Builder を Rust で

Builder は生成に関するデザインパターンの一つで、複雑なオブジェクトを段階的に構築することができます。

他の生成に関するパターンとは異なり、 Builder ではプロダクト（訳注：本パターンでは、生成されるモノのことを一般にプロダクトと呼びます）が共通のインターフェースを持つ必要はありません。このため、同じ構築の手続きを経て、異なるプロダクトを作成することができます。

Builder の詳細

Car & car manual builders

This slightly synthetic example illustrates how you can use the Builder pattern to construct totally different products using the same building process. For example, the trait Builder declares steps for assembling a car. However, depending on the builder implementation, a constructed object can be something different, for example, a car manual. The resulting manual will contain instructions from each building step, making it accurate and up-to-date.

The Builder design pattern is not the same as the Fluent Interface idiom (that relies on method chaining), although Rust developers sometimes use those terms interchangeably.

Fluent Interface is a way to chain methods for constructing or modifying an object using the following approach:
```
let car = Car::default().places(5).gas(30)
```
It's pretty elegant way to construct an object. Still, such a code may not be an instance of the Builder pattern.
While the Builder pattern also suggests constructing object step by step, it also lets you build different types of products using the same construction process.

builders: Builders

builders/mod.rs

mod car;
mod car_manual;

use crate::components::{CarType, Engine, GpsNavigator, Transmission};

/// Builder defines how to assemble a car.
pub trait Builder {
    type OutputType;
    fn set_car_type(&mut self, car_type: CarType);
    fn set_seats(&mut self, seats: u16);
    fn set_engine(&mut self, engine: Engine);
    fn set_transmission(&mut self, transmission: Transmission);
    fn set_gsp_navigator(&mut self, gps_navigator: GpsNavigator);
    fn build(self) -> Self::OutputType;
}

pub use car::CarBuilder;
pub use car_manual::CarManualBuilder;

builders/car.rs

use crate::{
    cars::Car,
    components::{CarType, Engine, GpsNavigator, Transmission},
};

use super::Builder;

pub const DEFAULT_FUEL: f64 = 5f64;

#[derive(Default)]
pub struct CarBuilder {
    car_type: Option<CarType>,
    engine: Option<Engine>,
    gps_navigator: Option<GpsNavigator>,
    seats: Option<u16>,
    transmission: Option<Transmission>,
}

impl Builder for CarBuilder {
    type OutputType = Car;

    fn set_car_type(&mut self, car_type: CarType) {
        self.car_type = Some(car_type);
    }

    fn set_engine(&mut self, engine: Engine) {
        self.engine = Some(engine);
    }

    fn set_gsp_navigator(&mut self, gps_navigator: GpsNavigator) {
        self.gps_navigator = Some(gps_navigator);
    }

    fn set_seats(&mut self, seats: u16) {
        self.seats = Some(seats);
    }

    fn set_transmission(&mut self, transmission: Transmission) {
        self.transmission = Some(transmission);
    }

    fn build(self) -> Car {
        Car::new(
            self.car_type.expect("Please, set a car type"),
            self.seats.expect("Please, set a number of seats"),
            self.engine.expect("Please, set an engine configuration"),
            self.transmission.expect("Please, set up transmission"),
            self.gps_navigator,
            DEFAULT_FUEL,
        )
    }
}

builders/car_manual.rs

use crate::{
    cars::Manual,
    components::{CarType, Engine, GpsNavigator, Transmission},
};

use super::Builder;

#[derive(Default)]
pub struct CarManualBuilder {
    car_type: Option<CarType>,
    engine: Option<Engine>,
    gps_navigator: Option<GpsNavigator>,
    seats: Option<u16>,
    transmission: Option<Transmission>,
}

/// Builds a car manual instead of an actual car.
impl Builder for CarManualBuilder {
    type OutputType = Manual;

    fn set_car_type(&mut self, car_type: CarType) {
        self.car_type = Some(car_type);
    }

    fn set_engine(&mut self, engine: Engine) {
        self.engine = Some(engine);
    }

    fn set_gsp_navigator(&mut self, gps_navigator: GpsNavigator) {
        self.gps_navigator = Some(gps_navigator);
    }

    fn set_seats(&mut self, seats: u16) {
        self.seats = Some(seats);
    }

    fn set_transmission(&mut self, transmission: Transmission) {
        self.transmission = Some(transmission);
    }

    fn build(self) -> Manual {
        Manual::new(
            self.car_type.expect("Please, set a car type"),
            self.seats.expect("Please, set a number of seats"),
            self.engine.expect("Please, set an engine configuration"),
            self.transmission.expect("Please, set up transmission"),
            self.gps_navigator,
        )
    }
}

cars: Products

cars/mod.rs

mod car;
mod manual;

pub use car::Car;
pub use manual::Manual;

cars/car.rs

use crate::components::{CarType, Engine, GpsNavigator, Transmission};

pub struct Car {
    car_type: CarType,
    seats: u16,
    engine: Engine,
    transmission: Transmission,
    gps_navigator: Option<GpsNavigator>,
    fuel: f64,
}

impl Car {
    pub fn new(
        car_type: CarType,
        seats: u16,
        engine: Engine,
        transmission: Transmission,
        gps_navigator: Option<GpsNavigator>,
        fuel: f64,
    ) -> Self {
        Self {
            car_type,
            seats,
            engine,
            transmission,
            gps_navigator,
            fuel,
        }
    }

    pub fn car_type(&self) -> CarType {
        self.car_type
    }

    pub fn fuel(&self) -> f64 {
        self.fuel
    }

    pub fn set_fuel(&mut self, fuel: f64) {
        self.fuel = fuel;
    }

    pub fn seats(&self) -> u16 {
        self.seats
    }

    pub fn engine(&self) -> &Engine {
        &self.engine
    }

    pub fn transmission(&self) -> &Transmission {
        &self.transmission
    }

    pub fn gps_navigator(&self) -> &Option<GpsNavigator> {
        &self.gps_navigator
    }
}

cars/manual.rs

use crate::components::{CarType, Engine, GpsNavigator, Transmission};

pub struct Manual {
    car_type: CarType,
    seats: u16,
    engine: Engine,
    transmission: Transmission,
    gps_navigator: Option<GpsNavigator>,
}

impl Manual {
    pub fn new(
        car_type: CarType,
        seats: u16,
        engine: Engine,
        transmission: Transmission,
        gps_navigator: Option<GpsNavigator>,
    ) -> Self {
        Self {
            car_type,
            seats,
            engine,
            transmission,
            gps_navigator,
        }
    }
}

impl std::fmt::Display for Manual {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        writeln!(f, "Type of car: {:?}", self.car_type)?;
        writeln!(f, "Count of seats: {}", self.seats)?;
        writeln!(
            f,
            "Engine: volume - {}; mileage - {}",
            self.engine.volume(),
            self.engine.mileage()
        )?;
        writeln!(f, "Transmission: {:?}", self.transmission)?;
        match self.gps_navigator {
            Some(_) => writeln!(f, "GPS Navigator: Functional")?,
            None => writeln!(f, "GPS Navigator: N/A")?,
        };
        Ok(())
    }
}

components.rs: Product components

#[derive(Copy, Clone, Debug)]
pub enum CarType {
    CityCar,
    SportsCar,
    Suv,
}

#[derive(Debug)]
pub enum Transmission {
    SingleSpeed,
    Manual,
    Automatic,
    SemiAutomatic,
}

pub struct Engine {
    volume: f64,
    mileage: f64,
    started: bool,
}

impl Engine {
    pub fn new(volume: f64, mileage: f64) -> Self {
        Self {
            volume,
            mileage,
            started: false,
        }
    }

    pub fn on(&mut self) {
        self.started = true;
    }

    pub fn off(&mut self) {
        self.started = false;
    }

    pub fn started(&self) -> bool {
        self.started
    }

    pub fn volume(&self) -> f64 {
        self.volume
    }

    pub fn mileage(&self) -> f64 {
        self.mileage
    }

    pub fn go(&mut self, mileage: f64) {
        if self.started() {
            self.mileage += mileage;
        } else {
            println!("Cannot go(), you must start engine first!");
        }
    }
}

pub struct GpsNavigator {
    route: String,
}

impl GpsNavigator {
    pub fn new() -> Self {
        Self::from_route(
            "221b, Baker Street, London  to Scotland Yard, 8-10 Broadway, London".into(),
        )
    }

    pub fn from_route(route: String) -> Self {
        Self { route }
    }

    pub fn route(&self) -> &String {
        &self.route
    }
}

director.rs: Directors

use crate::{
    builders::Builder,
    components::{CarType, Engine, GpsNavigator, Transmission},
};

/// Director knows how to build a car.
///
/// However, a builder can build a car manual instead of an actual car,
/// everything depends on the concrete builder.
pub struct Director;

impl Director {
    pub fn construct_sports_car(builder: &mut impl Builder) {
        builder.set_car_type(CarType::SportsCar);
        builder.set_seats(2);
        builder.set_engine(Engine::new(3.0, 0.0));
        builder.set_transmission(Transmission::SemiAutomatic);
        builder.set_gsp_navigator(GpsNavigator::new());
    }

    pub fn construct_city_car(builder: &mut impl Builder) {
        builder.set_car_type(CarType::CityCar);
        builder.set_seats(2);
        builder.set_engine(Engine::new(1.2, 0.0));
        builder.set_transmission(Transmission::Automatic);
        builder.set_gsp_navigator(GpsNavigator::new());
    }

    pub fn construct_suv(builder: &mut impl Builder) {
        builder.set_car_type(CarType::Suv);
        builder.set_seats(4);
        builder.set_engine(Engine::new(2.5, 0.0));
        builder.set_transmission(Transmission::Manual);
        builder.set_gsp_navigator(GpsNavigator::new());
    }
}

main.rs: Client code

#![allow(unused)]

mod builders;
mod cars;
mod components;
mod director;

use builders::{Builder, CarBuilder, CarManualBuilder};
use cars::{Car, Manual};
use director::Director;

fn main() {
    let mut car_builder = CarBuilder::default();

    // Director gets the concrete builder object from the client
    // (application code). That's because application knows better which
    // builder to use to get a specific product.
    Director::construct_sports_car(&mut car_builder);

    // The final product is often retrieved from a builder object, since
    // Director is not aware and not dependent on concrete builders and
    // products.
    let car: Car = car_builder.build();
    println!("Car built: {:?}\n", car.car_type());

    let mut manual_builder = CarManualBuilder::default();

    // Director may know several building recipes.
    Director::construct_city_car(&mut manual_builder);

    // The final car manual.
    let manual: Manual = manual_builder.build();
    println!("Car manual built:\n{}", manual);
}

Output

Car built: SportsCar

Car manual built:
Type of car: CityCar
Count of seats: 2
Engine: volume - 1.2; mileage - 0
Transmission: Automatic
GPS Navigator: Functional