Swift 6.2 字符串插值默认值特性

let name: String? = nil
print("Hi \(name ?? "Guest")") // 输出 "Hi Guest"

let count: Int? = nil
// 编译错误：Binary operator '??' cannot be applied to operands of type 'Int?' and 'String'
// print("Count: \(count ?? "Unknown")") // ❌ 旧写法无法编译

// 旧的解决方案通常需要手动转换类型，非常繁琐
print("Count: \(count.map(String.init) ?? "Unknown")") // ✅ 但很啰嗦

let count: Int? = nil
print("Count: \(count, default: "Unknown")") // ✅ 输出 "Count: Unknown"

struct User {
    var username: String?
    var email: String?
    var age: Int?
}

let user = User(username: nil, email: "jane@example.com", age: nil)

// Swift 6.2 新写法 (清晰简洁)
print("""
User Info:
- Username: \(user.username, default: "Guest")
- Email: \(user.email, default: "Not provided")
- Age: \(user.age, default: "Unknown")
""")

// 传统写法 (冗长且需要类型转换)
print("""
User Info:
- Username: \(user.username ?? "Guest")
- Email: \(user.email ?? "Not provided")
- Age: \(user.age.map(String.init) ?? "Unknown") // 需要额外 map 转换
""")

func logEvent(name: String?, duration: Double?, user: User?) {
    print("""
    Event '\(name, default: "Unnamed")' 
    ran for \(duration, default: "an unknown amount of time") 
    triggered by \(user?.username, default: "anonymous user")
    """)
}

logEvent(name: nil, duration: nil, user: nil)
// 输出: Event 'Unnamed' ran for an unknown amount of time triggered by anonymous user

let optionalInt: Int? = nil
let optionalDouble: Double? = 3.14
let optionalArray: [String]? = ["A", "B"]
let optionalBool: Bool? = nil

// 轻松处理各种类型，默认值直接作为字符串使用
let report = """
The value is \(optionalInt, default: "N/A").
Pi is approximately \(optionalDouble, default: "unknown").
The list contains \(optionalArray, default: "no items").
The flag is \(optionalBool, default: "not set").
"""
print(report)

extension String.StringInterpolation {
    /// 新的插值方法，接受一个可选值和一个默认字符串表达式
    /// - Parameters:
    ///   - value: 任意类型的可选值 (T?)
    ///   - defaultValue: 一个 @autoclosure，当 value 为 nil 时返回用作默认值的字符串
    mutating func appendInterpolation<T>(_ value: T?, default defaultValue: @autoclosure () -> String) {
        switch value {
        case .some(let concreteValue):
            // 如果值存在，使用原有的插值方法将其追加到字符串中
            appendInterpolation(concreteValue)
        case .none:
            // 如果值为 nil，将默认字符串作为字面量追加
            appendLiteral(defaultValue())
        }
    }
}

var scores = ["Alice": 10, "Bob": 20]
// 从字典中获取值，如果键不存在，则使用默认值
let bobsScore = scores["Bob", default: 0] // 20
let evesScore = scores["Eve", default: 0] // 0

import SwiftUI

// 假设在 SwiftUI 的 Text 中 (目前可能不支持)
let optionalName: String? = nil
// Text("Hello, \(optionalName, default: "Guest")") // ❌ 可能无法编译或工作不如预期

// 对于本地化和日志，目前仍需使用传统方式
let localizedText = String(format: NSLocalizedString("welcome", comment: ""), optionalName ?? "Guest")
Text(localizedText)

// 未来的潜在扩展 (目前尚未实现)
// "\(value, default: "absent", width: 20, alignment: .right, placeholder: "_")"
// 输出可能是: "______________absent"

func expensiveDefaultValue() -> String {
    print("Calculating expensive default...")
    return "Default"
}

let nonNilValue: Int? = 42
print("Value: \(nonNilValue, default: expensiveDefaultValue())") 
// 输出 "Value: 42"。不会打印 "Calculating expensive default..."，因为 expensiveDefaultValue 函数未被调用。

// 如果插值过于复杂
// let message = "Welcome \(userName, default: "Guest"), your role is \(userRole, default: "Unknown"), status: \(userStatus, default: "Inactive")"

// 考虑提前处理
let displayName = userName ?? "Guest"
let displayRole = userRole ?? "Unknown"
let displayStatus = userStatus ?? "Inactive"
let message = "Welcome \(displayName), your role is \(displayRole), status: \(displayStatus)"

extension String.StringInterpolation {
    // 一个自定义的货币格式化方法
    mutating func appendInterpolation(_ value: Double, asCurrency currencyCode: String) {
        let formatter = NumberFormatter()
        formatter.numberStyle = .currency
        formatter.currencyCode = currencyCode
        appendLiteral(formatter.string(from: NSNumber(value: value)) ?? "\(currencyCode) \(value)")
    }
}

let optionalPrice: Double? = 99.99
// 结合自定义格式化和默认值
print("The price is \(optionalPrice, default: "free", asCurrency: "USD")") 
// 如果 optionalPrice 为 nil，则输出 "The price is free"
// 如果 optionalPrice 有值，则输出 "The price is $99.99"

// 全局函数方案
func unwrap<T>(_ optional: T?, or defaultValue: String) -> String {
    guard let value = optional else { return defaultValue }
    return "\(value)"
}
print("Value: \(unwrap(optionalValue, or: "N/A"))")

// 计算属性方案 (扩展 Optional)
extension Optional {
    var descriptionWithDefault: String {
        return self.map(String.init(describing:)) ?? "N/A"
    }
}
print("Value: \(optionalValue.descriptionWithDefault)")

infix operator ???: NilCoalescingPrecedence

func ??? <T>(optional: T?, defaultValue: @autoclosure () -> String) -> String {
    return optional.map(String.init(describing:)) ?? defaultValue()
}

print("Value: \(optionalValue ??? "N/A")")