前端开发者的 Kotlin 之旅：元编程初探

// 你写的代码 (ES6+)
const fetchUser = async (id) => {
  const response = await fetch(`/users/${id}`);
  return response.json();
};

// Babel 转换后的代码 (ES5)
function fetchUser(id) {
  return regeneratorRuntime.async(function fetchUser$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
          _context.next = 2;
          return regeneratorRuntime.awrap(fetch("/users/" + id));
        // ... 更多生成的代码
      }
    }
  });
}

// React: 运行时读取组件元数据并据此渲染
function CustomRenderer() {
  // 1. 读取组件的元数据
  const componentMeta = MyComponent.__metadata || {};
  
  // 2. 基于元数据动态处理
  if (componentMeta.requiresAuth) {
    return <LoginRequired />;
  }
  
  // 3. 根据元数据调整行为
  const enhancedProps = componentMeta.enhance 
    ? enhanceProps(props) 
    : props;
    
  return <MyComponent {...enhancedProps} />;
}


// Angular: 依赖注入基于元数据
@Injectable() // ← 元数据：标记为可注入
class UserService {
  constructor(
    @Inject(HTTP_CLIENT) private http: HttpClient // ← 元数据：注入令牌
  ) {
    // Angular 在运行时读取这些元数据，
    // 然后基于元数据自动创建和注入依赖
  }
}

// GraphQL Code Generator
// 从 GraphQL Schema 自动生成 TypeScript 类型
type User = {
  id: string;
  name: string;
  email: string;
};

// Prisma Schema → TypeScript 客户端
const user = await prisma.user.findUnique({
  where: { id: '123' }
});

// JavaScript 风格
// Object.keys(user).forEach(key => console.log(key, user[key]))

// Kotlin 反射 - 类型安全的动态操作
data class User(val name: String, val age: Int, val email: String)

fun inspectObject(obj: Any) {
    val kClass = obj::class
    
    // 获取类信息
    println("类名: ${kClass.simpleName}")
    
    // 获取所有属性
    kClass.memberProperties.forEach { prop ->
        val value = prop.get(obj)
        println("${prop.name}: $value (${prop.returnType})")
    }
}

// 使用
val user = User("Alice", 25, "alice@example.com")
inspectObject(user)
// 输出:
// 类名: User
// name: Alice (kotlin.String)
// age: 25 (kotlin.Int)
// email: alice@example.com (kotlin.String)

// 定义注解
@Target(AnnotationTarget.PROPERTY)
annotation class JsonField(val name: String = "")

@Target(AnnotationTarget.CLASS)
annotation class ApiEndpoint(val path: String)

// 使用注解
@ApiEndpoint("/api/users")
data class User(
    @JsonField("user_id") val id: Long,
    @JsonField("user_name") val name: String,
    val email: String  // 使用默认字段名
)

// 基于注解处理数据
fun serializeToJson(obj: Any): String {
    val kClass = obj::class
    val fields = kClass.memberProperties.map { prop ->
        val jsonField = prop.findAnnotation<JsonField>()
        val fieldName = jsonField?.name?.ifEmpty { prop.name } ?: prop.name
        val value = prop.get(obj)
        "\"$fieldName\": \"$value\""
    }
    return "{${fields.joinToString(", ")}}"
}

// 源代码：简单的注解标记
@GenerateBuilder
data class Product(
    val id: Long,
    val name: String,
    val price: Double,
    val category: String?
)

// 编译时自动生成 Builder 类：
class ProductBuilder {
    private var id: Long? = null
    private var name: String? = null
    private var price: Double? = null
    private var category: String? = null
    
    fun id(id: Long) = apply { this.id = id }
    fun name(name: String) = apply { this.name = name }
    fun price(price: Double) = apply { this.price = price }
    fun category(category: String?) = apply { this.category = category }
    
    fun build(): Product {
        return Product(
            id = id ?: throw IllegalStateException("id is required"),
            name = name ?: throw IllegalStateException("name is required"),
            price = price ?: throw IllegalStateException("price is required"),
            category = category
        )
    }
}

// 使用生成的代码
val product = ProductBuilder()
    .id(1L)
    .name("iPhone")
    .price(999.0)
    .category("Electronics")
    .build()

// HTML DSL - 类型安全的 HTML 构建
fun createPage() = html {
    head {
        title("用户管理")
        meta(charset = "UTF-8")
    }
    body {
        h1("用户列表")
        div(classes = "user-list") {
            users.forEach { user ->
                div(classes = "user-card") {
                    h3(user.name)
                    p("邮箱: ${user.email}")
                    button("编辑") {
                        onClick = "editUser(${user.id})"
                    }
                }
            }
        }
    }
}

// SQL DSL - 类型安全的查询构建
val query = Users
    .select(Users.name, Users.email)
    .where { Users.age greater 18 }
    .and { Users.email like "%@gmail.com" }
    .orderBy(Users.name.asc())
    .limit(10)