Bridge を Rust で

Bridge は、構造に関するデザインパターンの一つで、ビジネス・ロジックや巨大なクラスを独立して開発可能なクラス階層に分割します。

階層の一つ（抽象化と呼ばれる）は、二つ目の階層（実装）のオブジェクトへの参照を持ちます。抽象化階層は、その呼び出しのいくつか（場合によっては大多数）を実装階層のオブジェクトに委任します。すべての実装は、共通のインターフェースを持っているので、抽象化の中で入れ替え可能です。

Bridge の詳細

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はじめに

Devices and Remotes

mod

basic

advanced

mod

radio

main

Devices and Remotes

This example illustrates how the Bridge pattern can help divide the monolithic code of an app that manages devices and their remote controls. The Device classes act as the implementation, whereas the Remotes act as the abstraction.

remotes/mod.rs

mod advanced;
mod basic;

pub use advanced::AdvancedRemote;
pub use basic::BasicRemote;

use crate::device::Device;

pub trait HasMutableDevice<D: Device> {
    fn device(&mut self) -> &mut D;
}

pub trait Remote<D: Device>: HasMutableDevice<D> {
    fn power(&mut self) {
        println!("Remote: power toggle");
        if self.device().is_enabled() {
            self.device().disable();
        } else {
            self.device().enable();
        }
    }

    fn volume_down(&mut self) {
        println!("Remote: volume down");
        let volume = self.device().volume();
        self.device().set_volume(volume - 10);
    }

    fn volume_up(&mut self) {
        println!("Remote: volume up");
        let volume = self.device().volume();
        self.device().set_volume(volume + 10);
    }

    fn channel_down(&mut self) {
        println!("Remote: channel down");
        let channel = self.device().channel();
        self.device().set_channel(channel - 1);
    }

    fn channel_up(&mut self) {
        println!("Remote: channel up");
        let channel = self.device().channel();
        self.device().set_channel(channel + 1);
    }
}

remotes/basic.rs

use crate::device::Device;

use super::{HasMutableDevice, Remote};

pub struct BasicRemote<D: Device> {
    device: D,
}

impl<D: Device> BasicRemote<D> {
    pub fn new(device: D) -> Self {
        Self { device }
    }
}

impl<D: Device> HasMutableDevice<D> for BasicRemote<D> {
    fn device(&mut self) -> &mut D {
        &mut self.device
    }
}

impl<D: Device> Remote<D> for BasicRemote<D> {}

remotes/advanced.rs

use crate::device::Device;

use super::{HasMutableDevice, Remote};

pub struct AdvancedRemote<D: Device> {
    device: D,
}

impl<D: Device> AdvancedRemote<D> {
    pub fn new(device: D) -> Self {
        Self { device }
    }

    pub fn mute(&mut self) {
        println!("Remote: mute");
        self.device.set_volume(0);
    }
}

impl<D: Device> HasMutableDevice<D> for AdvancedRemote<D> {
    fn device(&mut self) -> &mut D {
        &mut self.device
    }
}

impl<D: Device> Remote<D> for AdvancedRemote<D> {}

device/mod.rs

mod radio;
mod tv;

pub use radio::Radio;
pub use tv::Tv;

pub trait Device {
    fn is_enabled(&self) -> bool;
    fn enable(&mut self);
    fn disable(&mut self);
    fn volume(&self) -> u8;
    fn set_volume(&mut self, percent: u8);
    fn channel(&self) -> u16;
    fn set_channel(&mut self, channel: u16);
    fn print_status(&self);
}

device/radio.rs

use super::Device;

#[derive(Clone)]
pub struct Radio {
    on: bool,
    volume: u8,
    channel: u16,
}

impl Default for Radio {
    fn default() -> Self {
        Self {
            on: false,
            volume: 30,
            channel: 1,
        }
    }
}

impl Device for Radio {
    fn is_enabled(&self) -> bool {
        self.on
    }

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

    fn disable(&mut self) {
        self.on = false;
    }

    fn volume(&self) -> u8 {
        self.volume
    }

    fn set_volume(&mut self, percent: u8) {
        self.volume = std::cmp::min(percent, 100);
    }

    fn channel(&self) -> u16 {
        self.channel
    }

    fn set_channel(&mut self, channel: u16) {
        self.channel = channel;
    }

    fn print_status(&self) {
        println!("------------------------------------");
        println!("| I'm radio.");
        println!("| I'm {}", if self.on { "enabled" } else { "disabled" });
        println!("| Current volume is {}%", self.volume);
        println!("| Current channel is {}", self.channel);
        println!("------------------------------------\n");
    }
}

device/tv.rs

use super::Device;

#[derive(Clone)]
pub struct Tv {
    on: bool,
    volume: u8,
    channel: u16,
}

impl Default for Tv {
    fn default() -> Self {
        Self {
            on: false,
            volume: 30,
            channel: 1,
        }
    }
}

impl Device for Tv {
    fn is_enabled(&self) -> bool {
        self.on
    }

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

    fn disable(&mut self) {
        self.on = false;
    }

    fn volume(&self) -> u8 {
        self.volume
    }

    fn set_volume(&mut self, percent: u8) {
        self.volume = std::cmp::min(percent, 100);
    }

    fn channel(&self) -> u16 {
        self.channel
    }

    fn set_channel(&mut self, channel: u16) {
        self.channel = channel;
    }

    fn print_status(&self) {
        println!("------------------------------------");
        println!("| I'm TV set.");
        println!("| I'm {}", if self.on { "enabled" } else { "disabled" });
        println!("| Current volume is {}%", self.volume);
        println!("| Current channel is {}", self.channel);
        println!("------------------------------------\n");
    }
}

main.rs

mod device;
mod remotes;

use device::{Device, Radio, Tv};
use remotes::{AdvancedRemote, BasicRemote, HasMutableDevice, Remote};

fn main() {
    test_device(Tv::default());
    test_device(Radio::default());
}

fn test_device(device: impl Device + Clone) {
    println!("Tests with basic remote.");
    let mut basic_remote = BasicRemote::new(device.clone());
    basic_remote.power();
    basic_remote.device().print_status();

    println!("Tests with advanced remote.");
    let mut advanced_remote = AdvancedRemote::new(device);
    advanced_remote.power();
    advanced_remote.mute();
    advanced_remote.device().print_status();
}

Output

Tests with basic remote.
Remote: power toggle
------------------------------------
| I'm TV set.
| I'm enabled
| Current volume is 30%
| Current channel is 1
------------------------------------

Tests with advanced remote.
Remote: power toggle
Remote: mute
------------------------------------
| I'm TV set.
| I'm enabled
| Current volume is 0%
| Current channel is 1
------------------------------------

Tests with basic remote.
Remote: power toggle
------------------------------------
| I'm radio.
| I'm enabled
| Current volume is 30%
| Current channel is 1
------------------------------------

Tests with advanced remote.
Remote: power toggle
Remote: mute
------------------------------------
| I'm radio.
| I'm enabled
| Current volume is 0%
| Current channel is 1
------------------------------------```