deltachat-ios/deltachat-ios/Extensions/URL+Extension.swift

import Foundation
import AVKit

extension URL {
    enum ConversionError: Error {
        case runtimeError(String)
    }

    public var queryParameters: [String: String]? {
        guard
            let components = URLComponents(url: self, resolvingAgainstBaseURL: true),
            let queryItems = components.queryItems else { return nil }
        return queryItems.reduce(into: [String: String]()) { result, item in
            result[item.name] = item.value
        }
    }

    public func convertToMp4(completionHandler: ((URL?, Error?) -> Void)?) {
        let avAsset = AVURLAsset(url: self, options: nil)
        AVAssetExportSession.determineCompatibility(ofExportPreset: AVAssetExportPresetPassthrough,
                                                    with: avAsset,
                                                    outputFileType: .mp4,
                                                    completionHandler: { (isCompatible) in
            if !isCompatible {
                completionHandler?(nil, ConversionError.runtimeError("File has incompatible file format for mp4 conversion"))
                return
            }

            guard let exportSession = AVAssetExportSession(asset: avAsset, presetName: AVAssetExportPresetPassthrough) else {
                       completionHandler?(nil, ConversionError.runtimeError("Could not initiate AVAssertExportSession"))
                       return
            }

            let filename = self.deletingPathExtension().lastPathComponent.replacingOccurrences(of: ".", with: "-").appending(".mp4")
            let outputURL = FileManager.default.temporaryDirectory.appendingPathComponent(filename)

            if FileManager.default.fileExists(atPath: outputURL.path) {
                do {
                    try FileManager.default.removeItem(at: outputURL)
                } catch {
                    completionHandler?(nil, error)
                }
            }

            exportSession.outputURL = outputURL
            exportSession.outputFileType = AVFileType.mp4
            exportSession.shouldOptimizeForNetworkUse = true
            let start = CMTimeMakeWithSeconds(0.0, preferredTimescale: 0)
            let range = CMTimeRangeMake(start: start, duration: avAsset.duration)
            exportSession.timeRange = range

            exportSession.exportAsynchronously(completionHandler: {() -> Void in
                switch exportSession.status {
                case .failed:
                    completionHandler?(nil, exportSession.error)
                case .cancelled:
                    completionHandler?(nil, nil)
                case .completed:
                    completionHandler?(exportSession.outputURL, nil)
                default: break
                }
            })
         })
    }
}