You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: simple_async.rs
    path of file: ./repos/Relm4/examples
    
    the code of the file until where you have to start completion: use std::time::Duration;

use gtk::prelude::*;
use relm4::{
    component::{AsyncComponent, AsyncComponentParts, AsyncComponentSender},
    gtk,
    loading_widgets::LoadingWidgets,
    view, RelmApp, RelmWidgetExt,
};

struct App {
    counter: u8,
}

#[derive(Debug)]
enum Msg {
    Increment,
    Decrement,
}

#[relm4::component(async)]
impl
    
    fn init_loading_widgets(root: Self::Root) -> Option<LoadingWidgets> {
        view! {
            #[local]
            root {
                set_title: Some("Simple app"),
                set_default_size: (300, 100),

                // This will be removed automatically by
                // LoadingWidgets when the full view has loaded
                #[name(spinner)]
                gtk::Spinner {
                    start: (),
                    set_halign: gtk::Align::Center,
                }
            }
        }
        Some(LoadingWidgets::new(root, spinner))
    }