Rust 设计模式

本文档在 common/patterns.md 的基础上，补充了 Rust 特有的内容。

基于 Trait 的 Repository 模式

将数据访问封装在 trait 之后：

pub trait OrderRepository: Send + Sync {
    fn find_by_id(&self, id: u64) -> Result<Option<Order>, StorageError>;
    fn find_all(&self) -> Result<Vec<Order>, StorageError>;
    fn save(&self, order: &Order) -> Result<Order, StorageError>;
    fn delete(&self, id: u64) -> Result<(), StorageError>;
}

具体的实现负责处理存储细节（如 Postgres、SQLite，或用于测试的内存存储）。

服务层

业务逻辑位于服务结构体中；通过构造函数注入依赖：

pub struct OrderService {
    repo: Box<dyn OrderRepository>,
    payment: Box<dyn PaymentGateway>,
}

impl OrderService {
    pub fn new(repo: Box<dyn OrderRepository>, payment: Box<dyn PaymentGateway>) -> Self {
        Self { repo, payment }
    }

    pub fn place_order(&self, request: CreateOrderRequest) -> anyhow::Result<OrderSummary> {
        let order = Order::from(request);
        self.payment.charge(order.total())?;
        let saved = self.repo.save(&order)?;
        Ok(OrderSummary::from(saved))
    }
}

为类型安全使用 Newtype 模式

使用不同的包装类型防止参数混淆：

struct UserId(u64);
struct OrderId(u64);

fn get_order(user: UserId, order: OrderId) -> anyhow::Result<Order> {
    // Can't accidentally swap user and order IDs at call sites
    todo!()
}

枚举状态机

将状态建模为枚举 —— 使非法状态无法表示：

enum ConnectionState {
    Disconnected,
    Connecting { attempt: u32 },
    Connected { session_id: String },
    Failed { reason: String, retries: u32 },
}

fn handle(state: &ConnectionState) {
    match state {
        ConnectionState::Disconnected => connect(),
        ConnectionState::Connecting { attempt } if *attempt > 3 => abort(),
        ConnectionState::Connecting { .. } => wait(),
        ConnectionState::Connected { session_id } => use_session(session_id),
        ConnectionState::Failed { retries, .. } if *retries < 5 => retry(),
        ConnectionState::Failed { reason, .. } => log_failure(reason),
    }
}

始终进行穷尽匹配 —— 对于业务关键的枚举，不要使用通配符 _。

建造者模式

适用于具有多个可选参数的结构体：

pub struct ServerConfig {
    host: String,
    port: u16,
    max_connections: usize,
}

impl ServerConfig {
    pub fn builder(host: impl Into<String>, port: u16) -> ServerConfigBuilder {
        ServerConfigBuilder {
            host: host.into(),
            port,
            max_connections: 100,
        }
    }
}

pub struct ServerConfigBuilder {
    host: String,
    port: u16,
    max_connections: usize,
}

impl ServerConfigBuilder {
    pub fn max_connections(mut self, n: usize) -> Self {
        self.max_connections = n;
        self
    }

    pub fn build(self) -> ServerConfig {
        ServerConfig {
            host: self.host,
            port: self.port,
            max_connections: self.max_connections,
        }
    }
}

密封 Trait 以控制扩展性

使用私有模块来密封一个 trait，防止外部实现：

mod private {
    pub trait Sealed {}
}

pub trait Format: private::Sealed {
    fn encode(&self, data: &[u8]) -> Vec<u8>;
}

pub struct Json;
impl private::Sealed for Json {}
impl Format for Json {
    fn encode(&self, data: &[u8]) -> Vec<u8> { todo!() }
}

API 响应包装器

使用泛型枚举实现一致的 API 响应：

#[derive(Debug, serde::Serialize)]
#[serde(tag = "status")]
pub enum ApiResponse<T: serde::Serialize> {
    #[serde(rename = "ok")]
    Ok { data: T },
    #[serde(rename = "error")]
    Error { message: String },
}

参考资料

参见技能：rust-patterns，其中包含全面的模式，涵盖所有权、trait、泛型、并发和异步。

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