Add example_web app

example_web/Cargo.toml

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+[package]
+name = "example_web"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2018"
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+egui = { path = "../egui", features = ["serde"] }
+egui_web = { path = "../egui_web" }
+js-sys = "0.3"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+wasm-bindgen = "0.2"

example_web/src/example_app.rs

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+/// We derive Deserialize/Serialize so we can persist app state on shutdown.
+#[derive(serde::Deserialize, serde::Serialize)]
+pub struct ExampleApp {
+    name: String,
+    age: u32,
+}
+
+impl Default for ExampleApp {
+    fn default() -> Self {
+        Self {
+            name: "Arthur".to_owned(),
+            age: 42,
+        }
+    }
+}
+
+impl egui::app::App for ExampleApp {
+    /// Called each time the UI needs repainting, which may be many times per second.
+    /// Put your widgets into a `SidePanel`, `TopPanel`, `CentralPanel`, `Window` or `Area`.
+    fn ui(
+        &mut self,
+        ctx: &std::sync::Arc<egui::Context>,
+        integration_context: &mut egui::app::IntegrationContext,
+    ) {
+        let ExampleApp { name, age } = self;
+
+        // Example used in `README.md`.
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.heading("My Egui Application");
+
+            ui.horizontal(|ui| {
+                ui.label("Your name: ");
+                ui.text_edit_singleline(name);
+            });
+
+            ui.add(egui::Slider::u32(age, 0..=120).text("age"));
+            if ui.button("Click each year").clicked {
+                *age += 1;
+            }
+
+            ui.label(format!("Hello '{}', age {}", name, age));
+
+            ui.advance_cursor(16.0);
+            if ui.button("Quit").clicked {
+                integration_context.output.quit = true;
+            }
+        });
+    }
+}

example_web/src/lib.rs

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+#![forbid(unsafe_code)]
+#![deny(warnings)]
+#![warn(clippy::all)]
+
+mod example_app;
+
+use wasm_bindgen::prelude::*;
+
+/// This is the entry-point for all the web-assembly.
+/// This is called once from the HTML.
+/// It loads the app, installs some callbacks, then returns.
+/// You can add more callbacks like this if you want to call in to your code.
+#[wasm_bindgen]
+pub fn start(canvas_id: &str) -> Result<(), wasm_bindgen::JsValue> {
+    let app = example_app::ExampleApp::default();
+    let backend = egui_web::WebBackend::new(canvas_id)?;
+    let runner = egui_web::AppRunner::new(backend, Box::new(app))?;
+    egui_web::start(runner)?;
+    Ok(())
+}