نمط Builder

لماذا نستخدم Builder؟

البنية الهدف

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

نوع Builder منفصل

pub struct ServerConfigBuilder {
    host: String,
    port: Option<u16>,
    max_connections: Option<usize>,
    use_tls: bool,
}

بدء Builder

impl ServerConfig {
    pub fn builder(host: impl Into<String>) -> ServerConfigBuilder {
        ServerConfigBuilder {
            host: host.into(),
            port: None,
            max_connections: None,
            use_tls: false,
        }
    }
}

تستقبل دوال الإسناد self بالقيمة

impl ServerConfigBuilder {
    pub fn port(mut self, port: u16) -> Self {
        self.port = Some(port);
        self
    }
    pub fn use_tls(mut self, yes: bool) -> Self {
        self.use_tls = yes;
        self
    }
}

تطبيق القيم الافتراضية في build()

impl ServerConfigBuilder {
    pub fn build(self) -> ServerConfig {
        ServerConfig {
            host: self.host,
            port: self.port.unwrap_or(8080),
            max_connections: self.max_connections.unwrap_or(128),
            use_tls: self.use_tls,
        }
    }
}

الإنشاء السلس

fn main() {
    let cfg = ServerConfig::builder("localhost")
        .port(9000)
        .use_tls(true)
        .build();
    println!("{}:{} tls={}", cfg.host, cfg.port, cfg.use_tls);
}

مثال كامل قابل للتشغيل

struct Config { name: String, retries: u32 }
struct Builder { name: String, retries: Option<u32> }
impl Config {
    fn builder(name: &str) -> Builder {
        Builder { name: name.to_string(), retries: None }
    }
}
impl Builder {
    fn retries(mut self, n: u32) -> Self { self.retries = Some(n); self }
    fn build(self) -> Config {
        Config { name: self.name, retries: self.retries.unwrap_or(3) }
    }
}
fn main() {
    let c = Config::builder("job").build();
    println!("{} retries={}", c.name, c.retries);
}

إنشاء قابل للفشل باستخدام Result

impl ServerConfigBuilder {
    pub fn try_build(self) -> Result<ServerConfig, String> {
        let port = self.port.unwrap_or(8080);
        if port == 0 {
            return Err("port must be non-zero".into());
        }
        Ok(ServerConfig { host: self.host, port,
            max_connections: self.max_connections.unwrap_or(128),
            use_tls: self.use_tls })
    }
}

حزمة derive_builder

use derive_builder::Builder;

#[derive(Builder)]
struct Channel {
    #[builder(default = "8080")]
    port: u16,
    name: String,
}

Builders المملوكة مقابل Builders ذات المرجع القابل للتعديل

impl ServerConfigBuilder {
    pub fn port_ref(&mut self, port: u16) -> &mut Self {
        self.port = Some(port);
        self
    }
}

تحقق سريع

مراجعة