eframe: several windows in series (#1919)

* Add example of opening several eframe windows in series

* Reuse the same winit event loop

* Ignore events to the wrong window

* Run run_return again

examples/serial_windows/src/main.rs

@@ -0,0 +1,53 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+
+fn main() {
+    if cfg!(target_os = "macos") {
+        eprintln!("WARNING: this example does not work on Mac! See https://github.com/emilk/egui/issues/1918");
+    }
+
+    let options = eframe::NativeOptions {
+        run_and_return: true,
+        ..Default::default()
+    };
+
+    eprintln!("Starting first window…");
+    eframe::run_native(
+        "First Window",
+        options.clone(),
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+
+    std::thread::sleep(std::time::Duration::from_secs(2));
+
+    eprintln!("Starting second window…");
+    eframe::run_native(
+        "Second Window",
+        options.clone(),
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+
+    std::thread::sleep(std::time::Duration::from_secs(2));
+
+    eprintln!("Starting third window…");
+    eframe::run_native(
+        "Third Window",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+}
+
+#[derive(Default)]
+struct MyApp {}
+
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            if ui.button("Close").clicked() {
+                eprintln!("Pressed Close button");
+                frame.quit();
+            }
+        });
+    }
+}