examples: Add a requestAnimationFrame loop example

examples/request-animation-frame/src/lib.rs

@@ -0,0 +1,57 @@
+use std::cell::RefCell;
+use std::rc::Rc;
+use wasm_bindgen::prelude::*;
+use wasm_bindgen::JsCast;
+
+fn window() -> web_sys::Window {
+    web_sys::window().expect("no global `window` exists")
+}
+
+fn request_animation_frame(f: &Closure<FnMut()>) {
+    window()
+        .request_animation_frame(f.as_ref().unchecked_ref())
+        .expect("should register `requestAnimationFrame` OK");
+}
+
+fn document() -> web_sys::Document {
+    window()
+        .document()
+        .expect("should have a document on window")
+}
+
+fn body() -> web_sys::HtmlElement {
+    document().body().expect("document should have a body")
+}
+
+// This function is automatically invoked after the wasm module is instantiated.
+#[wasm_bindgen(start)]
+pub fn run() -> Result<(), JsValue> {
+    let f = Rc::new(RefCell::new(None));
+    let g = f.clone();
+
+    {
+        let mut i = 0;
+        *g.borrow_mut() = Some(Closure::wrap(Box::new(move || {
+            if i > 300 {
+                body().set_text_content(Some("All done!"));
+
+                // Drop our handle to this closure so that it will get cleaned
+                // up once we return.
+                let _ = f.borrow_mut().take();
+                return;
+            }
+
+            // Set the body's text content to how many times this
+            // requestAnimationFrame callback has fired.
+            i += 1;
+            let text = format!("requestAnimationFrame has been called {} times.", i);
+            body().set_text_content(Some(&text));
+
+            // Schedule ourself for another requestAnimationFrame callback.
+            request_animation_frame(f.borrow().as_ref().unwrap());
+        }) as Box<FnMut()>));
+    }
+
+    request_animation_frame(g.borrow().as_ref().unwrap());
+    Ok(())
+}