将AVAset压缩并编码为mp4

Question

我正在尝试压缩AVAsset以降低质量，然后将其导出为mp4。输入资产可以是相机胶卷允许的任何类型的视频。

我遇到的问题是，当我尝试将资产导出到AVAssetExportPresetMediumQuality时，当输入最初是mp4文件时，它在第一个保护声明中失败。但是，如果我将其更改为AVAssetExportPresetPassthrough，则可以使用，但不会压缩文件。是否有一种方法来压缩/编码最初可能是.mov或.mp4的资产？

func encodeVideo(asset: AVAsset, completionHandler: @escaping (URL?, String?) -> ())  
    guard let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetPassthrough) else 
        completionHandler(nil, nil)
        return
    
    let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0] as URL
    let filePath = documentsDirectory.appendingPathComponent("compressed-video.mp4")
    if FileManager.default.fileExists(atPath: filePath.path) 
        do 
            try FileManager.default.removeItem(at: filePath)
         catch 
            completionHandler(nil, nil)
        
    
    exportSession.outputURL = filePath
    exportSession.outputFileType = .mp4
    exportSession.shouldOptimizeForNetworkUse = true
    let start = CMTimeMakeWithSeconds(0.0, preferredTimescale: 0)
    let range = CMTimeRangeMake(start: start, duration: asset.duration)
    exportSession.timeRange = range

    exportSession.exportAsynchronously 
        switch exportSession.status 
        case .failed:
            print("Conversion Failed")
            completionHandler(nil, exportSession.error?.localizedDescription)
        case .cancelled:
            print("Conversion Cancelled")
            completionHandler(nil, exportSession.error?.localizedDescription)
        case .completed:
            completionHandler(exportSession.outputURL, nil)
            default: break