Swift - 从CMSampleBuffer获取kCMSampleBufferAttachmentKey_DroppedFrameReason

Question

我试图理解为什么我的AVCaptureOutput正在丢帧。在captureOutput(_ output: AVCaptureOutput, didDrop sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection)委托方法中，我得到一个CMSampleBuffer，其中应包含一个附件，说明删除框架的原因（doc）

原因应该是CFString之一：

kCMSampleBufferDroppedFrameReason_FrameWasLate // "FrameWasLate"
kCMSampleBufferDroppedFrameReason_OutOfBuffers // "OutOfBuffers"
kCMSampleBufferDroppedFrameReason_Discontinuity // "Discontinuity"

从文档来看，目前还不清楚如何获得这个价值。我尝试使用CMGetAttachment，但这会返回CMAttachmentMode又名UInt32：

func captureOutput(_ output: AVCaptureOutput, didDrop sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
  var reason: CMAttachmentMode = 0
  CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_DroppedFrameReason, &reason)
  print("reason \(reason)") // 1
}

我真的不知道如何将这个UInt32与CFString常量匹配

Answer 1

我愚蠢地没有看正确的输出：

var mode: CMAttachmentMode = 0
let reason = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_DroppedFrameReason, &mode)
print("reason \(String(describing: reason))") // Optional(OutOfBuffers)