跳到主要内容

URLSession 详解

问题​

URLSession 的架构和核心 API 是什么？如何发起网络请求？

答案​

URLSession 架构​

Configuration 类型​

类型说明
.default使用磁盘缓存、Cookie
.ephemeral无持久化（类似无痕模式）
.background后台下载/上传（App 被挂起仍继续）

基本请求（async/await）​

// GET 请求
func fetchUser(id: Int) async throws -> User {
    let url = URL(string: "https://api.example.com/users/\(id)")!
    let (data, response) = try await URLSession.shared.data(from: url)

    guard let httpResponse = response as? HTTPURLResponse,
          httpResponse.statusCode == 200 else {
        throw NetworkError.invalidResponse
    }

    return try JSONDecoder().decode(User.self, from: data)
}

// POST 请求
func createUser(_ user: User) async throws -> User {
    var request = URLRequest(url: URL(string: "https://api.example.com/users")!)
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.httpBody = try JSONEncoder().encode(user)

    let (data, _) = try await URLSession.shared.data(for: request)
    return try JSONDecoder().decode(User.self, from: data)
}

后台下载​

let config = URLSessionConfiguration.background(withIdentifier: "com.app.download")
let session = URLSession(configuration: config, delegate: self, delegateQueue: nil)

let task = session.downloadTask(with: url)
task.resume()

// 通过 delegate 接收进度和完成回调
extension DownloadManager: URLSessionDownloadDelegate {
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask,
                    didFinishDownloadingTo location: URL) {
        // 文件下载到临时路径，需要移动到目标路径
        try? FileManager.default.moveItem(at: location, to: destinationURL)
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask,
                    didWriteData bytesWritten: Int64, totalBytesWritten: Int64,
                    totalBytesExpectedToWrite: Int64) {
        let progress = Double(totalBytesWritten) / Double(totalBytesExpectedToWrite)
        print("Progress: \(progress)")
    }
}

请求超时与重试​

var request = URLRequest(url: url)
request.timeoutInterval = 15  // 请求超时 15 秒

let config = URLSessionConfiguration.default
config.timeoutIntervalForResource = 300  // 整体资源超时 5 分钟
config.waitsForConnectivity = true       // 等待网络恢复

常见面试问题​

Q1: URLSession.shared 和自定义 URLSession 的区别？​

答案：

  • URLSession.shared：单例，不可设置 delegate，不可自定义 configuration，适合简单请求
  • 自定义 URLSession：可设置缓存策略、Cookie 策略、后台任务、证书校验等

Q2: 如何实现断点续传？​

答案：

  1. 下载失败时保存 resumeData（包含已下载的范围信息）
  2. 恢复时调用 session.downloadTask(withResumeData: resumeData)
  3. 服务器需要支持 HTTP Range 请求

相关链接​