Swift 5.7多表单数据图片上传异常：3张图请求无响应

我在Swift 5.7环境下，尝试通过multipart/form-data请求向服务器发送3张图片及其他附加数据，但仅移除后两张图片时，请求才能正常触达服务器。以下是相关代码片段，寻求问题排查方向，实现3张图片同请求上传。

相关代码 #

Service.swift #

let networker = Networker()
let serverUrl = URL(string: "https://myurl.com")
var urlRequest = URLRequest(url: serverUrl!)

let boundary = UUID().uuidString

urlRequest.httpMethod = "POST"
urlRequest.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
urlRequest.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")

let requestData = networker.buildMultipartImageRequest(boundary: boundary, item: items)

URLSession.shared.uploadTask(with: urlRequest, from: requestData) {(data, response, error) in
    // 处理响应
}.resume()

Networker.swift #

func buildMultipartImageRequest(boundary: String, item: ItemForBackend, itemId:UInt = 0) -> Data {
    var data = Data()
    
    // 添加上传图片数据
    data.append("\r\n--\(boundary)\r\n".data(using: .utf8)!)
    data.append("Content-Disposition: form-data; name=\"images\"; filename=\"\(item.filename1)\"\r\n".data(using: .utf8)!)
    data.append("Content-Type: image/jpeg\r\n\r\n".data(using: .utf8)!)
    data.append(item.image1)

//    取消注释这两段后，请求完全无法发送！
//        data.append("\r\n--\(boundary)\r\n".data(using: .utf8)!)
//        data.append("Content-Disposition: form-data; name=\"images\"; filename=\"\(item.filename2)\"\r\n".data(using: .utf8)!)
//        data.append("Content-Type: image/jpeg\r\n\r\n".data(using: .utf8)!)
//        data.append(item.image2)
//        
//        data.append("\r\n--\(boundary)\r\n".data(using: .utf8)!)
//        data.append("Content-Disposition: form-data; name=\"images\"; filename=\"\(item.filename3)\"\r\n".data(using: .utf8)!)
//        data.append("Content-Type: image/jpeg\r\n\r\n".data(using: .utf8)!)
//        data.append(item.image3)
    
    // 添加附加参数
    let paramObj:[String: Any] = item.getParams()
    
    for(key, value) in paramObj {
        data.append("\r\n--\(boundary)\r\n".data(using: .utf8)!)
        data.append("Content-Disposition: form-data; name=\"\(key)\"\r\n\r\n".data(using: .utf8)!)
        data.append("\(value)\r\n".data(using: .utf8)!)
    }
    
    if itemId != 0 {
        data.append("\r\n--\(boundary)\r\n".data(using: .utf8)!)
        data.append("Content-Disposition: form-data; name=\"itemId\"\r\n\r\n".data(using: .utf8)!)
        data.append("\(itemId)\r\n".data(using: .utf8)!)
    }
    
    data.append("\r\n--\(boundary)--\r\n".data(using: .utf8)!)
    
    return data
}

排查方向与解决建议 #

• 检查请求数据大小限制：三张图片总大小可能超过服务器配置的最大请求体限制（比如Nginx的client_max_body_size），或者触发了客户端内存预警。可以打印requestData.count对比单图和三图时的数据量，同时联系后端确认服务器的大小限制。
• 修正multipart字段命名格式：部分后端框架（如PHP）要求多文件字段名带[]后缀才能识别，把图片字段的name=\"images\"改成name=\"images[]\"试试。
• 验证图片数据有效性：检查image2和image3是否为完整有效的JPEG数据，打印它们的字节数确认数据长度合理，避免空数据或损坏数据导致请求格式错误。
• 改用OutputStream构建请求数据：手动拼接大Data容易出现内存溢出或隐式拼接错误，用OutputStream流式构建数据更稳定，示例代码如下：

func buildMultipartImageRequest(boundary: String, item: ItemForBackend, itemId: UInt = 0) -> Data {
    let outputStream = OutputStream(toMemory: ())
    outputStream.open()
    
    func write(_ string: String) {
        if let data = string.data(using: .utf8) {
            data.withUnsafeBytes { buffer in
                outputStream.write(buffer.baseAddress!.assumingMemoryBound(to: UInt8.self), maxLength: data.count)
            }
        }
    }
    
    // 第一张图
    write("\r\n--\(boundary)\r\n")
    write("Content-Disposition: form-data; name=\"images[]\"; filename=\"\(item.filename1)\"\r\n")
    write("Content-Type: image/jpeg\r\n\r\n")
    item.image1.withUnsafeBytes { buffer in
        outputStream.write(buffer.baseAddress!.assumingMemoryBound(to: UInt8.self), maxLength: item.image1.count)
    }
    
    // 第二张图
    write("\r\n--\(boundary)\r\n")
    write("Content-Disposition: form-data; name=\"images[]\"; filename=\"\(item.filename2)\"\r\n")
    write("Content-Type: image/jpeg\r\n\r\n")
    item.image2.withUnsafeBytes { buffer in
        outputStream.write(buffer.baseAddress!.assumingMemoryBound(to: UInt8.self), maxLength: item.image2.count)
    }
    
    // 第三张图
    write("\r\n--\(boundary)\r\n")
    write("Content-Disposition: form-data; name=\"images[]\"; filename=\"\(item.filename3)\"\r\n")
    write("Content-Type: image/jpeg\r\n\r\n")
    item.image3.withUnsafeBytes { buffer in
        outputStream.write(buffer.baseAddress!.assumingMemoryBound(to: UInt8.self), maxLength: item.image3.count)
    }
    
    // 附加参数
    let paramObj = item.getParams()
    for (key, value) in paramObj {
        write("\r\n--\(boundary)\r\n")
        write("Content-Disposition: form-data; name=\"\(key)\"\r\n\r\n")
        write("\(value)\r\n")
    }
    
    if itemId != 0 {
        write("\r\n--\(boundary)\r\n")
        write("Content-Disposition: form-data; name=\"itemId\"\r\n\r\n")
        write("\(itemId)\r\n")
    }
    
    write("\r\n--\(boundary)--\r\n")
    
    outputStream.close()
    return outputStream.property(forKey: .dataWrittenToMemoryStreamKey) as! Data
}

• 更换URLSession上传方式：尝试用dataTask(with:request, completionHandler:)替代uploadTask，同时打印error信息，确认请求失败的具体原因。
• 确认服务器端接收逻辑：联系后端开发人员，确认服务器是否支持接收多个同名字段的文件，是否需要调整后端的文件接收配置。

内容的提问来源于stack exchange，提问作者Tank12