网络与 API

iOS 网络层以 URLSession 为核心，生态中有 Alamofire 简化请求。对照 /roadmaps/ios「Networking」节点：REST JSON 是主流，GraphQL 在部分 App 中使用。

学习目标

• 用 URLSession 发起 GET / POST 并解析 JSON
• 封装 API Client，统一错误处理与解码
• 了解 Alamofire、认证头与 REST 最佳实践
• 对照 Spring Boot REST 与 React fetch

核心概念

URLSession 基础

import Foundation

enum APIError: Error, LocalizedError {
    case invalidURL
    case httpStatus(Int)
    case decodingFailed

    var errorDescription: String? {
        switch self {
        case .invalidURL: return "URL 无效"
        case .httpStatus(let code): return "HTTP \(code)"
        case .decodingFailed: return "JSON 解析失败"
        }
    }
}

struct APIClient {
    let baseURL: URL
    let session: URLSession

    init(baseURL: URL, session: URLSession = .shared) {
        self.baseURL = baseURL
        self.session = session
    }

    func get<T: Decodable>(_ path: String, as type: T.Type) async throws -> T {
        guard let url = URL(string: path, relativeTo: baseURL) else {
            throw APIError.invalidURL
        }
        var request = URLRequest(url: url)
        request.setValue("application/json", forHTTPHeaderField: "Accept")
        let (data, response) = try await session.data(for: request)
        guard let http = response as? HTTPURLResponse else {
            throw APIError.httpStatus(-1)
        }
        guard (200..<300).contains(http.statusCode) else {
            throw APIError.httpStatus(http.statusCode)
        }
        do {
            return try JSONDecoder().decode(T.self, from: data)
        } catch {
            throw APIError.decodingFailed
        }
    }
}

// 使用
let client = APIClient(baseURL: URL(string: "https://api.coderoadmap.local")!)
let tracks: [Track] = try await client.get("/api/tracks", as: [Track].self)

async/await 版 URLSession（iOS 15+）替代传统 completion handler，代码更清晰。

POST 与认证

func login(email: String, password: String) async throws -> AuthToken {
    var request = URLRequest(url: baseURL.appendingPathComponent("/api/auth/login"))
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.httpBody = try JSONEncoder().encode(["email": email, "password": password])
    let (data, response) = try await session.data(for: request)
    // 解析 token，存入 Keychain
    return try JSONDecoder().decode(AuthToken.self, from: data)
}