add save function of explanation

ios/OralTrainer/Services/SentenceExplanationStore.swift

@@ -0,0 +1,47 @@
+import Foundation
+
+final class SentenceExplanationStore {
+    static let shared = SentenceExplanationStore()
+
+    private let root: URL
+
+    init() {
+        let documents = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
+        root = documents.appendingPathComponent("SentenceExplanations", isDirectory: true)
+        try? FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+    }
+
+    func load(mediaId: String, sentenceIndex: Int) -> String? {
+        let url = fileURL(mediaId: mediaId, sentenceIndex: sentenceIndex)
+        guard FileManager.default.fileExists(atPath: url.path) else { return nil }
+        let text = try? String(contentsOf: url, encoding: .utf8)
+        let trimmed = text?.trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
+        return trimmed.isEmpty ? nil : trimmed
+    }
+
+    @discardableResult
+    func save(mediaId: String, sentenceIndex: Int, explanation: String) -> Bool {
+        let trimmed = explanation.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty else { return false }
+        let url = fileURL(mediaId: mediaId, sentenceIndex: sentenceIndex)
+        try? FileManager.default.createDirectory(at: url.deletingLastPathComponent(), withIntermediateDirectories: true)
+        do {
+            try trimmed.write(to: url, atomically: true, encoding: .utf8)
+            return true
+        } catch {
+            return false
+        }
+    }
+
+    private func fileURL(mediaId: String, sentenceIndex: Int) -> URL {
+        let directory = root.appendingPathComponent(Self.sanitize(mediaId), isDirectory: true)
+        return directory.appendingPathComponent("\(sentenceIndex).txt")
+    }
+
+    static func sanitize(_ mediaId: String) -> String {
+        let allowed = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "._-"))
+        let filtered = mediaId.lowercased().unicodeScalars.map { allowed.contains($0) ? Character($0) : "_" }
+        let value = String(filtered).trimmingCharacters(in: CharacterSet(charactersIn: "_"))
+        return value.isEmpty || value == "." || value == ".." ? "unknown" : value
+    }
+}

ios/OralTrainer/Views/FullscreenPlayerView.swift

@@ -97,6 +97,7 @@ struct FullscreenPlayerView: View {
         }
         .sheet(item: $explanationSentence) { sentence in
             SentenceExplanationSheet(
+                mediaId: controller.currentItem?.id ?? "",
                 courseTitle: controller.currentItem?.title ?? "口语训练课程",
                 sentence: sentence
             )

ios/OralTrainer/Views/PlayerSectionView.swift

@@ -22,6 +22,7 @@ struct PlayerSectionView: View {
         .padding(.horizontal, 16)
         .sheet(item: $explanationSentence) { sentence in
             SentenceExplanationSheet(
+                mediaId: controller.currentItem?.id ?? "",
                 courseTitle: controller.currentItem?.title ?? "口语训练课程",
                 sentence: sentence
             )
@@ -96,7 +97,7 @@ struct PlayerSectionView: View {
                 Text(sentenceTextText)
                 .font(.system(size: 15))
                     .foregroundColor(.white)
-                    .lineLimit(2)
+                    .lineLimit(4)
                 Spacer()
                 Button {
                     guard let sentence = controller.currentSentence, !(sentence.text ?? "").isEmpty else {
@@ -230,12 +231,14 @@ struct PlayerSectionView: View {
 }
 
 struct SentenceExplanationSheet: View {
+    let mediaId: String
     let courseTitle: String
     let sentence: SentenceBoundary
 
     @Environment(\.dismiss) private var dismiss
     @State private var explanation = ""
     @State private var errorMessage: String?
+    @State private var saved = false
 
     var body: some View {
         NavigationView {
@@ -267,18 +270,35 @@ struct SentenceExplanationSheet: View {
             .navigationBarTitleDisplayMode(.inline)
             .toolbar {
                 ToolbarItem(placement: .navigationBarTrailing) {
-                    Button {
-                        dismiss()
-                    } label: {
-                        Image(systemName: "xmark.circle.fill")
-                            .font(.system(size: 22, weight: .semibold))
+                    HStack(spacing: 6) {
+                        Button(saved ? "已保存" : "保存") {
+                            saveExplanation()
+                        }
+                        .disabled(saved || explanation.isEmpty || errorMessage != nil)
+                        Text(saved ? "（已保存本地）" : "（下次直接看本地）")
+                            .font(.system(size: 12))
                             .foregroundColor(Theme.textMuted)
+                        Button {
+                            dismiss()
+                        } label: {
+                            Image(systemName: "xmark.circle.fill")
+                                .font(.system(size: 22, weight: .semibold))
+                                .foregroundColor(Theme.textMuted)
+                        }
+                        .accessibilityLabel("关闭句子讲解")
                     }
-                    .accessibilityLabel("关闭句子讲解")
                 }
             }
         }
         .task {
+            if let cached = SentenceExplanationStore.shared.load(
+                mediaId: mediaId,
+                sentenceIndex: sentence.index
+            ) {
+                explanation = cached
+                saved = true
+                return
+            }
             guard explanation.isEmpty, errorMessage == nil else { return }
             do {
                 explanation = try await SentenceBoundaryAPI.explain(
@@ -290,4 +310,13 @@ struct SentenceExplanationSheet: View {
             }
         }
     }
+
+    private func saveExplanation() {
+        guard !saved, !explanation.isEmpty, errorMessage == nil else { return }
+        saved = SentenceExplanationStore.shared.save(
+            mediaId: mediaId,
+            sentenceIndex: sentence.index,
+            explanation: explanation
+        )
+    }
 }