获取相册视频数据的URL时，因为获取到的对象不是AVURLAsset，而是AVComposition导致的，因为AVComposition没有URL属性

    func aaa() {
        if let asset = avAsset as? AVComposition,
            asset.tracks.count == 2 {// 慢动作视频
            self?.dealSVideo(asset, currentAsset)
        }
    }

    func dealSVideo(_ asset: AVComposition) {
        
        let creater: (URL) -> Void = { dir in
            if !FileManager.default.fileExists(atPath: dir.path) {
                do {
                    try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
                } catch {
                   //--
                }
            }
        }
        // 创建文件夹
        creater(videoDirectory)
        
        let path = videoDirectory.appendingPathComponent("\(fileName).mp4").path
        
        if FileManager.default.fileExists(atPath: path) {
            已导出过-直接使用path
        } else {
            *******HUD show(因为比较耗时)***
            let exporter = AVAssetExportSession(asset: asset,
                                                presetName: AVAssetExportPresetMediumQuality)
            exporter?.outputURL = URL(fileURLWithPath: path)
            exporter?.outputFileType = .mov
            exporter?.shouldOptimizeForNetworkUse = true
            exporter?.exportAsynchronously {
                DispatchQueue.main.async {
                    *******HUD dissmiss***
                    if exporter?.status == .completed {
                        已导出成功-直接使用path
                    }
                }
            }
        }
    }

参考链接：
https://www.jianshu.com/p/486de4f331db 本文章的swift版本
https://www.jianshu.com/p/f0b299a2f2d5 此文章兼容方式不可用