如何在 Rust 中实现观察者模式

我有一个可观察的集合和一个观察者.我希望观察者是trait Observer的特征实现.当某个事件发生时，可观察对象应该能够通知每个观察者.这应该可以解释我的意图:

struct A {
    observables: Vec<Observable>,
}

impl A {
    fn new() -> A {
        A {
            observables: vec![],
        }
    }
}

trait Observer {
    fn event(&mut self, _: &String);
}

impl Observer for A {
    fn event(&mut self, ev: &String) {
        println!("Got event from observable: {}", ev);
    }
}

struct Observable {
    observers: Vec<dyn Observer>, // How to contain references to observers? (this line is invalid)
}

impl Observable {
    fn new() -> Observable {
        Observable {
            observers: Vec::new(),
        }
    }

    fn add_observer(&mut self, o: &dyn Observer) {
        // incorrect line too
        self.observers.push(o);
    }

    fn remove_observer(&mut self, o: &dyn Observer) {
        // incorrect line too
        self.observers.remove(o);
    }

    fn notify_observers(&self, ev: &String) {
        for o in &mut self.observers {
            o.event(ev);
        }
    }
}

(Playground)

我得到了一个错误:

error[E0277]: the size for values of type `(dyn Observer + 'static)` cannot be known at compilation time
  --> src/lib.rs:24:5
   |
24 |     observers: Vec<dyn Observer>, // How to contain references to observers?
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `(dyn Observer + 'static)`
   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: required by `std::vec::Vec`

这只是我想做的一个模型.我在java、python和C++中有这样的代码，但是我不知道如何在鲁斯特中实现观察者模式.我认为我的问题是在可观察对象中存储对观察对象的引用.