How to Implement the Observer Pattern in Rust

Implement the Observer pattern in Rust by defining a Subject struct that holds a list of Box<dyn Observer> and an Observer trait with a notify method.

use std::cell::RefCell;
use std::rc::Rc;

trait Observer {
    fn update(&self, message: &str);
}

struct Subject {
    observers: RefCell<Vec<Rc<dyn Observer>>>,
}

impl Subject {
    fn new() -> Self { Subject { observers: RefCell::new(vec![]) } }
    fn attach(&self, observer: Rc<dyn Observer>) { self.observers.borrow_mut().push(observer); }
    fn notify(&self, message: &str) {
        for observer in self.observers.borrow().iter() { observer.update(message); }
    }
}

struct ConcreteObserver {
    name: String,
}

impl Observer for ConcreteObserver {
    fn update(&self, message: &str) { println!("{} received: {}", self.name, message); }
}

fn main() {
    let subject = Subject::new();
    let obs1 = Rc::new(ConcreteObserver { name: "Observer 1".to_string() });
    let obs2 = Rc::new(ConcreteObserver { name: "Observer 2".to_string() });
    subject.attach(obs1);
    subject.attach(obs2);
    subject.notify("Hello World");
}

The Observer pattern lets one object notify many others when its state changes, like a news subscription where the publisher sends updates to all subscribers. In Rust, you use traits to define the notification rule and smart pointers to manage the list of subscribers safely. This keeps your code flexible so you can add new observers without changing the subject.