use gpui::{prelude::*, App, Application, Context, Entity, EventEmitter}; struct Counter {    count: usize,} struct Change {    increment: usize,} impl EventEmitter<Change> for Counter {} fn main() {    Application::new().run(|cx: &mut App| {        let counter: Entity<Counter> = cx.new(|_cx| Counter { count: 0 });        let subscriber = cx.new(|cx: &mut Context<Counter>| {            cx.subscribe(&counter, |subscriber, _emitter, event, _cx| {                subscriber.count += event.increment * 2;            })            .detach();             Counter {                count: counter.read(cx).count * 2,            }        });         counter.update(cx, |counter, cx| {            counter.count += 2;            cx.notify();            cx.emit(Change { increment: 2 });        });         assert_eq!(subscriber.read(cx).count, 4);    });}