How to export slow motion video (240 fps) from camera roll in objective c?

Question

I am going to work with slow motion video (240fps) selected from camera roll.

I've used following code to load asset from camera roll.

PHVideoRequestOptions *options = [PHVideoRequestOptions new];
                options.version = PHVideoRequestOptionsVersionCurrent;
[[PHImageManager defaultManager] requestAVAssetForVideo:phasset options:options resultHandler:^(AVAsset * _Nullable asset, AVAudioMix * _Nullable audioMix, NSDictionary * _Nullable info) {
//use asset to check fps : exported asset's fps is not 240 fps for 240 fps I recorded using native iPhone camera. 
}

If I export asset's url to file then I can get slow motion effected video. But its fps is 30 or 43.33 fps.

What's happened ? Is there any way to keep fps for exported slow motion asset ?

I've tried to googling whole day but not solved this issue.

Any help would be appreciated. Thanks