Fix the web backend too

Cargo.lock

@@ -1272,7 +1272,9 @@ dependencies = [
  "log",
  "poll-promise",
  "serde",
+ "wasm-bindgen",
  "wasm-bindgen-futures",
+ "web-sys",
 ]
 
 [[package]]

crates/eframe/src/web/backend.rs

@@ -259,8 +259,8 @@ impl AppRunner {
         let needs_repaint: std::sync::Arc<NeedRepaint> = Default::default();
         {
             let needs_repaint = needs_repaint.clone();
-            egui_ctx.set_request_repaint_callback(move || {
-                needs_repaint.repaint_asap();
+            egui_ctx.set_request_repaint_callback(move |info| {
+                needs_repaint.repaint_after(info.after.as_secs_f64());
             });
         }
 

crates/eframe/src/web/events.rs

@@ -4,12 +4,15 @@ use egui::Key;
 
 use super::*;
 
-struct IsDestroyed(pub bool);
-
+/// Calls `request_animation_frame` to schedule repaint.
+///
+/// It will only paint if needed, but will always call `request_animation_frame` immediately.
 pub fn paint_and_schedule(
     runner_ref: &AppRunnerRef,
     panicked: Arc<AtomicBool>,
 ) -> Result<(), JsValue> {
+    struct IsDestroyed(pub bool);
+
     fn paint_if_needed(runner_ref: &AppRunnerRef) -> Result<IsDestroyed, JsValue> {
         let mut runner_lock = runner_ref.lock();
         let is_destroyed = runner_lock.is_destroyed.fetch();

crates/egui_demo_app/Cargo.toml

@@ -63,4 +63,6 @@ env_logger = "0.10"
 # web:
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 console_error_panic_hook = "0.1.6"
+wasm-bindgen = "=0.2.84"
 wasm-bindgen-futures = "0.4"
+web-sys = "0.3"

crates/egui_demo_app/src/backend_panel.rs

@@ -265,8 +265,7 @@ impl BackendPanel {
                     {
                         log::info!("Waiting 2s before requesting repaint...");
                         let ctx = ui.ctx().clone();
-                        std::thread::spawn(move || {
-                            std::thread::sleep(std::time::Duration::from_secs(2));
+                        call_after_delay(std::time::Duration::from_secs(2), move || {
                             log::info!("Request a repaint in 3s...");
                             ctx.request_repaint_after(std::time::Duration::from_secs(3));
                         });
@@ -385,3 +384,28 @@ impl EguiWindows {
             });
     }
 }
+
+// ----------------------------------------------------------------------------
+
+#[cfg(not(target_arch = "wasm32"))]
+fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + Send + 'static) {
+    std::thread::spawn(move || {
+        std::thread::sleep(delay);
+        f();
+    });
+}
+
+#[cfg(target_arch = "wasm32")]
+fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + Send + 'static) {
+    use wasm_bindgen::prelude::*;
+    let window = web_sys::window().unwrap();
+    let closure = Closure::once(f);
+    let delay_ms = delay.as_millis() as _;
+    window
+        .set_timeout_with_callback_and_timeout_and_arguments_0(
+            closure.as_ref().unchecked_ref(),
+            delay_ms,
+        )
+        .unwrap();
+    closure.forget(); // We must forget it, or else the callback is canceled on drop
+}