Observer を Rust で

Observer は、振る舞いに関するデザインパターンの一つで、オブジェクトが別のオブジェクトに状態の変化を通知できるようにします。

Observer パターンは、サブスクライバー・インターフェースを実装するいかなるオブジェクトのイベント通知の申し込みと停止をする方法を提供します。

Conceptual example

In Rust, a convenient way to define a subscriber is to have a function as a callable object with complex logic passing it to a event publisher.

In this Observer example, Subscribers are either a lambda function or an explicit function subscribed to the event. Explicit function objects could be also unsubscribed (although, there could be limitations for some function types).

editor.rs

use crate::observer::{Event, Publisher};

/// Editor has its own logic and it utilizes a publisher
/// to operate with subscribers and events.
#[derive(Default)]
pub struct Editor {
    publisher: Publisher,
    file_path: String,
}

impl Editor {
    pub fn events(&mut self) -> &mut Publisher {
        &mut self.publisher
    }

    pub fn load(&mut self, path: String) {
        self.file_path = path.clone();
        self.publisher.notify(Event::Load, path);
    }

    pub fn save(&self) {
        self.publisher.notify(Event::Save, self.file_path.clone());
    }
}

observer.rs

use std::collections::HashMap;

/// An event type.
#[derive(PartialEq, Eq, Hash, Clone)]
pub enum Event {
    Load,
    Save,
}

/// A subscriber (listener) has type of a callable function.
pub type Subscriber = fn(file_path: String);

/// Publisher sends events to subscribers (listeners).
#[derive(Default)]
pub struct Publisher {
    events: HashMap<Event, Vec<Subscriber>>,
}

impl Publisher {
    pub fn subscribe(&mut self, event_type: Event, listener: Subscriber) {
        self.events.entry(event_type.clone()).or_default();
        self.events.get_mut(&event_type).unwrap().push(listener);
    }

    pub fn unsubscribe(&mut self, event_type: Event, listener: Subscriber) {
        self.events
            .get_mut(&event_type)
            .unwrap()
            .retain(|&x| x != listener);
    }

    pub fn notify(&self, event_type: Event, file_path: String) {
        let listeners = self.events.get(&event_type).unwrap();
        for listener in listeners {
            listener(file_path.clone());
        }
    }
}

main.rs

use editor::Editor;
use observer::Event;

mod editor;
mod observer;

fn main() {
    let mut editor = Editor::default();

    editor.events().subscribe(Event::Load, |file_path| {
        let log = "/path/to/log/file.txt".to_string();
        println!("Save log to {}: Load file {}", log, file_path);
    });

    editor.events().subscribe(Event::Save, save_listener);

    editor.load("test1.txt".into());
    editor.load("test2.txt".into());
    editor.save();

    editor.events().unsubscribe(Event::Save, save_listener);
    editor.save();
}

fn save_listener(file_path: String) {
    let email = "admin@example.com".to_string();
    println!("Email to {}: Save file {}", email, file_path);
}

Output

Save log to /path/to/log/file.txt: Load file test1.txt
Save log to /path/to/log/file.txt: Load file test2.txt
Email to admin@example.com: Save file test2.txt