在某些情况下，找不到完整的Firebase swift write（toFile :)文件

Question

这个问题困扰着我。我想怀疑一个种族状况。但是我不能得出结论。

这是我的代码的简单版本：

override func viewDidLoad() {
  private func downloadAssets(completion: @escaping () -> Void) {
    getAssetsForCurrentUser {
      let images = Image.all()
      for image in images {
        // uiImage here sometimes is nil
        // and it causes a crash because I am not handling `nil`.
        // and restarting the app would fix itself
        // but reading my code over and over again still brought 
        // me to the conclusion that I am strictly reading the 
        // content of the file after it completed the download. 
        // So why would I be getting `nil` here?
        let uiImage = UIImage(contentOf: image.localPath)
      }
    }
  }

  private func getAssetsForCurrentUser(inProgress: @escaping (Float) -> Void, completion: @escaping () -> Void) {
      let assetsManager = AssetsManager()
      assetsManager.getAssetsForCurrentUser(inProgress: { progressPercentage in
          inProgress(Float(progressPercentage))
      }, completion: {
          completion()
      }, onError: { errMsg in
          print(errMsg)
      })
  }
}

class AssetManager {
  func getAssetsForCurrentUser(inProgress: @escaping (_ progressPercentage: Float) -> Void,
                             completion: @escaping () -> Void,
                             onError: @escaping (_ errMsg: String) -> Void) {
    API().getAssetsForUserWithId(userId, completion: { imagesAssetUrlStrings in
        self.downloadAssetsFromImagesAssetUrlStrings(imagesAssetUrlStrings, inProgress: { _ in
            inProgress(self.progressPercentage)
        }, completion: completion)
    }, onError: { error in
        onError("error when getting alubms for current user \(error)")
    })
  }

  private func downloadAssetsFromImagesAssetUrlStrings(_ imagesAssetUrlStrings: AlbumsAPI.AlbumDataType, inProgress: @escaping (_ progressPercentage: Float) -> Void, completion: @escaping () -> Void) {
    if imagesAssetUrlStrings.count == 0 {
        print("no image")
        completion()
    }

    totalTaskCount = imagesAssetUrlStrings.count

    for image in imagesAssetUrlStrings {            
        let imageDownloadTask = FileDownloader().downloadFile(.Image, from: imageRemoteUrlString, as: imageFileName, completion: { _ in
            print("Image download success from \(imageRemoteUrlString)")
            self.markTaskAsCompleted()

            if self.isAllTaskDone {
                completion()
            }
        })

        imageDownloadTask.observe(.progress) { snapshot in
            let completedUnitCount = snapshot.progress?.completedUnitCount ?? 0
            let totalUnitCount = snapshot.progress?.totalUnitCount ?? 0
            let taskId = imageDownloadTask.id
            inProgress(self.progressPercentage)
            self.recordProgress(taskId: taskId, completedUnitCount: completedUnitCount, totalUnitCount: totalUnitCount)
        }
     }
  }
}

class FileDownloader {
  func downloadFile(_ fileType: FileType, from urlString: String, as fileName: String, completion: @escaping (String?) -> Void) -> DownloadTask {
    let filePath = LocalStorage.documentPath(of: fileType).appendingPathComponent(fileName)
    if !LocalStorage.fileExists(of: fileType, fileName: fileName) {
        print("file not found at \(filePath)")

        let storage = Storage.storage()
        let storageRef = storage.reference(forURL: urlString)

        let downloadTask = storageRef.write(toFile: filePath) { (localUrl, error) in
            if let err = error {
                Rollbar.error(
                    withMessage: "Error writing file",
                    data: [
                        "error": err,
                        "localUrl": localUrl,
                        "fileName": fileName
                    ]
                )
                print("Error writing file: \(err)")
            } else {
                print("file saved at \(filePath.path)")
                completion(filePath.relativePath)
            }
        }
        return downloadTask
    } else {
        print("file found at \(filePath)")
        completion(filePath.relativePath)
        return DownloadTask(firebaseDownloadTask: nil)
    }
  }
}