egui/examples/confirm_exit/src/main.rs

#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release

use eframe::egui;

fn main() -> Result<(), eframe::EframeError> {
    let options = eframe::NativeOptions {
        initial_window_size: Some(egui::vec2(320.0, 240.0)),
        ..Default::default()
    };
    eframe::run_native(
        "Confirm exit",
        options,
        Box::new(|_cc| Box::new(MyApp::default())),
    )
}

#[derive(Default)]
struct MyApp {
    allowed_to_close: bool,
    show_confirmation_dialog: bool,
}

impl eframe::App for MyApp {
    fn on_close_event(&mut self) -> bool {
        self.show_confirmation_dialog = true;
        self.allowed_to_close
    }

    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading("Try to close the window");
        });

        if self.show_confirmation_dialog {
            // Show confirmation dialog:
            egui::Window::new("Do you want to quit?")
                .collapsible(false)
                .resizable(false)
                .show(ctx, |ui| {
                    ui.horizontal(|ui| {
                        if ui.button("Cancel").clicked() {
                            self.show_confirmation_dialog = false;
                        }

                        if ui.button("Yes!").clicked() {
                            self.allowed_to_close = true;
                            frame.close();
                        }
                    });
                });
        }
    }
}