Files manager that allow sending background POST requests and retry if the user has been force-quitting the app

FilesManager.swift

//
//  FilesManager.swift
//
//  Created by Ido Monzon on 26.6.2018.
//  Copyright © 2018 Ido Monzon. All rights reserved.
//

import UIKit

class FilesManager: NSObject {
    static let uploadUrl = URL(string: "https://...")!
    static func fullPath(for path: String?) -> String {
        let documentsPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
        return "\(documentsPath)/\(path ?? "fileName.jpg.multipart")"
    }

    static let multipartFileNameSuffix = ".multipart"
    static let urlSessionIdentifierPrefix = "\(Bundle.main.bundleIdentifier ?? "com.app.bundle").uploadfile."
    static func urlSessionConfiguration(for id: String) -> URLSessionConfiguration {
        let id = id.ltrim(text: FilesManager.urlSessionIdentifierPrefix) // Trim the identifier prefix if `id` containing it
        let configuration = URLSessionConfiguration.background(withIdentifier: FilesManager.urlSessionIdentifierPrefix + id)
        configuration.sharedContainerIdentifier = FilesManager.urlSessionIdentifierPrefix
        return configuration
    }
}

// MARK: - Utils

extension FilesManager {
    /// Recreate URLSession with same configuration for file to initiate urlSession(_:task:didCompleteWithError:)
    func recreateSession(id: String) {
        let multipartFileName = id.ltrim(text: FilesManager.urlSessionIdentifierPrefix)
        _ = URLSession(configuration: FilesManager.urlSessionConfiguration(for: multipartFileName), delegate: self, delegateQueue: nil)
    }

    func uploadFile(name fileName: String) {
        var urlRequest = URLRequest(url: FilesManager.uploadUrl)
        urlRequest.httpMethod = "POST"

        let boundary = urlRequest.generateBoundary()
        urlRequest.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")

        let session = URLSession(configuration: FilesManager.urlSessionConfiguration(for: fileName), delegate: self, delegateQueue: nil)
        let filePath = FilesManager.fullPath(for: fileName)
        DispatchQueue.main.async {
            let fileUrl = NSURL(fileURLWithPath: filePath) as URL
            if FileManager.default.fileExists(atPath: fileUrl.path) {
                session.uploadTask(with: urlRequest, fromFile: fileUrl).resume()
            }

            FilesManager.addFileUploadTask(id: session.configuration.identifier, fileName: fileName)
        }
    }

    static func addFileUploadTask(id: String?, fileName: String) {
        guard let sessionId = id else { return }
        var filesUploadTasks = UserDefaults.standard.stringArray(forKey: UserDefaultsKeys.filesUploadTasks) ?? []
        if filesUploadTasks.firstIndex(of: sessionId) == nil { // Append only if doesn't already exists
            filesUploadTasks.append(sessionId)
        }
        UserDefaults.standard.set(filesUploadTasks, forKey: UserDefaultsKeys.filesUploadTasks)
    }

    static func removeFileUploadTask(id: String?) {
        guard let sessionId = id else { return }
        var filesUploadTasks = UserDefaults.standard.stringArray(forKey: UserDefaultsKeys.filesUploadTasks)
        if let index = filesUploadTasks?.firstIndex(of: sessionId) {
            filesUploadTasks?.remove(at: index)
        }
        UserDefaults.standard.set(filesUploadTasks, forKey: UserDefaultsKeys.filesUploadTasks)
    }

    static func taskFinishedCompletion(id: String?) {
        do {
            let fileName = id?.ltrim(text: FilesManager.urlSessionIdentifierPrefix)
            let filePath = FilesManager.fullPath(for: fileName)
            FilesManager.removeFileUploadTask(id: id)

            let fileManager = FileManager.default
            if fileManager.fileExists(atPath: filePath) {
                try fileManager.removeItem(atPath: filePath)
            }
        } catch _ {
        }
    }
}

// MARK: - URLSessionTaskDelegate

extension FilesManager: URLSessionDataDelegate, URLSessionDelegate {
    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        guard let multipartFileName = session.configuration.identifier?.ltrim(text: FilesManager.urlSessionIdentifierPrefix) else { return }

        if let error = error {
            let nsError = (error as NSError)
            guard nsError.userInfo[NSURLErrorBackgroundTaskCancelledReasonKey] as? Int == NSURLErrorCancelledReasonUserForceQuitApplication else { return }

            // File didn't finished upload because of force quit
            // Try uploading again from saved file
            uploadFile(name: multipartFileName)
        } else {
            FilesManager.taskFinishedCompletion(id: session.configuration.identifier)
        }
    }
}

Helpers.swift

import UIKit

extension String {
    /// trim spaces & enters from both sides of the string
    func trim() -> String {
        return trimmingCharacters(in: .whitespaces).trimmingCharacters(in: .newlines)
    }

    /// trim specific text from the beginning
    func ltrim(text: String) -> String {
        return replacingOccurrences(of: text, with: "", options: .anchored)
    }

    /// trim specific text from the ending
    func rtrim(text: String) -> String {
        return replacingOccurrences(of: text, with: "", options: [.anchored, .backwards])
    }
}

extension URLRequest {
    func generateBoundary() -> String {
        return "Boundary-\(userId)"
    }
}