Add a comment about memory management

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

@@ -26,10 +26,22 @@ fn body() -> web_sys::HtmlElement {
 // This function is automatically invoked after the wasm module is instantiated.
 #[wasm_bindgen(start)]
 pub fn run() -> Result<(), JsValue> {
+    // Here we want to call `requestAnimationFrame` in a loop, but only a fixed
+    // number of times. After it's done we want all our resources cleaned up. To
+    // achieve this we're using an `Rc`. The `Rc` will eventually store the
+    // closure we want to execute on each frame, but to start out it contains
+    // `None`.
+    //
+    // After the `Rc` is made we'll actually create the closure, and the closure
+    // will reference one of the `Rc` instances. The other `Rc` reference is
+    // used to store the closure, request the first frame, and then is dropped
+    // by this function.
+    //
+    // Inside the closure we've got a persistent `Rc` reference, which we use
+    // for all future iterations of the loop
     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 {
@@ -50,7 +62,6 @@ pub fn run() -> Result<(), JsValue> {
         // 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(())