fix(reader): stabilize pagination display and page curl reuse

Sources/RDReaderView/ReaderView/RDReaderView.swift

@@ -624,19 +624,31 @@ public class RDReaderView: UIView {
     /// 获取用于显示的页面视图
     /// 优先从缓存中获取，其次从预加载缓存中获取，最后新建
     private func pageViewForDisplay(pageNum: Int) -> UIView {
-        if let cached = pageCurlCachedViews[pageNum] {
-            return cached
-        }
-        if let preloaded = preloadedPageViews.removeValue(forKey: pageNum) {
-            preloaded.removeFromSuperview()
-            pageCurlCachedViews[pageNum] = preloaded
-            return preloaded
-        }
-        let view = dataSource?.pageContentView(readerView: self, pageNum: pageNum, containerView: nil) ?? UIView()
+        let reusableView = detachedReusablePageView(for: pageNum)
+        let view = dataSource?.pageContentView(readerView: self, pageNum: pageNum, containerView: reusableView)
+            ?? reusableView
+            ?? UIView()
         pageCurlCachedViews[pageNum] = view
         return view
     }
 
+    /// 返回可安全复用的 page view。
+    /// 只有已经脱离视图层级的 view 才能复用，避免 pageCurl 转场期间同一个 UIView
+    /// 被多个 child controller 争用，从而导致前一个页面瞬时空白。
+    private func detachedReusablePageView(for pageNum: Int) -> UIView? {
+        if let preloaded = preloadedPageViews.removeValue(forKey: pageNum) {
+            preloaded.removeFromSuperview()
+            return preloaded
+        }
+
+        guard let cached = pageCurlCachedViews[pageNum], cached.superview == nil else {
+            return nil
+        }
+
+        pageCurlCachedViews.removeValue(forKey: pageNum)
+        return cached
+    }
+
     /// 裁剪缓存，只保留指定页码集合中的页面
     /// 不在保留集合中的页面会被从视图层级中移除
     private func trimCachedPageViews(keeping pageNumbers: Set<Int>) {
@@ -705,7 +717,7 @@ public class RDReaderView: UIView {
         preloadHostView.frame = bounds
 
         for targetPage in targets {
-            let existing = preloadedPageViews[targetPage] ?? pageCurlCachedViews.removeValue(forKey: targetPage)
+            let existing = detachedReusablePageView(for: targetPage)
             let contentView = dataSource?.pageContentView(readerView: self, pageNum: targetPage, containerView: existing) ?? existing ?? UIView()
             preloadedPageViews[targetPage] = contentView
             if contentView.superview !== preloadHostView {