Update serial window example (#2756)

* Update to make it more clear what will happen

* Provide an overview in readme of expectations

* Update screenshot to match new code

examples/serial_windows/README.md

@@ -1,5 +1,12 @@
 Demonstrates how to open several windows after each other.
 
+Expected order of execution:
+
+- When the example runs a first window will be shown.
+- Once the first window is closed after a delay a second window will be shown.
+- Similarly, when the second window is closed after a delay a third will be shown.
+- Once the third is closed the program will stop.
+
 NOTE: this doesn't work on Mac due to <https://github.com/rust-windowing/winit/issues/2431>.
 See also <https://github.com/emilk/egui/issues/1918>.
 

examples/serial_windows/src/main.rs

@@ -17,7 +17,7 @@ fn main() -> Result<(), eframe::Error> {
     eframe::run_native(
         "First Window",
         options.clone(),
-        Box::new(|_cc| Box::new(MyApp::default())),
+        Box::new(|_cc| Box::new(MyApp { has_next: true })),
     )?;
 
     std::thread::sleep(std::time::Duration::from_secs(2));
@@ -26,7 +26,7 @@ fn main() -> Result<(), eframe::Error> {
     eframe::run_native(
         "Second Window",
         options.clone(),
-        Box::new(|_cc| Box::new(MyApp::default())),
+        Box::new(|_cc| Box::new(MyApp { has_next: true })),
     )?;
 
     std::thread::sleep(std::time::Duration::from_secs(2));
@@ -35,16 +35,23 @@ fn main() -> Result<(), eframe::Error> {
     eframe::run_native(
         "Third Window",
         options,
-        Box::new(|_cc| Box::new(MyApp::default())),
+        Box::new(|_cc| Box::new(MyApp { has_next: false })),
     )
 }
 
-#[derive(Default)]
-struct MyApp {}
+struct MyApp {
+    pub(crate) has_next: bool,
+}
 
 impl eframe::App for MyApp {
     fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
         egui::CentralPanel::default().show(ctx, |ui| {
+            let label_text = if self.has_next {
+                "When this window is closed the next will be opened after a short delay"
+            } else {
+                "This is the last window. Program will end when closed"
+            };
+            ui.label(label_text);
             if ui.button("Close").clicked() {
                 eprintln!("Pressed Close button");
                 frame.close();