Type Aliases in Kotlin Explained

Type Aliases in Kotlin Explained

Type aliases in Kotlin provide a way to create alternative names for existing types, making your code more readable and maintainable.

Basic Usage

Simple Type Aliases

typealias Name = String
typealias Age = Int
typealias UserId = String

data class User(
    val id: UserId,
    val name: Name,
    val age: Age
)

Function Type Aliases

typealias ClickHandler = (View) -> Unit
typealias StringFilter = (String) -> Boolean
typealias DataCallback = (Result<Data>) -> Unit

class Button {
    private var onClick: ClickHandler? = null

    fun setOnClickListener(listener: ClickHandler) {
        onClick = listener
    }
}

Advanced Usage

Generic Type Aliases

typealias StringMap = Map<String, String>
typealias NumberList = List<Number>
typealias ResultCallback<T> = (Result<T>) -> Unit

fun processData(data: StringMap, callback: ResultCallback<String>) {
    // Implementation
}

Complex Type Aliases

typealias UserMap = Map<UserId, User>
typealias UserList = List<User>
typealias UserCallback = (User) -> Unit
typealias UserFilter = (User) -> Boolean

class UserRepository {
    private val users: UserMap = mutableMapOf()

    fun findUsers(filter: UserFilter): UserList {
        return users.values.filter(filter)
    }
}

Best Practices

1. Use descriptive names for type aliases
2. Keep aliases close to their usage
3. Use aliases for complex types
4. Consider package-level aliases for common types
5. Document complex type aliases

Common Patterns

API Response Types

typealias ApiResponse<T> = Result<T>
typealias ApiCallback<T> = (ApiResponse<T>) -> Unit
typealias ApiError = String

interface ApiClient {
    fun <T> get(url: String, callback: ApiCallback<T>)
    fun <T> post(url: String, data: T, callback: ApiCallback<T>)
}

Database Types

typealias DbId = Long
typealias DbTimestamp = Long
typealias DbQuery = String
typealias DbResult<T> = Result<T>

class Database {
    fun <T> query(query: DbQuery): DbResult<T> {
        // Implementation
    }
}

Use Cases

UI Components

typealias ViewId = Int
typealias Color = Int
typealias Dimension = Float
typealias ResourceId = Int

class CustomView {
    fun setBackgroundColor(@ColorRes color: Color) {
        // Implementation
    }
}

Network Types

typealias HttpMethod = String
typealias HttpHeaders = Map<String, String>
typealias HttpBody = String
typealias HttpStatusCode = Int

class HttpClient {
    fun request(
        method: HttpMethod,
        headers: HttpHeaders,
        body: HttpBody?
    ): HttpStatusCode {
        // Implementation
    }
}

Performance Considerations

• Type aliases are resolved at compile time
• No runtime overhead
• Helps with code organization
• Improves code readability

Conclusion

Type aliases are a powerful feature in Kotlin that helps make your code more readable and maintainable. Use them to simplify complex types and create more meaningful names for your types.