Swift: Swift 枚举与模式匹配教程：从基础关联值到递归枚举

// 不优雅的做法：用多个可选变量表示状态
var data: NewsResponse? = nil
var error: Error? = nil
var isLoading = false

enum NetworkResult<T> {
    case success(T)
    case failure(Error)
    case loading
}

// ============================================
// 使用原始值的 HTTP 状态码枚举
// ============================================
enum HTTPStatus: Int {
    case ok = 200
    case created = 201
    case badRequest = 400
    case unauthorized = 401
    case notFound = 404
    case serverError = 500
}
let status = HTTPStatus.notFound
print("Code: \(status.rawValue)")
print("Name: \(status)")
// 从原始值反向创建
let parsed = HTTPStatus(rawValue: 200)
print("Parsed: \(parsed ?? .serverError)")

// ============================================
// 枚举自定义方法：判断状态类别
// ============================================
enum HTTPStatus: Int {
    case ok = 200
    case created = 201
    case badRequest = 400
    case unauthorized = 401
    case notFound = 404
    case serverError = 500
    var isSuccess: Bool {
        rawValue >= 200 && rawValue < 300
    }
    var description: String {
        switch self {
        case .ok: return "OK"
        case .created: return "Created"
        case .badRequest: return "Bad Request"
        case .unauthorized: return "Unauthorized"
        case .notFound: return "Not Found"
        case .serverError: return "Internal Server Error"
        }
    }
}
let code = HTTPStatus.ok
print("\(code.rawValue) \(code.description) - Success: \(code.isSuccess)")

// ============================================
// 关联值枚举处理 API 响应
// ============================================
enum NetworkResult<T> {
    case success(T)
    case failure(String)
    case loading
}
func handleResponse<T>(_ result: NetworkResult<T>) {
    switch result {
    case .success(let data):
        print("成功获取数据: \(data)")
    case .failure(let error):
        print("请求失败: \(error)")
    case .loading:
        print("加载中...")
    }
}
handleResponse(NetworkResult.success("用户信息"))
handleResponse(NetworkResult.failure("网络连接超时"))
handleResponse(NetworkResult.loading)

// ============================================
// if-case 和 guard-case 的简洁用法
// ============================================
enum Measurement {
    case weight(Double)
    case height(Double)
    case count(Int)
}
let record = Measurement.weight(75.5)
// if-case 只关心一种情况
if case .weight(let kg) = record, kg > 70 {
    print("体重大于 70kg: \(kg)")
}
// guard-case 提前退出
func process(_ m: Measurement) {
    guard case .count(let n) = m else {
        print("不是计数类型")
        return
    }
    print("计数为 \(n)")
}
process(.weight(65))
process(.count(42))

// ============================================
// 递归枚举：数学表达式树
// ============================================
indirect enum ArithmeticExpression {
    case number(Int)
    case addition(ArithmeticExpression, ArithmeticExpression)
    case multiplication(ArithmeticExpression, ArithmeticExpression)
}
func evaluate(_ expr: ArithmeticExpression) -> Int {
    switch expr {
    case .number(let value):
        return value
    case .addition(let left, let right):
        return evaluate(left) + evaluate(right)
    case .multiplication(let left, let right):
        return evaluate(left) * evaluate(right)
    }
}
// 构建表达式 (3 + 5) * 2
let three = ArithmeticExpression.number(3)
let five = ArithmeticExpression.number(5)
let sum = ArithmeticExpression.addition(three, five)
let product = ArithmeticExpression.multiplication(sum, .number(2))
print("(3 + 5) * 2 = \(evaluate(product))")

// ============================================
// 完整示例：电商订单状态机
// 功能：订单状态流转 + 数据处理
// ============================================
import Foundation
// 1. 订单状态枚举（关联值）
enum OrderStatus {
    case pending          // 待支付
    case paid(Date)       // 已支付（含支付时间）
    case shipped(trackingNumber: String)  // 已发货（含运单号）
    case delivered(Date)  // 已送达（含签收时间）
    case cancelled(reason: String)  // 已取消（含原因）
    var description: String {
        switch self {
        case .pending: return "待支付"
        case .paid(let date): return "已支付 (\(date.formatted()))"
        case .shipped(let tracking): return "已发货 (运单: \(tracking))"
        case .delivered(let date): return "已送达 (\(date.formatted()))"
        case .cancelled(let reason): return "已取消: \(reason)"
        }
    }
    var isFinalState: Bool {
        if case .delivered = self { return true }
        if case .cancelled = self { return true }
        return false
    }
}
// 2. 使用示例
let now = Date()
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
let statuses: [OrderStatus] = [
    .pending,
    .paid(now),
    .shipped(trackingNumber: "SF1234567890"),
    .delivered(now),
    .cancelled(reason: "用户取消")
]
for status in statuses {
    let final = status.isFinalState ? "（终态）" : ""
    print("\(status.description) \(final)")
}