Swift Library Design

Patterns and best practices for designing high-quality Swift libraries and frameworks.

Core Principles

1. Protocol-Oriented Design

Design around protocols with associated types for flexibility and testability. Use generics for type safety with runtime polymorphism.

2. Compile-Time Safety

Leverage Swift's type system to catch errors at compile time. Use noncopyable types, generics, and protocol constraints to make invalid states unrepresentable.

3. Performance by Default

Design APIs that are efficient by default. Use @inlinable for hot paths, avoid unnecessary allocations, and provide zero-copy options.

4. Progressive Disclosure

Simple things should be simple, complex things should be possible. Provide sensible defaults while allowing customization.

Key Patterns

Protocol with Associated Types

public protocol Handler<Context>: Sendable {
    associatedtype Context
    func handle(input: Input, context: Context) async throws -> Output
}

// Consumers accept any conforming type
public struct Pipeline<Context> {
    public func add<H: Handler>(handler: H) where H.Context == Context {
        // Type-safe composition
    }
}

ResponseGenerator Pattern

Allow multiple return types with automatic conversion:

public protocol ResponseGenerator: Sendable {
    func response(from request: Request, context: some RequestContext) throws -> Response
}

// Extend standard types
extension String: ResponseGenerator {
    public func response(from request: Request, context: some RequestContext) -> Response {
        Response(status: .ok, body: .init(string: self))
    }
}

extension HTTPResponse.Status: ResponseGenerator {
    public func response(from request: Request, context: some RequestContext) -> Response {
        Response(status: self)
    }
}

Result Builder for DSLs

@resultBuilder
public enum PipelineBuilder<Context> {
    public static func buildExpression<M: Middleware>(_ m: M) -> M
        where M.Context == Context { m }

    public static func buildPartialBlock<M0: Middleware, M1: Middleware>(
        accumulated m0: M0, next m1: M1
    ) -> CombinedMiddleware<M0, M1> {
        CombinedMiddleware(m0, m1)
    }
}

// Usage
let pipeline = PipelineBuilder {
    LoggingMiddleware()
    AuthMiddleware()
    RateLimitMiddleware()
}

Static Builder Methods

public struct ServerBuilder: Sendable {
    private let build: @Sendable () throws -> Server

    public static func http1(config: HTTP1Config = .init()) -> ServerBuilder {
        .init { HTTP1Server(config: config) }
    }

    public static func http2(tlsConfig: TLSConfiguration) -> ServerBuilder {
        .init { HTTP2Server(tlsConfig: tlsConfig) }
    }
}

// Fluent usage
let server = Application(server: .http1(config: .init(idleTimeout: .seconds(30))))

Reference Files

• references/api-patterns.md - Protocol design, result builders, builder pattern, error handling, parameter extraction
• references/performance.md - Inlining, noncopyable types, data structures, lock-free patterns, state machines