fix a bug about ios

ios/OralTrainer/Player/PlayerController.swift

@@ -18,6 +18,8 @@ final class PlayerController: ObservableObject {
 
     private var mediaItem: TrainingMediaItem?
     private var lastSentenceIndex: Int?
+    private var pendingSeekPositionMs: Int64?
+    private var seekGeneration = 0
     private var stopAtSentenceEnd = false
     private var stopAtSentenceEndIndex: Int?
     private var speedBeforeLongPress: Float?
@@ -67,6 +69,7 @@ final class PlayerController: ObservableObject {
 
     func load(_ item: TrainingMediaItem) {
         lastSentenceIndex = nil
+        pendingSeekPositionMs = nil
         stopAtSentenceEnd = false
         stopAtSentenceEndIndex = nil
         mediaItem = item
@@ -109,18 +112,17 @@ final class PlayerController: ObservableObject {
         stopAtSentenceEnd = false
         stopAtSentenceEndIndex = nil
         player.pause()
-        player.seek(to: .zero, toleranceBefore: .zero, toleranceAfter: .zero)
+        seek(to: .zero)
     }
 
     func seek(to positionMs: Int64) {
         stopAtSentenceEnd = false
         stopAtSentenceEndIndex = nil
-        let time = CMTime(seconds: Double(max(0, positionMs)) / 1000, preferredTimescale: 600)
-        player.seek(to: time, toleranceBefore: .zero, toleranceAfter: .zero)
+        performSeek(to: positionMs)
     }
 
     func seekRelative(_ deltaMs: Int64) {
-        seek(to: snapshot.positionMs + deltaMs)
+        seek(to: referencePositionMs() + deltaMs)
     }
 
     func rewind() {
@@ -138,7 +140,7 @@ final class PlayerController: ObservableObject {
         guard let item = mediaItem, !item.sentences.isEmpty else {
             return false
         }
-        let position = snapshot.positionMs
+        let position = referencePositionMs()
         guard let target = item.sentences.reversed().first(where: { $0.startMs < position - 250 })
             ?? item.sentences.first else {
             return false
@@ -152,7 +154,7 @@ final class PlayerController: ObservableObject {
         guard let item = mediaItem, !item.sentences.isEmpty else {
             return false
         }
-        let position = snapshot.positionMs
+        let position = referencePositionMs()
         guard let target = item.sentences.first(where: { $0.startMs > position + 150 }) else {
             return false
         }
@@ -177,8 +179,7 @@ final class PlayerController: ObservableObject {
         guard let sentence = currentSentence else {
             return
         }
-        let time = CMTime(seconds: Double(sentence.startMs) / 1000, preferredTimescale: 600)
-        player.seek(to: time, toleranceBefore: .zero, toleranceAfter: .zero)
+        performSeek(to: sentence.startMs)
         stopAtSentenceEnd = true
         stopAtSentenceEndIndex = sentence.index
         play()
@@ -277,9 +278,14 @@ final class PlayerController: ObservableObject {
             snapshot = PlaybackSnapshot(mediaId: mediaItem?.id, state: .idle, speed: currentSpeed())
             return
         }
-        let position = currentPositionMs()
-        enforceSentenceBoundary(positionMs: position)
-        let finalPosition = currentPositionMs()
+        let position = referencePositionMs()
+        if pendingSeekPositionMs == nil, lastSentenceIndex == nil {
+            lastSentenceIndex = sentence(at: position)?.index
+        }
+        if pendingSeekPositionMs == nil {
+            enforceSentenceBoundary(positionMs: position)
+        }
+        let finalPosition = referencePositionMs()
         let sentence = sentence(at: finalPosition)
         if sentence?.index != lastSentenceIndex {
             lastSentenceIndex = sentence?.index
@@ -307,6 +313,34 @@ final class PlayerController: ObservableObject {
         return max(0, Int64(seconds * 1000))
     }
 
+    /// 连续手势/快进快退时使用 pending seek 的目标位置，避免仍基于旧的播放位置计算。
+    private func referencePositionMs() -> Int64 {
+        pendingSeekPositionMs ?? currentPositionMs()
+    }
+
+    /// AVPlayer 的 seek 是异步的；所有 seek 必须统一走这里，避免播放循环、
+    /// 用户手势和 seek 完成回调交错后把位置拉回旧句子。
+    private func performSeek(to positionMs: Int64) {
+        let targetPositionMs = max(0, positionMs)
+        seekGeneration += 1
+        let generation = seekGeneration
+        pendingSeekPositionMs = targetPositionMs
+        // 立即切换句子锚点。否则 periodic observer 仍可能以旧句子为锚点，
+        // 在 seek 完成前后把新位置拉回旧句开头。
+        lastSentenceIndex = sentence(at: targetPositionMs)?.index
+        let time = CMTime(seconds: Double(targetPositionMs) / 1000, preferredTimescale: 600)
+        player.seek(to: time, toleranceBefore: .zero, toleranceAfter: .zero) { [weak self] _ in
+            DispatchQueue.main.async {
+                guard let self, self.seekGeneration == generation else {
+                    return
+                }
+                self.pendingSeekPositionMs = nil
+                self.tick()
+            }
+        }
+        tick()
+    }
+
     private func durationMs(of item: AVPlayerItem) -> Int64 {
         let seconds = item.duration.seconds
         guard seconds.isFinite, seconds > 0 else {
@@ -393,8 +427,7 @@ final class PlayerController: ObservableObject {
     }
 
     private func seekRaw(_ positionMs: Int64) {
-        let time = CMTime(seconds: Double(max(0, positionMs)) / 1000, preferredTimescale: 600)
-        player.seek(to: time, toleranceBefore: .zero, toleranceAfter: .zero)
+        performSeek(to: positionMs)
     }
 
     private func handleEnded() {
@@ -402,7 +435,7 @@ final class PlayerController: ObservableObject {
             tick()
             return
         }
-        player.seek(to: .zero, toleranceBefore: .zero, toleranceAfter: .zero)
+        performSeek(to: .zero)
         player.play()
         tick()
     }