Add a simple example of the viewports API

examples/multiple_viewports/src/main.rs

@@ -0,0 +1,43 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+
+fn main() -> Result<(), eframe::Error> {
+    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
+    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::<MyApp>::default()),
+    )
+}
+
+#[derive(Default)]
+struct MyApp {
+    show_child_viewport: bool,
+}
+
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.label("Hello from the root viewport");
+
+            ui.checkbox(&mut self.show_child_viewport, "Show secondary viewport");
+        });
+
+        if self.show_child_viewport {
+            ctx.show_viewport(
+                egui::ViewportBuilder::new(egui::ViewportId::from_hash_of("secondary_viewport"))
+                    .with_title("Secondary Viewport"),
+                |ctx| {
+                    egui::CentralPanel::default().show(ctx, |ui| {
+                        ui.label("Hello from secondary viewport");
+                    });
+                },
+            );
+        }
+    }
+}