ios - How to set frame rate when using iOS VTCompression to encode video

Question

In my work, I try to use iOS VideoToolBox to encode my video from capture. It worked . But when I try to set the fps, it can not work, the video encode stream's fps always is 30.

In my code, I used this code to try to set fps (but it not work)

status =VTSessionSetProperty(EncodingSession, kVTCompressionPropertyKey_ProfileLevel,
kVTProfileLevel_H264_Main_AutoLevel);

status =VTSessionSetProperty(EncodingSession, kVTCompressionPropertyKey_AverageBitRate,(__bridge CFTypeRef _Nonnull)(@500000));
status =VTSessionSetProperty(EncodingSession, kVTCompressionPropertyKey_MaxKeyFrameInterval, (__bridge CFTypeRef _Nonnull)(@15));

status =VTSessionSetProperty(EncodingSession, kVTCompressionPropertyKey_H264EntropyMode, kVTH264EntropyMode_CABAC);

status = VTSessionSetProperty(EncodingSession, kVTCompressionPropertyKey_RealTime, kCFBooleanTrue);