Options Pattern 完全解説 — 無名関数でコンフィグを組み立てるデザインパターンの全貌

builder.Services.AddAuthentication(options =>
{
    options.DefaultScheme = "Bearer";
    options.DefaultChallengeScheme = "Bearer";
});

server := NewServer(
    WithPort(8080),
    WithTimeout(30 * time.Second),
    WithLogger(myLogger),
)

func NewServer(port int) *Server {
    return &Server{port: port}
}

func NewServer(
    port int,
    host string,
    timeout time.Duration,
    maxConnections int,
    logger Logger,
    tlsConfig *tls.Config,
    middleware []Middleware,
    readTimeout time.Duration,
    writeTimeout time.Duration,
    idleTimeout time.Duration,
) *Server {
    // ...
}

type ServerConfig struct {
    Port           int
    Host           string
    Timeout        time.Duration
    MaxConnections int
    Logger         Logger
    // ...
}
 
func NewServer(cfg ServerConfig) *Server { ... }

package server
 
import (
    "log"
    "time"
)
 
// Server は HTTP サーバーの本体
type Server struct {
    port         int
    host         string
    timeout      time.Duration
    maxConns     int
    logger       *log.Logger
}
 
// Option は Server の設定を変更する関数型
type Option func(*Server)
 
// WithPort はポート番号を設定する Option
func WithPort(port int) Option {
    return func(s *Server) {
        s.port = port
    }
}
 
// WithHost はホスト名を設定する Option
func WithHost(host string) Option {
    return func(s *Server) {
        s.host = host
    }
}
 
// WithTimeout はタイムアウトを設定する Option
func WithTimeout(d time.Duration) Option {
    return func(s *Server) {
        s.timeout = d
    }
}
 
// WithMaxConnections は最大接続数を設定する Option
func WithMaxConnections(n int) Option {
    return func(s *Server) {
        s.maxConns = n
    }
}
 
// WithLogger はロガーを設定する Option
func WithLogger(l *log.Logger) Option {
    return func(s *Server) {
        s.logger = l
    }
}
 
// NewServer は Option を適用して Server を生成する
func NewServer(opts ...Option) *Server {
    // 1. デフォルト値で初期化
    s := &Server{
        port:     8080,
        host:     "localhost",
        timeout:  30 * time.Second,
        maxConns: 100,
        logger:   log.Default(),
    }
 
    // 2. Option 関数を順に適用
    for _, opt := range opts {
        opt(s)
    }
 
    return s
}

// デフォルト設定
s1 := server.NewServer()
 
// カスタム設定
s2 := server.NewServer(
    server.WithPort(9090),
    server.WithTimeout(60 * time.Second),
    server.WithLogger(customLogger),
)
 
// プリセット: 本番環境向け設定をまとめておく
var ProductionDefaults = []server.Option{
    server.WithPort(443),
    server.WithTimeout(10 * time.Second),
    server.WithMaxConnections(10000),
}
 
s3 := server.NewServer(ProductionDefaults...)

// Option はエラーを返せるバージョン
type Option func(*Server) error
 
func WithPort(port int) Option {
    return func(s *Server) error {
        if port < 0 || port > 65535 {
            return fmt.Errorf("invalid port: %d", port)
        }
        s.port = port
        return nil
    }
}
 
func NewServer(opts ...Option) (*Server, error) {
    s := &Server{
        port:     8080,
        host:     "localhost",
        timeout:  30 * time.Second,
        maxConns: 100,
    }
 
    for _, opt := range opts {
        if err := opt(s); err != nil {
            return nil, fmt.Errorf("applying option: %w", err)
        }
    }
 
    return s, nil
}

// Options クラスの定義
public class SmtpOptions
{
    public string Host { get; set; } = "localhost";
    public int Port { get; set; } = 25;
    public bool UseSsl { get; set; } = false;
    public string? Username { get; set; }
    public string? Password { get; set; }
}
 
// サービス登録時に Action<T> で設定
builder.Services.Configure<SmtpOptions>(options =>
{
    options.Host = "smtp.example.com";
    options.Port = 587;
    options.UseSsl = true;
});

// 基本設定
builder.Services.Configure<SmtpOptions>(options =>
{
    options.Host = "smtp.example.com";
    options.Port = 587;
});
 
// 追加設定（上書き可能）
builder.Services.Configure<SmtpOptions>(options =>
{
    options.UseSsl = true;
});
 
// appsettings.json からもバインド可能
builder.Services.Configure<SmtpOptions>(
    builder.Configuration.GetSection("Smtp"));

// AddSmtp 拡張メソッドの実装
public static class SmtpServiceCollectionExtensions
{
    public static IServiceCollection AddSmtp(
        this IServiceCollection services,
        Action<SmtpOptions> configure)
    {
        // 1. Options インフラを登録
        services.AddOptions<SmtpOptions>()
            .Configure(configure)        // ユーザーの設定を適用
            .ValidateDataAnnotations()   // バリデーション
            .ValidateOnStart();          // 起動時検証
 
        // 2. サービス本体を登録
        services.AddSingleton<ISmtpClient, SmtpClient>();
 
        return services;
    }
}
 
// 使用側
builder.Services.AddSmtp(options =>
{
    options.Host = "smtp.example.com";
    options.Port = 587;
    options.UseSsl = true;
});

builder.Services.AddOptions<SmtpOptions>()
    // Stage 1: Configure — 設定値を注入
    .Configure(o => o.Host = "smtp.example.com")
 
    // Stage 2: PostConfigure — すべての Configure 後に実行される
    .PostConfigure(o =>
    {
        // SSL が有効なのにポートがデフォルトなら 465 に変更
        if (o.UseSsl && o.Port == 25)
            o.Port = 465;
    })
 
    // Stage 3: Validate — 最終的なバリデーション
    .Validate(o => !string.IsNullOrEmpty(o.Host),
              "SMTP host must not be empty")
    .ValidateDataAnnotations()
    .ValidateOnStart();

// "Primary" と "Backup" の 2 つの SMTP 設定
builder.Services.Configure<SmtpOptions>("Primary", options =>
{
    options.Host = "smtp-primary.example.com";
    options.Port = 587;
});
 
builder.Services.Configure<SmtpOptions>("Backup", options =>
{
    options.Host = "smtp-backup.example.com";
    options.Port = 2525;
});
 
// 使用側
public class EmailService
{
    private readonly SmtpOptions _primary;
    private readonly SmtpOptions _backup;
 
    public EmailService(IOptionsSnapshot<SmtpOptions> optionsSnapshot)
    {
        _primary = optionsSnapshot.Get("Primary");
        _backup = optionsSnapshot.Get("Backup");
    }
}

// OptionsFactory<TOptions>.Create() の内部実装（簡略化）
public TOptions Create(string name)
{
    // 1. 新しいインスタンスを生成（デフォルト値）
    TOptions options = new TOptions();
 
    // 2. すべての IConfigureOptions<T> を実行
    foreach (var setup in _setups)
    {
        if (setup is IConfigureNamedOptions<TOptions> namedSetup)
            namedSetup.Configure(name, options);
        else if (name == Options.DefaultName)
            setup.Configure(options);
    }
 
    // 3. すべての IPostConfigureOptions<T> を実行
    foreach (var post in _postConfigures)
        post.PostConfigure(name, options);
 
    // 4. すべての IValidateOptions<T> を実行（エラーは集約）
    var failures = new List<string>();
    foreach (var validate in _validations)
    {
        var result = validate.Validate(name, options);
        if (result.Failed)
            failures.AddRange(result.Failures);
    }
    if (failures.Count > 0)
        throw new OptionsValidationException(name, typeof(TOptions), failures);
 
    return options;
}

interface ServerOptions {
  port: number;
  host: string;
  timeout: number;
  maxConnections: number;
}
 
const defaults: ServerOptions = {
  port: 8080,
  host: "localhost",
  timeout: 30_000,
  maxConnections: 100,
};
 
function createServer(overrides?: Partial<ServerOptions>): Server {
  const options = { ...defaults, ...overrides };
  return new Server(options);
}
 
// 使用
const server = createServer({ port: 9090, timeout: 60_000 });

interface ServerConfig {
  port: number;
  host: string;
  timeout: number;
  maxConnections: number;
  logger: Logger;
}
 
type ServerOption = (config: ServerConfig) => void;
 
function withPort(port: number): ServerOption {
  return (config) => {
    if (port < 0 || port > 65535) throw new RangeError(`Invalid port: ${port}`);
    config.port = port;
  };
}
 
function withHost(host: string): ServerOption {
  return (config) => { config.host = host; };
}
 
function withTimeout(ms: number): ServerOption {
  return (config) => { config.timeout = ms; };
}
 
function withMaxConnections(n: number): ServerOption {
  return (config) => { config.maxConnections = n; };
}
 
function createServer(...opts: ServerOption[]): Server {
  const config: ServerConfig = {
    port: 8080,
    host: "localhost",
    timeout: 30_000,
    maxConnections: 100,
    logger: console,
  };
 
  for (const opt of opts) {
    opt(config);
  }
 
  return new Server(config);
}
 
// 使用
const server = createServer(
  withPort(9090),
  withTimeout(60_000),
  withMaxConnections(5000),
);
 
// プリセット
const productionDefaults: ServerOption[] = [
  withPort(443),
  withTimeout(10_000),
  withMaxConnections(10000),
];
 
const prodServer = createServer(...productionDefaults);

function createServer(configure: (options: ServerConfig) => void): Server {
  const config: ServerConfig = {
    port: 8080,
    host: "localhost",
    timeout: 30_000,
    maxConnections: 100,
    logger: console,
  };
 
  configure(config);
 
  return new Server(config);
}
 
// 使用
const server = createServer((options) => {
  options.port = 9090;
  options.timeout = 60_000;
  options.maxConnections = 5000;
});

function createServer(
  configure?: (options: ServerConfig) => void,
  ...opts: ServerOption[]
): Server {
  const config: ServerConfig = {
    port: 3000, host: "localhost",
    timeout: 30_000, maxConnections: 100, logger: console.log,
  };
  configure?.(config);
  for (const opt of opts) opt(config);
  return new Server(config);
}

pub struct ServerOptions {
    pub port: u16,
    pub host: String,
    pub timeout: std::time::Duration,
    pub max_connections: usize,
}
 
impl Default for ServerOptions {
    fn default() -> Self {
        Self {
            port: 8080,
            host: "localhost".into(),
            timeout: std::time::Duration::from_secs(30),
            max_connections: 100,
        }
    }
}
 
// Option は FnOnce で表現する
type ServerOption = Box<dyn FnOnce(&mut ServerOptions)>;
 
pub fn with_port(port: u16) -> ServerOption {
    Box::new(move |opts| { opts.port = port; })
}
 
pub fn with_host(host: impl Into<String>) -> ServerOption {
    let host = host.into();
    Box::new(move |opts| { opts.host = host; })
}
 
pub fn with_timeout(duration: std::time::Duration) -> ServerOption {
    Box::new(move |opts| { opts.timeout = duration; })
}
 
pub struct Server { /* ... */ }
 
impl Server {
    pub fn new(opts: impl IntoIterator<Item = ServerOption>) -> Self {
        let mut options = ServerOptions::default();
        for opt in opts {
            opt(&mut options);
        }
        Server::from_options(options)
    }
 
    fn from_options(_options: ServerOptions) -> Self {
        Server { /* ... */ }
    }
}
 
// 使い方
let server = Server::new([
    with_port(9090),
    with_timeout(std::time::Duration::from_secs(60)),
]);

from dataclasses import dataclass
from typing import Callable
 
@dataclass
class ServerOptions:
    port: int = 8080
    host: str = "localhost"
    timeout: float = 30.0
    max_connections: int = 100
 
# Option 型
Option = Callable[[ServerOptions], None]
 
def with_port(port: int) -> Option:
    def apply(opts: ServerOptions) -> None:
        if not 0 <= port <= 65535:
            raise ValueError(f"Invalid port: {port}")
        opts.port = port
    return apply
 
def with_host(host: str) -> Option:
    def apply(opts: ServerOptions) -> None:
        opts.host = host
    return apply
 
def with_timeout(seconds: float) -> Option:
    def apply(opts: ServerOptions) -> None:
        opts.timeout = seconds
    return apply
 
class Server:
    def __init__(self, *opts: Option):
        self.options = ServerOptions()
        for opt in opts:
            opt(self.options)
 
# 使い方
server = Server(
    with_port(9090),
    with_timeout(60.0),
)
 
# プリセット
production_defaults: list[Option] = [
    with_port(443),
    with_timeout(10.0),
]
 
prod_server = Server(*production_defaults)

// 複数の Option を 1 つにまとめる
func ComposeOptions(opts ...Option) Option {
    return func(s *Server) {
        for _, opt := range opts {
            opt(s)
        }
    }
}
 
// プリセットの定義
var HighPerformance = ComposeOptions(
    WithMaxConnections(50000),
    WithTimeout(5 * time.Second),
)
 
var SecureDefaults = ComposeOptions(
    WithPort(443),
    WithTimeout(10 * time.Second),
)
 
// プリセットの合成
s := NewServer(
    HighPerformance,
    SecureDefaults,
    WithHost("api.example.com"), // 個別の上書き
)

// 条件付きで Option を適用する
func WithPortIf(port int, condition bool) Option {
    return func(s *Server) {
        if condition {
            s.port = port
        }
    }
}
 
// 環境変数に基づく設定
func WithEnvConfig() Option {
    return func(s *Server) {
        if port := os.Getenv("PORT"); port != "" {
            s.port, _ = strconv.Atoi(port)
        }
        if host := os.Getenv("HOST"); host != "" {
            s.host = host
        }
    }
}

type Option func(*Server) Option
 
func WithPort(port int) Option {
    return func(s *Server) Option {
        prev := s.port   // 以前の値を保存
        s.port = port     // 新しい値を設定
        return WithPort(prev) // 以前の値に戻す Option を返す
    }
}
 
// ロールバックの例
func temporarilyChangePort(s *Server, port int) {
    restore := WithPort(port)(s)  // ポートを変更
    defer restore(s)               // 関数終了時に元に戻す
    // ... 一時的に変更されたポートで処理 ...
}