most resource in the visible spread.
@@ -575,6 +587,7 @@ open class EPUBNavigatorViewController: UIViewController,
         let locator = locator ?? currentLocation
 
         guard
+            let paginationView = paginationView,
             // Already loaded with the expected amount of spreads?
             force || spreads.first?.spread != viewModel.spreadEnabled,
             on(.load(locator))
@@ -607,7 +620,7 @@ open class EPUBNavigatorViewController: UIViewController,
     }
 
     private func loadedSpreadViewForHREF<T: URLConvertible>(_ href: T) -> EPUBSpreadView? {
-        paginationView.loadedViews
+        paginationView?.loadedViews
             .compactMap { _, view in view as? EPUBSpreadView }
             .first { $0.spread.links.firstWithHREF(href) != nil }
     }
@@ -640,7 +653,7 @@ open class EPUBNavigatorViewController: UIViewController,
             return pendingLocator
         }
 
-        guard let spreadView = paginationView.currentView as? EPUBSpreadView else {
+        guard let spreadView = paginationView?.currentView as? EPUBSpreadView else {
             return nil
         }
 
@@ -670,7 +683,7 @@ open class EPUBNavigatorViewController: UIViewController,
     }
 
     public func firstVisibleElementLocator() async -> Locator? {
-        guard let spreadView = paginationView.currentView as? EPUBSpreadView else {
+        guard let spreadView = paginationView?.currentView as? EPUBSpreadView else {
             return nil
         }
         return await spreadView.findFirstVisibleElementLocator()
@@ -705,6 +718,7 @@ open class EPUBNavigatorViewController: UIViewController,
         let locator = publication.normalizeLocator(locator)
 
         guard
+            let paginationView = paginationView,
             let spreadIndex = spreads.firstIndexWithHREF(locator.href),
             on(.jump(locator))
         else {
@@ -759,6 +773,10 @@ open class EPUBNavigatorViewController: UIViewController,
     }
 
     public func clearSelection() {
+        guard let paginationView = paginationView else {
+            return
+        }
+
         for (_, pageView) in paginationView.loadedViews {
             (pageView as? EPUBSpreadView)?.webView.clearSelection()
         }
@@ -779,6 +797,10 @@ open class EPUBNavigatorViewController: UIViewController,
         Task {
             await initialized()
 
+            guard let paginationView = paginationView else {
+                return
+            }
+
             await withTaskGroup(of: Void.self) { tasks in
                 let source = self.decorations[group] ?? []
                 let target = decorations.map {
@@ -821,6 +843,10 @@ open class EPUBNavigatorViewController: UIViewController,
         Task {
             await initialized()
 
+            guard let paginationView = paginationView else {
+                return
+            }
+
             await withTaskGroup(of: Void.self) { tasks in
                 for (_, view) in paginationView.loadedViews {
                     tasks.addTask {
@@ -854,7 +880,7 @@ open class EPUBNavigatorViewController: UIViewController,
         }
 
         view.backgroundColor = settings.effectiveBackgroundColor.uiColor
-        paginationView.isScrollEnabled = isPaginationViewScrollingEnabled
+        paginationView?.isScrollEnabled = isPaginationViewScrollingEnabled
     }
 
     // MARK: - User interactions
@@ -872,7 +898,7 @@ open class EPUBNavigatorViewController: UIViewController,
     /// Evaluates the given JavaScript on the currently visible HTML resource.
     @discardableResult
     public func evaluateJavaScript(_ script: String) async -> Result<Any, Error> {
-        guard let spreadView = paginationView.currentView as? EPUBSpreadView else {
+        guard let spreadView = paginationView?.currentView as? EPUBSpreadView else {
             return .failure(EPUBError.spreadNotLoaded)
         }
         return await spreadView.evaluateScript(script)
@@ -913,15 +939,18 @@ open class EPUBNavigatorViewController: UIViewController,
 extension EPUBNavigatorViewController: EPUBNavigatorViewModelDelegate {
     func epubNavigatorViewModelInvalidatePaginationView(_ viewModel: EPUBNavigatorViewModel) {
         Task {
-            paginationView.isScrollEnabled = isPaginationViewScrollingEnabled
-            await reloadSpreads(force: true)
+            await invalidatePaginationView()
         }
     }
 
     func epubNavigatorViewModel(_ viewModel: EPUBNavigatorViewModel, runScript script: String, in scope: EPUBScriptScope) {
         Task {
             await initialized()
 
+            guard let paginationView = paginationView else {
+                return
+            }
+
             switch scope {
             case .currentResource:
                 await (paginationView.currentView as? EPUBSpreadView)?.evaluateScript(script)
@@ -1159,7 +1188,7 @@ extension EPUBNavigatorViewController: EPUBSpreadViewDelegate {
     }
 
     func spreadViewPagesDidChange(_ spreadView: EPUBSpreadView) {
-        if paginationView.currentView == spreadView {
+        if paginationView?.currentView == spreadView {
             updateCurrentLocation()
         }
     }