在URLSession.uploadTask上設定邊界：with:fromFile

我正在嘗試將影像上傳到彈簧后端。它應該在后臺作業，因此我只能使用session.uploadTask函式 我的問題是后端希望我設定Content-Type標題。一個關鍵部分是boundary在我的請求正文中相應地定義和使用它，但是我應該如何boundary在影像上設定我的？

我見過的大多數教程都使用該session.uploadData功能來執行此操作，當您想在后臺執行操作時，該功能不可用。在那里我可以簡單地將 附加boundary到資料。

總結一下：boundary上傳圖片時如何正確使用標題欄位uploadTask(with request: URLRequest, fromFile fileURL: URL)？

我從春天收到這個錯誤：

org.springframework.web.multipart.MultipartException: Current request is not a multipart request

我的代碼：

let boundary = UUID().uuidString
// A background upload task must specify a file
var imageURLRequest = URLRequest(url: uploadURL)
imageURLRequest.httpMethod = "Post"
imageURLRequest.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
                
let imageTask = URLBackgroundSession.shared.uploadTask(with: imageURLRequest, fromFile: URL(fileURLWithPath: imagePath))
imageTask.resume()

uj5u.com熱心網友回復：

在您找到的那些其他示例中（例如，在 Swift 中使用引數上傳影像），他們構建了一個Data符合格式正確的multipart/form-data請求并在請求正文中使用的請求。

你必須在這里做同樣的事情，除了不是構建一個Data，你將創建一個臨時檔案，將所有這些寫入該檔案，然后在你的uploadTask.

例如：

func uploadImage(from imageURL: URL, filePathKey: String, to uploadURL: URL) throws {
    let boundary = UUID().uuidString
    var imageURLRequest = URLRequest(url: uploadURL)
    imageURLRequest.httpMethod = "POST"
    imageURLRequest.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")

    let folder = URL(filePath: NSTemporaryDirectory()).appending(path: "uploads")
    try FileManager.default.createDirectory(at: folder, withIntermediateDirectories: true)
    let fileURL = folder.appendingPathExtension(boundary)

    guard let outputStream = OutputStream(url: fileURL, append: false) else {
        throw OutputStream.OutputStreamError.unableToCreateFile(fileURL)
    }

    outputStream.open()
    try outputStream.write("--\(boundary)\r\n")
    try outputStream.write("Content-Disposition: form-data; name=\"\(filePathKey)\"; filename=\"\(imageURL.lastPathComponent)\"\r\n")
    try outputStream.write("Content-Type: \(imageURL.mimeType)\r\n\r\n")
    try outputStream.write(contentsOf: imageURL)
    try outputStream.write("\r\n")
    try outputStream.write("--\(boundary)--\r\n")
    outputStream.close()

    let imageTask = URLBackgroundSession.shared.uploadTask(with: imageURLRequest, fromFile: fileURL)
    imageTask.resume()
}

您可能應該洗掉urlSession(_:task:didCompleteWithError:).

FWIW，上面使用以下擴展來簡化的生成OutputStream：

extension OutputStream {
    enum OutputStreamError: Error {
        case stringConversionFailure
        case unableToCreateFile(URL)
        case bufferFailure
        case writeFailure
        case readFailure
    }

    /// Write `String` to `OutputStream`
    ///
    /// - parameter string:                The `String` to write.
    /// - parameter encoding:              The `String.Encoding` to use when writing the string. This will default to `.utf8`.
    /// - parameter allowLossyConversion:  Whether to permit lossy conversion when writing the string. Defaults to `false`.

    func write(_ string: String, encoding: String.Encoding = .utf8, allowLossyConversion: Bool = false) throws {
        guard let data = string.data(using: encoding, allowLossyConversion: allowLossyConversion) else {
            throw OutputStreamError.stringConversionFailure
        }
        try write(data)
    }

    /// Write `Data` to `OutputStream`
    ///
    /// - parameter data:                  The `Data` to write.

    func write(_ data: Data) throws {
        try data.withUnsafeBytes { (buffer: UnsafeRawBufferPointer) throws in
            guard var pointer = buffer.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
                throw OutputStreamError.bufferFailure
            }

            var bytesRemaining = buffer.count

            while bytesRemaining > 0 {
                let bytesWritten = write(pointer, maxLength: bytesRemaining)
                if bytesWritten < 0 {
                    throw OutputStreamError.writeFailure
                }

                bytesRemaining -= bytesWritten
                pointer  = bytesWritten
            }
        }
    }

    /// Write contents of local `URL` to `OutputStream`
    ///
    /// - parameter fileURL:                  The `URL` of the file to written to this output stream.

    func write(contentsOf fileURL: URL) throws {
        guard let inputStream = InputStream(url: fileURL) else {
            throw OutputStreamError.readFailure
        }

        inputStream.open()

        let bufferSize = 1024
        let buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: bufferSize)
        defer { buffer.deallocate() }

        while inputStream.hasBytesAvailable {
            let length = inputStream.read(buffer, maxLength: bufferSize)
            if length > 0 {
                write(buffer, maxLength: length)
            }
        }

        inputStream.close()
    }
}

順便說一句，使用檔案而不是檔案上傳和下載的優點之一Data是記憶體占用更小，避免在任何給定時間將整個資產加載到記憶體中。因此，本著這種精神，我使用一個小緩沖區將影像的內容寫入臨時檔案。這在上傳圖片時可能并不重要，但在上傳更大的資產（如視頻）時可能變得必不可少。

無論如何，以上內容還確定了使用此擴展的資產的 mimetype：

extension URL {
    /// Mime type for the URL
    ///
    /// Requires `import UniformTypeIdentifiers` for iOS 14 solution.
    /// Requires `import MobileCoreServices` for pre-iOS 14 solution

    var mimeType: String {
        if #available(iOS 14.0, *) {
            return UTType(filenameExtension: pathExtension)?.preferredMIMEType ?? "application/octet-stream"
        } else {
            guard
                let identifier = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, pathExtension as CFString, nil)?.takeRetainedValue(),
                let mimeType = UTTypeCopyPreferredTagWithClass(identifier, kUTTagClassMIMEType)?.takeRetainedValue() as String?
            else {
                return "application/octet-stream"
            }

            return mimeType
        }
    }
}

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