regenerate something when the sentences changed

ios/OralTrainer/Services/SentenceExplanationStore.swift

@@ -33,11 +33,31 @@ final class SentenceExplanationStore {
         }
     }
 
+    func sync(mediaId: String, sentences: [SentenceBoundary]) {
+        guard !sentences.isEmpty else { return }
+        let directory = root.appendingPathComponent(Self.sanitize(mediaId), isDirectory: true)
+        let revision = Self.revision(for: sentences)
+        let marker = directory.appendingPathComponent("revision.txt")
+        let previous = (try? String(contentsOf: marker, encoding: .utf8))?
+            .trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
+        guard previous != revision else { return }
+        try? FileManager.default.removeItem(at: directory)
+        try? FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
+        try? revision.write(to: marker, atomically: true, encoding: .utf8)
+    }
+
     private func fileURL(mediaId: String, sentenceIndex: Int) -> URL {
         let directory = root.appendingPathComponent(Self.sanitize(mediaId), isDirectory: true)
         return directory.appendingPathComponent("\(sentenceIndex).txt")
     }
 
+    static func revision(for sentences: [SentenceBoundary]) -> String {
+        sentences.map { sentence in
+            let text = (sentence.text ?? "").trimmingCharacters(in: .whitespacesAndNewlines)
+            return "\(sentence.index)\t\(text)"
+        }.joined(separator: "\n")
+    }
+
     static func sanitize(_ mediaId: String) -> String {
         let allowed = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "._-"))
         let filtered = mediaId.lowercased().unicodeScalars.map { allowed.contains($0) ? Character($0) : "_" }

ios/OralTrainer/Views/CatalogView.swift

@@ -45,6 +45,10 @@ final class CatalogViewModel: ObservableObject {
             do {
                 let document = try await SentenceBoundaryAPI.fetch(videoHash: video.videoHash)
                 boundaryDocuments[document.videoHash] = document
+                SentenceExplanationStore.shared.sync(
+                    mediaId: document.videoHash,
+                    sentences: document.sentences
+                )
                 updatedVideos[index] = video.copy(
                     durationMs: document.durationMs,
                     sentenceCount: document.sentences.count
@@ -75,6 +79,10 @@ final class CatalogViewModel: ObservableObject {
             return
         }
         if let document = boundaryDocuments[video.videoHash] {
+            SentenceExplanationStore.shared.sync(
+                mediaId: document.videoHash,
+                sentences: document.sentences
+            )
             let item = TrainingMediaItem(
                 id: video.videoHash,
                 title: video.title,
@@ -98,6 +106,10 @@ final class CatalogViewModel: ObservableObject {
                     return
                 }
                 boundaryDocuments[document.videoHash] = document
+                SentenceExplanationStore.shared.sync(
+                    mediaId: document.videoHash,
+                    sentences: document.sentences
+                )
                 controller.updateBoundaries(document.sentences)
                 controller.boundaryState = .loaded(count: document.sentences.count)
                 controller.showTransientMessage("课程已就绪：\(document.sentences.count) 句")
@@ -142,6 +154,10 @@ final class CatalogViewModel: ObservableObject {
                 guard activeVideoHash == hash else {
                     return
                 }
+                SentenceExplanationStore.shared.sync(
+                    mediaId: document.videoHash,
+                    sentences: document.sentences
+                )
                 controller.updateBoundaries(document.sentences)
                 controller.boundaryState = .loaded(count: document.sentences.count)
                 controller.showTransientMessage("句子边界已加载：\(document.sentences.count) 句")

ios/OralTrainer/Views/FullscreenPlayerView.swift

@@ -29,7 +29,8 @@ struct FullscreenPlayerView: View {
             SentenceExplanationSheet(
                 mediaId: controller.currentItem?.id ?? "",
                 courseTitle: controller.currentItem?.title ?? "口语训练课程",
-                sentence: sentence
+                sentence: sentence,
+                sentences: controller.currentItem?.sentences ?? []
             )
         }
     }

ios/OralTrainer/Views/PlayerSectionView.swift

@@ -27,7 +27,8 @@ struct PlayerSectionView: View {
             SentenceExplanationSheet(
                 mediaId: controller.currentItem?.id ?? "",
                 courseTitle: controller.currentItem?.title ?? "口语训练课程",
-                sentence: sentence
+                sentence: sentence,
+                sentences: controller.currentItem?.sentences ?? []
             )
         }
     }
@@ -273,6 +274,7 @@ struct SentenceExplanationSheet: View {
     let mediaId: String
     let courseTitle: String
     let sentence: SentenceBoundary
+    var sentences: [SentenceBoundary] = []
 
     @Environment(\.dismiss) private var dismiss
     @State private var explanation = ""
@@ -327,6 +329,7 @@ struct SentenceExplanationSheet: View {
             }
         }
         .task {
+            SentenceExplanationStore.shared.sync(mediaId: mediaId, sentences: sentences)
             if let cached = SentenceExplanationStore.shared.load(
                 mediaId: mediaId,
                 sentenceIndex: sentence.index