How to flip a video using AVFoundation

Swift 5. AVCaptureSession:

let movieFileOutput = AVCaptureMovieFileOutput()
let connection = movieFileOutput.connection(with: .video)

if connection?.isVideoMirroringSupported ?? false {
    connection?.isVideoMirrored = true
}

Same for PhotoOutput.

You can use -[AVMutableVideoCompositionLayerInstruction setTransform:atTime:]

CGAffineTransform transform = CGAffineTransformMakeTranslation(self.config.videoSize, 0);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
[videoCompositionLayerInstruction setTransform:transform atTime:videoTime];

// then append video tracks
// [compositionTrack insertTimeRange:timeRange ofTrack:track atTime:atTime error:&error];

// apply instructions
videoCompositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, composition.duration);
videoCompositionInstruction.layerInstructions = @[videoCompositionLayerInstruction];

videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.renderSize = CGSizeMake(self.config.videoSize, self.config.videoSize);
videoComposition.frameDuration = CMTimeMake(1, self.config.videoFrameRate);
videoComposition.instructions = @[videoCompositionInstruction];

https://github.com/ElfSundae/AVDemo/tree/ef2ca437d0d8dcb3dd41c5a272c8754a29d8a936/AVSimpleEditoriOS

Export composition:

AVAssetExportSession *exportSession = [AVAssetExportSession exportSessionWithAsset:composition presetName:presetName];
exportSession.outputFileType = AVFileTypeMPEG4;
exportSession.outputURL = outputURL;
exportSession.shouldOptimizeForNetworkUse = YES;
// videoComposition contains transform instructions for video tracks
exportSession.videoComposition = videoComposition;
// audioMix contains background music for audio tracks
exportSession.audioMix = audioMix;

[exportSession exportAsynchronouslyWithCompletionHandler:^{
    AVAssetExportSessionStatus status = exportSession.status;
    if (status != AVAssetExportSessionStatusCompleted) {
        // exportSession.error
    } else {
        // exportSession.outputURL
    }
}];

Theres no indication on what you are using to record the video, ill assume AVCaptureSession + AVCaptureVideoDataOutput

lazy var videoFileOutput: AVCaptureVideoDataOutput = AVCaptureVideoDataOutput()
let v = videoFileOutput.connectionWithMediaType(AVMediaTypeVideo)
v.videoOrientation = .Portrait
v.videoMirrored = true