progressReportAndWaitExample.swift

// NOTE: important to be aware that your reporting that your async is on is not on main thread
// see also https://developer.apple.com/library/content/samplecode/avexporter/Listings/Swift_AVFoundationExporter_main_swift.html

// An Interested party such as a UIViewController (the call site of embedWatermark, for example) can register as delegate 
// and update progress indicators (like a UIProgressView or UILabel).
protocol VideoProcessingProgressDelegate {
    func watermarkProgressUpdated(progress: Float)
}

class VideoProcessingProgressExample {

    public func embedWatermark(progressDelegate: VideoProcessingProgressDelegate, success: @escaping (() -> Void)?) {
      
      DispatchQueue.init(label: "watermarkDispatchQueue").async {
        let mixComposition = AVMutableComposition()
        //...
        guard let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality) else { return }
        // configuration work preppring watermark embedding into video ...

        // Kick off export asynchronous operation & use dispatch group to facilitate wait and progress reporting during processing
        let group = DispatchGroup()
        group.enter()

        exporter.exportAsynchronously(completionHandler: {
            group.leave()
        })

        self.reportProgressForAsyncExportSessionAndWait(exportSession: exporter, group: group, progressDelegate: watermarkProgressDelegate)

        if exporter.status == AVAssetExportSessionStatus.completed {
            let outputURL = exporter.outputURL
            success?() // output params could be passed here
            // success?(AVAssetExportSessionStatus.completed, exporter, outputURL!)
        } else {
            // Error
            if let systemError = exporter.error {
                print("Error: \(systemError)")
            }
        } // end if/else
        
      } // end dispatch async
    } // end embedWatermark
  
    // facilitates progress updates
    private class func reportProgressForAsyncExportSessionAndWait(exportSession: AVAssetExportSession, group: DispatchGroup, progressDelegate: VideoProcessingProgressDelegate?) {
        while exportSession.status == .waiting || exportSession.status == .exporting {
            progressDelegate?.watermarkProgressUpdated(progressFloat: exportSession.progress)
            group.wait(timeout: DispatchTime(uptimeNanoseconds: 1000 * NSEC_PER_MSEC))
        }

        print("WATERMARK COMPLETED!")
    }
  
}


// Call Site Example
// =====================================
// MEANWHILE & ELSEWHERE IN ANOTHER FILE
// =====================================

Class VideoEditViewController: UIViewController {
    // ...
    
    @IBOutlet weak var progressView: UIProgressView!
    
    // call site
    func addWatermarkToVideo() {
        VideoProcessingProgressExample.embedWatermark(progressDelegate: self, success: {
            // processing completed successfully
        })
    }
}

extention VideoEditViewController: VideoProcessingProgressDelegate {
    func watermarkProgressUpdated(progress: Float) {
        DispatchQueue.main.async {
            self.progressView.setProgress(progress, animated: true)
        }
    }
}