fix: 视频只有一帧+音频超长——重写录制时间基准和音频限速

视频修复:
- startSession(atSourceTime: .zero) 替代 itemTime
- 之前 session 起点是绝对 itemTime(如120s)，但 sample 时间戳
  是相对的(0, 1/30...)，writer 因 0<120 丢弃帧
- 现在统一用 player.currentTime() 作为共享时间基准

音频修复:
- 用 DispatchSourceTimer(20ms) 替代 requestMediaDataWhenReady
- 每 20ms 检查实时进度(elapsed wall-clock)，只写入不超前的音频
- 超前的 sample 缓存到 pendingAudioSample 等下次 tick
- 停止时 cancel timer + cancel reader，音频长度精确对齐视频

音视频同步:
- 两者共享 recordStartTime = player.currentTime()
- 视频: itemTime - recordStartTime
- 音频: audioPTS - recordStartTime

Sources/PlayerRecorder.swift

@@ -12,16 +12,22 @@ final class PlayerRecorder: NSObject {
     private var timer: Timer?
     private var startDate: Date?
     private var weakOutput: AVPlayerItemVideoOutput?
-    private var firstFrameTime: CMTime?
+    nonisolated(unsafe) private var isRunning = false
-    private var audioReader: AVAssetReader?
-    private var audioReaderOutput: AVAssetReaderTrackOutput?
-    private var audioCaptureQueue: DispatchQueue?
-    private var isRunning = false
     
     // 视频：缓存上一帧，确保静态场景也能持续写入
     private var lastPixelBuffer: CVPixelBuffer?
     private var captureFrameCount: Int = 0
     
+    // 音频
+    private var audioReader: AVAssetReader?
+    private var audioReaderOutput: AVAssetReaderTrackOutput?
+    private nonisolated(unsafe) var audioFeedTimer: DispatchSourceTimer?
+    
+    // 共享时间基准：player.currentTime() at record start
+    nonisolated(unsafe) private var recordStartTime: CMTime = .zero
+    nonisolated(unsafe) private var audioStartHostTime: CFTimeInterval = 0
+    nonisolated(unsafe) private var pendingAudioSample: CMSampleBuffer?
+    
     @Published var isRecording = false
     @Published var durationText = "00:00"
     
@@ -37,15 +43,17 @@ final class PlayerRecorder: NSObject {
         return dir
     }
     
-    /// 开始录制
+    // MARK: - 开始录制
     func startRecording(from output: AVPlayerItemVideoOutput, sourceURL: URL, startTime: CMTime) {
         guard !isRecording else { return }
         
         weakOutput = output
-        firstFrameTime = nil
         lastPixelBuffer = nil
         captureFrameCount = 0
         isRunning = false
+        recordStartTime = startTime  // 共享时间基准
+        audioStartHostTime = 0
+        pendingAudioSample = nil
         
         // 临时文件
         let formatter = DateFormatter()
@@ -91,6 +99,8 @@ final class PlayerRecorder: NSObject {
         }
         
         writer.startWriting()
+        // session 起点为 0，视频和音频都用相对时间戳
+        writer.startSession(atSourceTime: .zero)
         
         // 启动视频抓帧
         isRecording = true
@@ -98,11 +108,11 @@ final class PlayerRecorder: NSObject {
         startTimer()
         startCaptureLoop()
         
-        // 启动音频读取（从播放位置开始）
+        // 启动音频读取
         startAudioCapture(sourceURL: sourceURL, startTime: startTime, audioWriterInput: aInput)
     }
     
-    /// 停止录制
+    // MARK: - 停止录制
     func stopRecording() {
         guard isRecording else { return }
         isRecording = false
@@ -111,10 +121,15 @@ final class PlayerRecorder: NSObject {
         captureTimer = nil
         stopTimer()
         
-        // 停止音频：先 cancel reader，再 markAsFinished
+        // 停止音频 timer
+        audioFeedTimer?.cancel()
+        audioFeedTimer = nil
+        
+        // 停止音频 reader
         audioReader?.cancelReading()
         audioReader = nil
         audioReaderOutput = nil
+        pendingAudioSample = nil
         
         videoInput?.markAsFinished()
         audioInput?.markAsFinished()
@@ -123,6 +138,8 @@ final class PlayerRecorder: NSObject {
             Task { @MainActor in
                 guard let self = self else { return }
                 if let url = self.tempURL {
+                    let count = self.captureFrameCount
+                    print("[Recorder] Finish writing: \(count) video frames")
                     self.showSaveDialog(tempURL: url)
                 }
             }
@@ -131,7 +148,7 @@ final class PlayerRecorder: NSObject {
         lastPixelBuffer = nil
     }
     
-    // MARK: - 视频抓帧
+    // MARK: - 视频抓帧 (30fps Timer)
     private var captureTimer: Timer?
     
     private func startCaptureLoop() {
@@ -149,21 +166,15 @@ final class PlayerRecorder: NSObject {
         let hostTime = CACurrentMediaTime()
         let itemTime = output.itemTime(forHostTime: hostTime)
         
-        // 尝试获取新帧，否则复用上一帧（确保静态场景也持续写入）
+        // 尝试获取新帧，否则复用上一帧
         if let pb = output.copyPixelBuffer(forItemTime: itemTime, itemTimeForDisplay: nil) {
             lastPixelBuffer = pb
         }
         
         guard let pb = lastPixelBuffer else { return }
         
-        // 第一帧：启动 session
+        // 相对时间戳：itemTime - recordStartTime
-        if firstFrameTime == nil {
+        let relativeTime = CMTimeSubtract(itemTime, recordStartTime)
-            firstFrameTime = itemTime
-            writer?.startSession(atSourceTime: itemTime)
-        }
-        
-        guard let firstTime = firstFrameTime else { return }
-        let relativeTime = CMTimeSubtract(itemTime, firstTime)
         guard relativeTime.seconds >= 0 else { return }
         
         let sampleBuffer = Self.createSampleBuffer(from: pb, time: relativeTime)
@@ -190,7 +201,7 @@ final class PlayerRecorder: NSObject {
         return sampleBuffer
     }
     
-    // MARK: - 音频读取（从源 URL，用 requestMediaDataWhenReady 控制节奏）
+    // MARK: - 音频读取（DispatchSourceTimer 限速到实时）
     private func startAudioCapture(sourceURL: URL, startTime: CMTime, audioWriterInput: AVAssetWriterInput) {
         let asset = AVURLAsset(url: sourceURL)
         
@@ -225,14 +236,20 @@ final class PlayerRecorder: NSObject {
                     self.audioReader = reader
                     self.audioReaderOutput = readerOutput
                     
-                    // 用 requestMediaDataWhenReady 控制读取节奏（writer 需要数据时才读）
+                    // 记录 wall-clock 起始时间，用于实时限速
-                    let queue = DispatchQueue(label: "recorder.audio.capture")
+                    self.audioStartHostTime = CACurrentMediaTime()
-                    self.audioCaptureQueue = queue
                     
-                    audioWriterInput.requestMediaDataWhenReady(on: queue) { [weak self] in
+                    // 用 DispatchSourceTimer 每 20ms 喂一次音频，限速到实时
-                        guard let self = self else { return }
+                    let queue = DispatchQueue(label: "recorder.audio.feed")
-                        self.feedAudioSamples(writerInput: audioWriterInput, readerOutput: readerOutput, reader: reader)
+                    let feedTimer = DispatchSource.makeTimerSource(queue: queue)
+                    feedTimer.schedule(deadline: .now() + .milliseconds(10), repeating: .milliseconds(20))
+                    feedTimer.setEventHandler { [weak self] in
+                        self?.feedAudioSamplesRealtime(writerInput: audioWriterInput,
+                                                       readerOutput: readerOutput,
+                                                       reader: reader)
                     }
+                    self.audioFeedTimer = feedTimer
+                    feedTimer.resume()
                 }
             } catch {
                 print("[Recorder] Audio capture setup failed: \(error)")
@@ -240,40 +257,60 @@ final class PlayerRecorder: NSObject {
         }
     }
     
-    /// 由 requestMediaDataWhenReady 回调，writer 需要数据时才读取
+    /// 每 20ms 调用，只写入不超过当前实时进度的音频
-    private func feedAudioSamples(writerInput: AVAssetWriterInput,
+    private nonisolated func feedAudioSamplesRealtime(writerInput: AVAssetWriterInput,
-                                   readerOutput: AVAssetReaderTrackOutput,
+                                                       readerOutput: AVAssetReaderTrackOutput,
-                                   reader: AVAssetReader) {
+                                                       reader: AVAssetReader) {
-        while writerInput.isReadyForMoreMediaData {
+        guard isRunning, reader.status == .reading else {
-            guard isRunning, reader.status == .reading,
+            if isRunning && reader.status != .reading {
-                  let sampleBuffer = readerOutput.copyNextSampleBuffer() else {
+                writerInput.markAsFinished()
-                // 录制已停止或读取完毕
+                audioFeedTimer?.cancel()
-                if isRunning {
+            }
+            return
+        }
+        guard writerInput.isReadyForMoreMediaData else { return }
+        
+        // 实时进度：录制开始以来经过的秒数
+        let elapsed = CACurrentMediaTime() - audioStartHostTime
+        let maxAudioTime = elapsed + 0.05  // 允许 50ms 预读
+        
+        // 先处理上次缓存的超前 sample
+        if let pending = pendingAudioSample {
+            let pts = CMSampleBufferGetPresentationTimeStamp(pending)
+            let relative = CMTimeSubtract(pts, recordStartTime)
+            if relative.seconds > maxAudioTime {
+                return  // 还没到时间，等下次 tick
+            }
+            pendingAudioSample = nil
+            if let nb = Self.remapAudioBuffer(pending, offset: recordStartTime) {
+                _ = writerInput.append(nb)
+            }
+        }
+        
+        // 继续读取，直到追上实时进度
+        while isRunning, reader.status == .reading, writerInput.isReadyForMoreMediaData {
+            guard let sampleBuffer = readerOutput.copyNextSampleBuffer() else {
+                // 源文件音频读完
+                if reader.status != .reading {
                     writerInput.markAsFinished()
+                    audioFeedTimer?.cancel()
                 }
                 return
             }
             
-            // 重映射时间戳：源文件时间 → 相对于录制开始的时间
+            let pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
-            guard let firstTime = self.firstFrameTime else {
+            let relative = CMTimeSubtract(pts, recordStartTime)
-                // 视频第一帧还没到，丢弃这个 buffer
+            
-                continue
+            // 超出实时进度 → 缓存，等下次 tick
+            if relative.seconds > maxAudioTime {
+                pendingAudioSample = sampleBuffer
+                return
             }
             
-            var count: CMItemCount = 0
+            // 跳过负时间戳（音频 pts 在 recordStartTime 之前）
-            CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: 0, arrayToFill: nil, entriesNeededOut: &count)
+            guard relative.seconds >= 0 else { continue }
-            var timingInfo = [CMSampleTimingInfo](repeating: CMSampleTimingInfo(), count: count)
-            CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: count, arrayToFill: &timingInfo, entriesNeededOut: nil)
             
-            for i in 0..<count {
+            if let nb = Self.remapAudioBuffer(sampleBuffer, offset: recordStartTime) {
-                timingInfo[i].decodeTimeStamp = .invalid
-                timingInfo[i].presentationTimeStamp = CMTimeSubtract(timingInfo[i].presentationTimeStamp, firstTime)
-            }
-            
-            var newBuffer: CMSampleBuffer?
-            CMSampleBufferCreateCopyWithNewTiming(allocator: kCFAllocatorDefault, sampleBuffer: sampleBuffer, sampleTimingEntryCount: count, sampleTimingArray: &timingInfo, sampleBufferOut: &newBuffer)
-            
-            if let nb = newBuffer {
                 if !writerInput.append(nb) {
                     return
                 }
@@ -281,7 +318,26 @@ final class PlayerRecorder: NSObject {
         }
     }
     
-    // MARK: - Timer
+    /// 重映射音频 sample buffer 的时间戳为相对时间
+    private nonisolated static func remapAudioBuffer(_ sampleBuffer: CMSampleBuffer, offset: CMTime) -> CMSampleBuffer? {
+        var count: CMItemCount = 0
+        CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: 0, arrayToFill: nil, entriesNeededOut: &count)
+        guard count > 0 else { return sampleBuffer }
+        
+        var timingInfo = [CMSampleTimingInfo](repeating: CMSampleTimingInfo(), count: count)
+        CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: count, arrayToFill: &timingInfo, entriesNeededOut: nil)
+        
+        for i in 0..<count {
+            timingInfo[i].decodeTimeStamp = .invalid
+            timingInfo[i].presentationTimeStamp = CMTimeSubtract(timingInfo[i].presentationTimeStamp, offset)
+        }
+        
+        var newBuffer: CMSampleBuffer?
+        CMSampleBufferCreateCopyWithNewTiming(allocator: kCFAllocatorDefault, sampleBuffer: sampleBuffer, sampleTimingEntryCount: count, sampleTimingArray: &timingInfo, sampleBufferOut: &newBuffer)
+        return newBuffer
+    }
+    
+    // MARK: - Timer (显示录制时长)
     private func startTimer() {
         startDate = Date()
         durationText = "00:00"