Add a simple example of the viewports API

examples/multiple_viewports/Cargo.toml

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+[package]
+name = "multiple_viewports"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <[email protected]>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.70"
+publish = false
+
+
+[dependencies]
+eframe = { path = "../../crates/eframe", features = [
+    "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
+] }
+env_logger = "0.10"

examples/multiple_viewports/README.md

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+Example how to show multiple viewports (native windows) can be created in `egui` when using the `eframe` backend.
+
+```sh
+cargo run -p multiple_viewports
+```
+
+For a more advanced example, see [../test_viewports].

examples/multiple_viewports/src/main.rs

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+#![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");
+                    });
+                },
+            );
+        }
+    }
+}

examples/test_viewports/README.md

@@ -1 +1,3 @@
 This is a test of the viewports feature of eframe and egui, where we show off using multiple windows.
+
+For a simple example, see [../multiple_viewports].