SwiftUI中发起HTTP多部分表单数据请求时出现异常错误

我用以下Swift代码实现POST请求+form-data格式上传文件：

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

let fileName:String = String(request.fileURLForFormData!.path().split(separator: "/").last ?? "")
let fileData = try Data(contentsOf: request.fileURLForFormData!)
var uploadData = Data()

// Append file data
uploadData.append("--\(boundary)\r\n".data(using: .utf8)!)
uploadData.append("Content-Disposition: form-data; name=\"image\"; filename=\"\(fileName)\"\r\n".data(using: .utf8)!)
uploadData.append("Content-Type: \(fileName.mimeType())\r\n\r\n".data(using: .utf8)!)
uploadData.append(fileData)
uploadData.append("\r\n".data(using: .utf8)!)

// Close the form data
uploadData.append("--\(boundary)--\r\n".data(using: .utf8)!)

// Set the request body
urlRequest.httpBody = uploadData
let (data, response) = try await URLSession.shared.upload(for: urlRequest, from: Data(contentsOf: request.fileURLForFormData!))

return .success((data, response))

运行时触发错误：

上传任务的请求不应包含请求体或请求体流，请使用upload(for:fromFile:)、upload(for:from:)，或通过urlSession(_:needNewBodyStreamForTask:)代理方法提供请求体流。

错误核心原因 #

你同时执行了两个冲突操作：给urlRequest设置了httpBody，又调用了URLSession的upload(for:from:)方法。upload系列方法的设计逻辑是自行负责构建请求体，不允许提前给请求对象设置httpBody，否则就会触发该报错。

两种修复方案 #

方案1：改用data方法发送（适合已手动组装好form-data的场景） #

既然已经手动构建了完整的multipart/form-data请求体，直接用data系列方法发送即可，无需调用upload：

let contentType = "multipart/form-data; boundary=\(boundary)"
urlRequest.setValue(contentType, forHTTPHeaderField: "Content-Type")
urlRequest.httpMethod = "POST" // 确保设置POST请求方法

let fileName:String = String(request.fileURLForFormData!.path().split(separator: "/").last ?? "")
let fileData = try Data(contentsOf: request.fileURLForFormData!)
var uploadData = Data()

// Append file data
uploadData.append("--\(boundary)\r\n".data(using: .utf8)!)
uploadData.append("Content-Disposition: form-data; name=\"image\"; filename=\"\(fileName)\"\r\n".data(using: .utf8)!)
uploadData.append("Content-Type: \(fileName.mimeType())\r\n\r\n".data(using: .utf8)!)
uploadData.append(fileData)
uploadData.append("\r\n".data(using: .utf8)!)

// Close the form data
uploadData.append("--\(boundary)--\r\n".data(using: .utf8)!)

// Set the request body
urlRequest.httpBody = uploadData
// 改用data方法发送请求
let (data, response) = try await URLSession.shared.data(for: urlRequest)

return .success((data, response))

方案2：使用upload方法（适合直接上传文件的场景） #

如果后端支持直接接收文件流，无需手动构建form-data，可以去掉httpBody设置，用upload(for:fromFile:)更高效（避免把大文件读入内存）：

// 注意：使用upload方法时，不要设置urlRequest.httpBody
urlRequest.httpMethod = "POST"
// 根据文件类型设置Content-Type
let mimeType = request.fileURLForFormData!.path().mimeType()
urlRequest.setValue(mimeType, forHTTPHeaderField: "Content-Type")

// 直接传入文件URL，无需读入内存
let (data, response) = try await URLSession.shared.upload(for: urlRequest, fromFile: request.fileURLForFormData!)

return .success((data, response))

内容的提问来源于stack exchange，提问作者sumit kumar Pradhan