在输入视频帧之前，为AVAssetWriterInput添加一个空的CMSampleBuffer，以确保捕获的音频数据和视频帧都能与完整的持续时间对应。

示例代码：

//定义一些变量 AVAssetWriterInput *videoInput; AVAssetWriterInput *audioInput; AVAssetWriter *writer; CMTime videoTime; CMSampleBufferRef emptySampleBuffer; CMTime audioStartTime; CMTime videoStartTime;

//初始化AVAssetWriter和输入流 //...

//为videoInput设置一个初始时间戳 videoStartTime = CMTimeMake(0, videoTrack.naturalTimeScale); [videoInput setMediaTimeScale:videoTrack.naturalTimeScale]; [videoInput setMediaTimeRange:CMTimeRangeMake(kCMTimeZero, videoDuration)]; [writer startWriting]; [writer startSessionAtSourceTime:videoStartTime];

//添加一个空的CMSampleBuffer //该样本缓冲区保证视频帧和音频数据都能与持续时间相对应 emptySampleBuffer = [self createEmptySampleBufferForAudioTrack:audioTrack atTime:audioStartTime]; if ([audioInput isReadyForMoreMediaData]) { [audioInput appendSampleBuffer:emptySampleBuffer]; }

//添加视频帧和音频数据 //...

//创建一个空的CMSampleBuffer

• (CMSampleBufferRef)createEmptySampleBufferForAudioTrack:(AVAssetTrack *)audioTrack atTime:(CMTime)time { CMSampleBufferRef sampleBuffer = NULL; CMBlockBufferRef blockBuffer = NULL;

  OSStatus status = CMBlockBufferCreateEmpty(kCFAllocatorDefault, 0, kCMBlockBufferAssureMemoryNowFlag, &blockBuffer); if (status != noErr) { NSLog(@"Error creating empty CMSampleBuffer for audio: %d", status); return NULL; }

  AudioStreamBasicDescription audioFormat = *(audioTrack.formatDescriptions[0]); CMSampleTimingInfo timingInfo = {.duration = kCMTimeInvalid, .presentationTimeStamp = time, .decodeTimeStamp = kCMTimeInvalid};

  CMSampleBufferCreate(kCFAllocatorDefault, blockBuffer, true, NULL, NULL, audioTrack.formatDescriptionsCount, 1, &timingInfo, 0, NULL, &sampleBuffer); CFRelease(blockBuffer);

  return sampleBuffer; }