eframe web: Better panic handling (#2942)

* Refactor: remove extra store of events

* Remove unnecessary extra function

* Refactor: simplify event registering

* Store panic summary

* egui_demo_app: move web-part to own module

* index.html: await

* Properly unsubscribe from events on panic

* Better error handling

* Demo app html: hide the wasm canvas and show an error message on panic

* egui_demo_app: add panic button to test panic response on web

* fix typo

* Use a constructor to create WebHandle

* Refactor: less use of locks in the interfaces

* More consistent naming

crates/eframe/src/lib.rs

@@ -81,61 +81,17 @@ pub use epi::*;
 // ----------------------------------------------------------------------------
 // When compiling for web
 
-#[cfg(target_arch = "wasm32")]
-pub mod web;
-
 #[cfg(target_arch = "wasm32")]
 pub use wasm_bindgen;
 
-#[cfg(target_arch = "wasm32")]
-use web::AppRunnerRef;
-
 #[cfg(target_arch = "wasm32")]
 pub use web_sys;
 
-/// Install event listeners to register different input events
-/// and start running the given app.
-///
-/// ``` no_run
-/// #[cfg(target_arch = "wasm32")]
-/// use wasm_bindgen::prelude::*;
-///
-/// /// This is the entry-point for all the web-assembly.
-/// /// This is called from the HTML.
-/// /// It loads the app, installs some callbacks, then returns.
-/// /// It returns a handle to the running app that can be stopped calling `AppRunner::stop_web`.
-/// /// You can add more callbacks like this if you want to call in to your code.
-/// #[cfg(target_arch = "wasm32")]
-/// #[wasm_bindgen]
-/// pub struct WebHandle {
-///     handle: AppRunnerRef,
-/// }
-/// #[cfg(target_arch = "wasm32")]
-/// #[wasm_bindgen]
-/// pub async fn start(canvas_id: &str) -> Result<WebHandle, eframe::wasm_bindgen::JsValue> {
-///     let web_options = eframe::WebOptions::default();
-///     eframe::start_web(
-///         canvas_id,
-///         web_options,
-///         Box::new(|cc| Box::new(MyEguiApp::new(cc))),
-///     )
-///     .await
-///     .map(|handle| WebHandle { handle })
-/// }
-/// ```
-///
-/// # Errors
-/// Failing to initialize WebGL graphics.
 #[cfg(target_arch = "wasm32")]
-pub async fn start_web(
+pub mod web;
-    canvas_id: &str,
-    web_options: WebOptions,
-    app_creator: AppCreator,
-) -> std::result::Result<AppRunnerRef, wasm_bindgen::JsValue> {
-    let handle = web::start(canvas_id, web_options, app_creator).await?;
 
-    Ok(handle)
+#[cfg(target_arch = "wasm32")]
-}
+pub use web::start_web;
 
 // ----------------------------------------------------------------------------
 // When compiling natively

crates/eframe/src/web/backend.rs

@@ -1,7 +1,6 @@
-use egui::{
+use std::{cell::RefCell, rc::Rc};
-    mutex::{Mutex, MutexGuard},
+
-    TexturesDelta,
+use egui::{mutex::Mutex, TexturesDelta};
-};
 
 use crate::{epi, App};
 
@@ -170,6 +169,64 @@ fn test_parse_query() {
 
 // ----------------------------------------------------------------------------
 
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen(js_namespace = console)]
+    fn error(msg: String);
+
+    type Error;
+
+    #[wasm_bindgen(constructor)]
+    fn new() -> Error;
+
+    #[wasm_bindgen(structural, method, getter)]
+    fn stack(error: &Error) -> String;
+}
+
+#[derive(Clone, Debug)]
+pub struct PanicSummary {
+    message: String,
+    callstack: String,
+}
+
+impl PanicSummary {
+    pub fn new(info: &std::panic::PanicInfo<'_>) -> Self {
+        let message = info.to_string();
+        let callstack = Error::new().stack();
+        Self { message, callstack }
+    }
+
+    pub fn message(&self) -> String {
+        self.message.clone()
+    }
+
+    pub fn callstack(&self) -> String {
+        self.callstack.clone()
+    }
+}
+
+/// Handle to information about any panic than has occurred
+#[derive(Clone, Default)]
+pub struct PanicHandler {
+    summary: Option<PanicSummary>,
+}
+
+impl PanicHandler {
+    pub fn has_panicked(&self) -> bool {
+        self.summary.is_some()
+    }
+
+    pub fn panic_summary(&self) -> Option<PanicSummary> {
+        self.summary.clone()
+    }
+
+    pub fn on_panic(&mut self, info: &std::panic::PanicInfo<'_>) {
+        self.summary = Some(PanicSummary::new(info));
+    }
+}
+
+// ----------------------------------------------------------------------------
+
 pub struct AppRunner {
     pub(crate) frame: epi::Frame,
     egui_ctx: egui::Context,
@@ -183,7 +240,6 @@ pub struct AppRunner {
     pub(crate) text_cursor_pos: Option<egui::Pos2>,
     pub(crate) mutable_text_under_cursor: bool,
     textures_delta: TexturesDelta,
-    pub events_to_unsubscribe: Vec<EventToUnsubscribe>,
 }
 
 impl Drop for AppRunner {
@@ -277,7 +333,6 @@ impl AppRunner {
             text_cursor_pos: None,
             mutable_text_under_cursor: false,
             textures_delta: Default::default(),
-            events_to_unsubscribe: Default::default(),
         };
 
         runner.input.raw.max_texture_side = Some(runner.painter.max_texture_side());
@@ -333,21 +388,13 @@ impl AppRunner {
         Ok(())
     }
 
-    pub fn destroy(&mut self) -> Result<(), JsValue> {
+    fn destroy(&mut self) {
-        let is_destroyed_already = self.is_destroyed.fetch();
+        if self.is_destroyed.fetch() {
-
-        if is_destroyed_already {
             log::warn!("App was destroyed already");
-            Ok(())
         } else {
             log::debug!("Destroying");
-            for x in self.events_to_unsubscribe.drain(..) {
-                x.unsubscribe()?;
-            }
-
             self.painter.destroy();
             self.is_destroyed.set_true();
-            Ok(())
         }
     }
 
@@ -439,7 +486,130 @@ impl AppRunner {
 
 // ----------------------------------------------------------------------------
 
-pub type AppRunnerRef = Arc<Mutex<AppRunner>>;
+/// This is how we access the [`AppRunner`].
+/// This is cheap to clone.
+#[derive(Clone)]
+pub struct AppRunnerRef {
+    /// If we ever panic during running, this mutex is poisoned.
+    /// So before we use it, we need to check `panic_handler`.
+    runner: Rc<RefCell<AppRunner>>,
+
+    /// Have we ever panicked?
+    panic_handler: Arc<Mutex<PanicHandler>>,
+
+    /// In case of a panic, unsubscribe these.
+    /// They have to be in a separate `Arc` so that we don't need to pass them to
+    /// the panic handler, since they aren't `Send`.
+    events_to_unsubscribe: Rc<RefCell<Vec<EventToUnsubscribe>>>,
+}
+
+impl AppRunnerRef {
+    pub fn new(runner: AppRunner) -> Self {
+        Self {
+            runner: Rc::new(RefCell::new(runner)),
+            panic_handler: Arc::new(Mutex::new(Default::default())),
+            events_to_unsubscribe: Rc::new(RefCell::new(Default::default())),
+        }
+    }
+
+    /// Returns true if there has been a panic.
+    fn unsubscribe_if_panicked(&self) {
+        if self.panic_handler.lock().has_panicked() {
+            // Unsubscribe from all events so that we don't get any more callbacks
+            // that will try to access the poisoned runner.
+            let events_to_unsubscribe: Vec<_> =
+                std::mem::take(&mut *self.events_to_unsubscribe.borrow_mut());
+            if !events_to_unsubscribe.is_empty() {
+                log::debug!(
+                    "Unsubscribing from {} events due to panic",
+                    events_to_unsubscribe.len()
+                );
+                for x in events_to_unsubscribe {
+                    if let Err(err) = x.unsubscribe() {
+                        log::error!("Failed to unsubscribe from event: {err:?}");
+                    }
+                }
+            }
+        }
+    }
+
+    /// Returns true if there has been a panic.
+    pub fn has_panicked(&self) -> bool {
+        self.unsubscribe_if_panicked();
+        self.panic_handler.lock().has_panicked()
+    }
+
+    /// Returns `Some` if there has been a panic.
+    pub fn panic_summary(&self) -> Option<PanicSummary> {
+        self.unsubscribe_if_panicked();
+        self.panic_handler.lock().panic_summary()
+    }
+
+    pub fn destroy(&self) {
+        if let Some(mut runner) = self.try_lock() {
+            runner.destroy();
+        }
+    }
+
+    /// Returns `None` if there has been a panic, or if we have been destroyed.
+    /// In that case, just return to JS.
+    pub fn try_lock(&self) -> Option<std::cell::RefMut<'_, AppRunner>> {
+        if self.has_panicked() {
+            None
+        } else {
+            let lock = self.runner.borrow_mut();
+            if lock.is_destroyed.fetch() {
+                None
+            } else {
+                Some(lock)
+            }
+        }
+    }
+
+    /// Convenience function to reduce boilerplate and ensure that all event handlers
+    /// are dealt with in the same way
+    pub fn add_event_listener<E: wasm_bindgen::JsCast>(
+        &self,
+        target: &EventTarget,
+        event_name: &'static str,
+        mut closure: impl FnMut(E, &mut AppRunner) + 'static,
+    ) -> Result<(), JsValue> {
+        // Create a JS closure based on the FnMut provided
+        let closure = Closure::wrap({
+            // Clone atomics
+            let runner_ref = self.clone();
+
+            Box::new(move |event: web_sys::Event| {
+                // Only call the wrapped closure if the egui code has not panicked
+                if let Some(mut runner_lock) = runner_ref.try_lock() {
+                    // Cast the event to the expected event type
+                    let event = event.unchecked_into::<E>();
+
+                    closure(event, &mut runner_lock);
+                }
+            }) as Box<dyn FnMut(web_sys::Event)>
+        });
+
+        // Add the event listener to the target
+        target.add_event_listener_with_callback(event_name, closure.as_ref().unchecked_ref())?;
+
+        let handle = TargetEvent {
+            target: target.clone(),
+            event_name: event_name.to_owned(),
+            closure,
+        };
+
+        // Remember it so we unsubscribe on panic.
+        // Otherwise we get calls into `self.runner` after it has been poisoned by a panic.
+        self.events_to_unsubscribe
+            .borrow_mut()
+            .push(EventToUnsubscribe::TargetEvent(handle));
+
+        Ok(())
+    }
+}
+
+// ----------------------------------------------------------------------------
 
 pub struct TargetEvent {
     target: EventTarget,
@@ -454,7 +624,7 @@ pub struct IntervalHandle {
 
 pub enum EventToUnsubscribe {
     TargetEvent(TargetEvent),
-    #[allow(dead_code)]
+
     IntervalHandle(IntervalHandle),
 }
 
@@ -477,61 +647,42 @@ impl EventToUnsubscribe {
     }
 }
 
-pub struct AppRunnerContainer {
-    pub runner: AppRunnerRef,
-
-    /// Set to `true` if there is a panic.
-    /// Used to ignore callbacks after a panic.
-    pub panicked: Arc<AtomicBool>,
-    pub events: Vec<EventToUnsubscribe>,
-}
-
-impl AppRunnerContainer {
-    /// Convenience function to reduce boilerplate and ensure that all event handlers
-    /// are dealt with in the same way
-    pub fn add_event_listener<E: wasm_bindgen::JsCast>(
-        &mut self,
-        target: &EventTarget,
-        event_name: &'static str,
-        mut closure: impl FnMut(E, MutexGuard<'_, AppRunner>) + 'static,
-    ) -> Result<(), JsValue> {
-        // Create a JS closure based on the FnMut provided
-        let closure = Closure::wrap({
-            // Clone atomics
-            let runner_ref = self.runner.clone();
-            let panicked = self.panicked.clone();
-
-            Box::new(move |event: web_sys::Event| {
-                // Only call the wrapped closure if the egui code has not panicked
-                if !panicked.load(Ordering::SeqCst) {
-                    // Cast the event to the expected event type
-                    let event = event.unchecked_into::<E>();
-
-                    closure(event, runner_ref.lock());
-                }
-            }) as Box<dyn FnMut(web_sys::Event)>
-        });
-
-        // Add the event listener to the target
-        target.add_event_listener_with_callback(event_name, closure.as_ref().unchecked_ref())?;
-
-        let handle = TargetEvent {
-            target: target.clone(),
-            event_name: event_name.to_owned(),
-            closure,
-        };
-
-        self.events.push(EventToUnsubscribe::TargetEvent(handle));
-
-        Ok(())
-    }
-}
-
 // ----------------------------------------------------------------------------
 
 /// Install event listeners to register different input events
 /// and start running the given app.
-pub async fn start(
+///
+/// ``` no_run
+/// #[cfg(target_arch = "wasm32")]
+/// use wasm_bindgen::prelude::*;
+///
+/// /// This is the entry-point for all the web-assembly.
+/// /// This is called from the HTML.
+/// /// It loads the app, installs some callbacks, then returns.
+/// /// It returns a handle to the running app that can be stopped calling `AppRunner::stop_web`.
+/// /// You can add more callbacks like this if you want to call in to your code.
+/// #[cfg(target_arch = "wasm32")]
+/// #[wasm_bindgen]
+/// pub struct WebHandle {
+///     handle: AppRunnerRef,
+/// }
+/// #[cfg(target_arch = "wasm32")]
+/// #[wasm_bindgen]
+/// pub async fn start(canvas_id: &str) -> Result<WebHandle, eframe::wasm_bindgen::JsValue> {
+///     let web_options = eframe::WebOptions::default();
+///     eframe::start_web(
+///         canvas_id,
+///         web_options,
+///         Box::new(|cc| Box::new(MyEguiApp::new(cc))),
+///     )
+///     .await
+///     .map(|handle| WebHandle { handle })
+/// }
+/// ```
+///
+/// # Errors
+/// Failing to initialize WebGL graphics.
+pub async fn start_web(
     canvas_id: &str,
     web_options: crate::WebOptions,
     app_creator: epi::AppCreator,
@@ -544,43 +695,36 @@ pub async fn start(
 
     let mut runner = AppRunner::new(canvas_id, web_options, app_creator).await?;
     runner.warm_up()?;
-    start_runner(runner, follow_system_theme)
+    let runner_ref = AppRunnerRef::new(runner);
-}
-
-/// Install event listeners to register different input events
-/// and starts running the given [`AppRunner`].
-fn start_runner(app_runner: AppRunner, follow_system_theme: bool) -> Result<AppRunnerRef, JsValue> {
-    let mut runner_container = AppRunnerContainer {
-        runner: Arc::new(Mutex::new(app_runner)),
-        panicked: Arc::new(AtomicBool::new(false)),
-        events: Vec::with_capacity(20),
-    };
-
-    super::events::install_canvas_events(&mut runner_container)?;
-    super::events::install_document_events(&mut runner_container)?;
-    super::events::install_window_events(&mut runner_container)?;
-    text_agent::install_text_agent(&mut runner_container)?;
 
+    // Install events:
+    {
+        super::events::install_canvas_events(&runner_ref)?;
+        super::events::install_document_events(&runner_ref)?;
+        super::events::install_window_events(&runner_ref)?;
+        text_agent::install_text_agent(&runner_ref)?;
         if follow_system_theme {
-        super::events::install_color_scheme_change_event(&mut runner_container)?;
+            super::events::install_color_scheme_change_event(&runner_ref)?;
+        }
+        super::events::paint_and_schedule(&runner_ref)?;
     }
 
-    super::events::paint_and_schedule(&runner_container.runner, runner_container.panicked.clone())?;
+    // Instal panic handler:
-
+    {
         // Disable all event handlers on panic
         let previous_hook = std::panic::take_hook();
-
+        let panic_handler = runner_ref.panic_handler.clone();
-    runner_container.runner.lock().events_to_unsubscribe = runner_container.events;
 
         std::panic::set_hook(Box::new(move |panic_info| {
-        log::info!("egui disabled all event handlers due to panic");
+            log::info!("eframe detected a panic");
-        runner_container.panicked.store(true, SeqCst);
+            panic_handler.lock().on_panic(panic_info);
 
             // Propagate panic info to the previously registered panic hook
             previous_hook(panic_info);
         }));
+    }
 
-    Ok(runner_container.runner)
+    Ok(runner_ref)
 }
 
 // ----------------------------------------------------------------------------

crates/eframe/src/web/events.rs

@@ -1,99 +1,80 @@
-use std::sync::atomic::{AtomicBool, Ordering};
-
-use egui::Key;
-
 use super::*;
 
 /// 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(
+pub fn paint_and_schedule(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
-    runner_ref: &AppRunnerRef,
+    fn paint_if_needed(runner: &mut AppRunner) -> Result<(), JsValue> {
-    panicked: Arc<AtomicBool>,
+        if runner.needs_repaint.when_to_repaint() <= now_sec() {
-) -> Result<(), JsValue> {
+            runner.needs_repaint.clear();
-    struct IsDestroyed(pub bool);
+            let (repaint_after, clipped_primitives) = runner.logic()?;
-
+            runner.paint(&clipped_primitives)?;
-    fn paint_if_needed(runner_ref: &AppRunnerRef) -> Result<IsDestroyed, JsValue> {
+            runner
-        let mut runner_lock = runner_ref.lock();
-        let is_destroyed = runner_lock.is_destroyed.fetch();
-
-        if !is_destroyed && runner_lock.needs_repaint.when_to_repaint() <= now_sec() {
-            runner_lock.needs_repaint.clear();
-            let (repaint_after, clipped_primitives) = runner_lock.logic()?;
-            runner_lock.paint(&clipped_primitives)?;
-            runner_lock
                 .needs_repaint
                 .repaint_after(repaint_after.as_secs_f64());
-            runner_lock.auto_save_if_needed();
+            runner.auto_save_if_needed();
+        }
+        Ok(())
     }
 
-        Ok(IsDestroyed(is_destroyed))
+    fn request_animation_frame(runner_ref: AppRunnerRef) -> Result<(), JsValue> {
-    }
-
-    fn request_animation_frame(
-        runner_ref: AppRunnerRef,
-        panicked: Arc<AtomicBool>,
-    ) -> Result<(), JsValue> {
         let window = web_sys::window().unwrap();
-        let closure = Closure::once(move || paint_and_schedule(&runner_ref, panicked));
+        let closure = Closure::once(move || paint_and_schedule(&runner_ref));
         window.request_animation_frame(closure.as_ref().unchecked_ref())?;
         closure.forget(); // We must forget it, or else the callback is canceled on drop
         Ok(())
     }
 
     // Only paint and schedule if there has been no panic
-    if !panicked.load(Ordering::SeqCst) {
+    if let Some(mut runner_lock) = runner_ref.try_lock() {
-        let is_destroyed = paint_if_needed(runner_ref)?;
+        paint_if_needed(&mut runner_lock)?;
-        if !is_destroyed.0 {
+        drop(runner_lock);
-            request_animation_frame(runner_ref.clone(), panicked)?;
+        request_animation_frame(runner_ref.clone())?;
-        }
     }
 
     Ok(())
 }
 
-pub fn install_document_events(runner_container: &mut AppRunnerContainer) -> Result<(), JsValue> {
+pub fn install_document_events(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
     let document = web_sys::window().unwrap().document().unwrap();
 
     {
         // Avoid sticky modifier keys on alt-tab:
         for event_name in ["blur", "focus"] {
-            let closure =
+            let closure = move |_event: web_sys::MouseEvent, runner: &mut AppRunner| {
-                move |_event: web_sys::MouseEvent,
-                      mut runner_lock: egui::mutex::MutexGuard<'_, AppRunner>| {
                 let has_focus = event_name == "focus";
 
                 if !has_focus {
                     // We lost focus - good idea to save
-                        runner_lock.save();
+                    runner.save();
                 }
 
-                    runner_lock.input.on_web_page_focus_change(has_focus);
+                runner.input.on_web_page_focus_change(has_focus);
-                    runner_lock.egui_ctx().request_repaint();
+                runner.egui_ctx().request_repaint();
                 // log::debug!("{event_name:?}");
             };
 
-            runner_container.add_event_listener(&document, event_name, closure)?;
+            runner_ref.add_event_listener(&document, event_name, closure)?;
         }
     }
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &document,
         "keydown",
-        |event: web_sys::KeyboardEvent, mut runner_lock| {
+        |event: web_sys::KeyboardEvent, runner| {
             if event.is_composing() || event.key_code() == 229 {
                 // https://web.archive.org/web/20200526195704/https://www.fxsitecompat.dev/en-CA/docs/2018/keydown-and-keyup-events-are-now-fired-during-ime-composition/
                 return;
             }
 
             let modifiers = modifiers_from_event(&event);
-            runner_lock.input.raw.modifiers = modifiers;
+            runner.input.raw.modifiers = modifiers;
 
             let key = event.key();
             let egui_key = translate_key(&key);
 
             if let Some(key) = egui_key {
-                runner_lock.input.raw.events.push(egui::Event::Key {
+                runner.input.raw.events.push(egui::Event::Key {
                     key,
                     pressed: true,
                     repeat: false, // egui will fill this in for us!
@@ -106,18 +87,18 @@ pub fn install_document_events(runner_container: &mut AppRunnerContainer) -> Res
                 // When text agent is shown, it sends text event instead.
                 && text_agent::text_agent().hidden()
             {
-                runner_lock.input.raw.events.push(egui::Event::Text(key));
+                runner.input.raw.events.push(egui::Event::Text(key));
             }
-            runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
 
-            let egui_wants_keyboard = runner_lock.egui_ctx().wants_keyboard_input();
+            let egui_wants_keyboard = runner.egui_ctx().wants_keyboard_input();
 
             #[allow(clippy::if_same_then_else)]
-            let prevent_default = if egui_key == Some(Key::Tab) {
+            let prevent_default = if egui_key == Some(egui::Key::Tab) {
                 // Always prevent moving cursor to url bar.
                 // egui wants to use tab to move to the next text field.
                 true
-            } else if egui_key == Some(Key::P) {
+            } else if egui_key == Some(egui::Key::P) {
                 #[allow(clippy::needless_bool)]
                 if modifiers.ctrl || modifiers.command || modifiers.mac_cmd {
                     true // Prevent ctrl-P opening the print dialog. Users may want to use it for a command palette.
@@ -152,35 +133,35 @@ pub fn install_document_events(runner_container: &mut AppRunnerContainer) -> Res
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &document,
         "keyup",
-        |event: web_sys::KeyboardEvent, mut runner_lock| {
+        |event: web_sys::KeyboardEvent, runner| {
             let modifiers = modifiers_from_event(&event);
-            runner_lock.input.raw.modifiers = modifiers;
+            runner.input.raw.modifiers = modifiers;
             if let Some(key) = translate_key(&event.key()) {
-                runner_lock.input.raw.events.push(egui::Event::Key {
+                runner.input.raw.events.push(egui::Event::Key {
                     key,
                     pressed: false,
                     repeat: false,
                     modifiers,
                 });
             }
-            runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
         },
     )?;
 
     #[cfg(web_sys_unstable_apis)]
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &document,
         "paste",
-        |event: web_sys::ClipboardEvent, mut runner_lock| {
+        |event: web_sys::ClipboardEvent, runner| {
             if let Some(data) = event.clipboard_data() {
                 if let Ok(text) = data.get_data("text") {
                     let text = text.replace("\r\n", "\n");
                     if !text.is_empty() {
-                        runner_lock.input.raw.events.push(egui::Event::Paste(text));
+                        runner.input.raw.events.push(egui::Event::Paste(text));
-                        runner_lock.needs_repaint.repaint_asap();
+                        runner.needs_repaint.repaint_asap();
                     }
                     event.stop_propagation();
                     event.prevent_default();
@@ -190,76 +171,54 @@ pub fn install_document_events(runner_container: &mut AppRunnerContainer) -> Res
     )?;
 
     #[cfg(web_sys_unstable_apis)]
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&document, "cut", |_: web_sys::ClipboardEvent, runner| {
-        &document,
+        runner.input.raw.events.push(egui::Event::Cut);
-        "cut",
+        runner.needs_repaint.repaint_asap();
-        |_: web_sys::ClipboardEvent, mut runner_lock| {
+    })?;
-            runner_lock.input.raw.events.push(egui::Event::Cut);
-            runner_lock.needs_repaint.repaint_asap();
-        },
-    )?;
 
     #[cfg(web_sys_unstable_apis)]
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&document, "copy", |_: web_sys::ClipboardEvent, runner| {
-        &document,
+        runner.input.raw.events.push(egui::Event::Copy);
-        "copy",
+        runner.needs_repaint.repaint_asap();
-        |_: web_sys::ClipboardEvent, mut runner_lock| {
+    })?;
-            runner_lock.input.raw.events.push(egui::Event::Copy);
-            runner_lock.needs_repaint.repaint_asap();
-        },
-    )?;
 
     Ok(())
 }
 
-pub fn install_window_events(runner_container: &mut AppRunnerContainer) -> Result<(), JsValue> {
+pub fn install_window_events(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
     let window = web_sys::window().unwrap();
 
     // Save-on-close
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&window, "onbeforeunload", |_: web_sys::Event, runner| {
-        &window,
+        runner.save();
-        "onbeforeunload",
+    })?;
-        |_: web_sys::Event, mut runner_lock| {
-            runner_lock.save();
-        },
-    )?;
 
     for event_name in &["load", "pagehide", "pageshow", "resize"] {
-        runner_container.add_event_listener(
+        runner_ref.add_event_listener(&window, event_name, |_: web_sys::Event, runner| {
-            &window,
+            runner.needs_repaint.repaint_asap();
-            event_name,
+        })?;
-            |_: web_sys::Event, runner_lock| {
-                runner_lock.needs_repaint.repaint_asap();
-            },
-        )?;
     }
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&window, "hashchange", |_: web_sys::Event, runner| {
-        &window,
-        "hashchange",
-        |_: web_sys::Event, mut runner_lock| {
         // `epi::Frame::info(&self)` clones `epi::IntegrationInfo`, but we need to modify the original here
-            runner_lock.frame.info.web_info.location.hash = location_hash();
+        runner.frame.info.web_info.location.hash = location_hash();
-        },
+    })?;
-    )?;
 
     Ok(())
 }
 
-pub fn install_color_scheme_change_event(
+pub fn install_color_scheme_change_event(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
-    runner_container: &mut AppRunnerContainer,
-) -> Result<(), JsValue> {
     let window = web_sys::window().unwrap();
 
     if let Some(media_query_list) = prefers_color_scheme_dark(&window)? {
-        runner_container.add_event_listener::<web_sys::MediaQueryListEvent>(
+        runner_ref.add_event_listener::<web_sys::MediaQueryListEvent>(
             &media_query_list,
             "change",
-            |event, mut runner_lock| {
+            |event, runner| {
                 let theme = theme_from_dark_mode(event.matches());
-                runner_lock.frame.info.system_theme = Some(theme);
+                runner.frame.info.system_theme = Some(theme);
-                runner_lock.egui_ctx().set_visuals(theme.egui_visuals());
+                runner.egui_ctx().set_visuals(theme.egui_visuals());
-                runner_lock.needs_repaint.repaint_asap();
+                runner.needs_repaint.repaint_asap();
             },
         )?;
     }
@@ -267,8 +226,8 @@ pub fn install_color_scheme_change_event(
     Ok(())
 }
 
-pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Result<(), JsValue> {
+pub fn install_canvas_events(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
-    let canvas = canvas_element(runner_container.runner.lock().canvas_id()).unwrap();
+    let canvas = canvas_element(runner_ref.try_lock().unwrap().canvas_id()).unwrap();
 
     {
         let prevent_default_events = [
@@ -280,190 +239,151 @@ pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Resul
         ];
 
         for event_name in prevent_default_events {
-            let closure =
+            let closure = move |event: web_sys::MouseEvent, _runner: &mut AppRunner| {
-                move |event: web_sys::MouseEvent,
-                      mut _runner_lock: egui::mutex::MutexGuard<'_, AppRunner>| {
                 event.prevent_default();
                 // event.stop_propagation();
                 // log::debug!("Preventing event {event_name:?}");
             };
 
-            runner_container.add_event_listener(&canvas, event_name, closure)?;
+            runner_ref.add_event_listener(&canvas, event_name, closure)?;
         }
     }
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &canvas,
         "mousedown",
-        |event: web_sys::MouseEvent, mut runner_lock: egui::mutex::MutexGuard<'_, AppRunner>| {
+        |event: web_sys::MouseEvent, runner: &mut AppRunner| {
             if let Some(button) = button_from_mouse_event(&event) {
-                let pos = pos_from_mouse_event(runner_lock.canvas_id(), &event);
+                let pos = pos_from_mouse_event(runner.canvas_id(), &event);
-                let modifiers = runner_lock.input.raw.modifiers;
+                let modifiers = runner.input.raw.modifiers;
-                runner_lock
+                runner.input.raw.events.push(egui::Event::PointerButton {
-                    .input
-                    .raw
-                    .events
-                    .push(egui::Event::PointerButton {
                     pos,
                     button,
                     pressed: true,
                     modifiers,
                 });
-                runner_lock.needs_repaint.repaint_asap();
+                runner.needs_repaint.repaint_asap();
             }
             event.stop_propagation();
             // Note: prevent_default breaks VSCode tab focusing, hence why we don't call it here.
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &canvas,
         "mousemove",
-        |event: web_sys::MouseEvent, mut runner_lock| {
+        |event: web_sys::MouseEvent, runner| {
-            let pos = pos_from_mouse_event(runner_lock.canvas_id(), &event);
+            let pos = pos_from_mouse_event(runner.canvas_id(), &event);
-            runner_lock
+            runner.input.raw.events.push(egui::Event::PointerMoved(pos));
-                .input
+            runner.needs_repaint.repaint_asap();
-                .raw
-                .events
-                .push(egui::Event::PointerMoved(pos));
-            runner_lock.needs_repaint.repaint_asap();
             event.stop_propagation();
             event.prevent_default();
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&canvas, "mouseup", |event: web_sys::MouseEvent, runner| {
-        &canvas,
-        "mouseup",
-        |event: web_sys::MouseEvent, mut runner_lock| {
         if let Some(button) = button_from_mouse_event(&event) {
-                let pos = pos_from_mouse_event(runner_lock.canvas_id(), &event);
+            let pos = pos_from_mouse_event(runner.canvas_id(), &event);
-                let modifiers = runner_lock.input.raw.modifiers;
+            let modifiers = runner.input.raw.modifiers;
-                runner_lock
+            runner.input.raw.events.push(egui::Event::PointerButton {
-                    .input
-                    .raw
-                    .events
-                    .push(egui::Event::PointerButton {
                 pos,
                 button,
                 pressed: false,
                 modifiers,
             });
-                runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
 
-                text_agent::update_text_agent(runner_lock);
+            text_agent::update_text_agent(runner);
         }
         event.stop_propagation();
         event.prevent_default();
-        },
+    })?;
-    )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &canvas,
         "mouseleave",
-        |event: web_sys::MouseEvent, mut runner_lock| {
+        |event: web_sys::MouseEvent, runner| {
-            runner_lock.input.raw.events.push(egui::Event::PointerGone);
+            runner.input.raw.events.push(egui::Event::PointerGone);
-            runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
             event.stop_propagation();
             event.prevent_default();
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &canvas,
         "touchstart",
-        |event: web_sys::TouchEvent, mut runner_lock| {
+        |event: web_sys::TouchEvent, runner| {
-            let mut latest_touch_pos_id = runner_lock.input.latest_touch_pos_id;
+            let mut latest_touch_pos_id = runner.input.latest_touch_pos_id;
-            let pos =
+            let pos = pos_from_touch_event(runner.canvas_id(), &event, &mut latest_touch_pos_id);
-                pos_from_touch_event(runner_lock.canvas_id(), &event, &mut latest_touch_pos_id);
+            runner.input.latest_touch_pos_id = latest_touch_pos_id;
-            runner_lock.input.latest_touch_pos_id = latest_touch_pos_id;
+            runner.input.latest_touch_pos = Some(pos);
-            runner_lock.input.latest_touch_pos = Some(pos);
+            let modifiers = runner.input.raw.modifiers;
-            let modifiers = runner_lock.input.raw.modifiers;
+            runner.input.raw.events.push(egui::Event::PointerButton {
-            runner_lock
-                .input
-                .raw
-                .events
-                .push(egui::Event::PointerButton {
                 pos,
                 button: egui::PointerButton::Primary,
                 pressed: true,
                 modifiers,
             });
 
-            push_touches(&mut runner_lock, egui::TouchPhase::Start, &event);
+            push_touches(runner, egui::TouchPhase::Start, &event);
-            runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
             event.stop_propagation();
             event.prevent_default();
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &canvas,
         "touchmove",
-        |event: web_sys::TouchEvent, mut runner_lock| {
+        |event: web_sys::TouchEvent, runner| {
-            let mut latest_touch_pos_id = runner_lock.input.latest_touch_pos_id;
+            let mut latest_touch_pos_id = runner.input.latest_touch_pos_id;
-            let pos =
+            let pos = pos_from_touch_event(runner.canvas_id(), &event, &mut latest_touch_pos_id);
-                pos_from_touch_event(runner_lock.canvas_id(), &event, &mut latest_touch_pos_id);
+            runner.input.latest_touch_pos_id = latest_touch_pos_id;
-            runner_lock.input.latest_touch_pos_id = latest_touch_pos_id;
+            runner.input.latest_touch_pos = Some(pos);
-            runner_lock.input.latest_touch_pos = Some(pos);
+            runner.input.raw.events.push(egui::Event::PointerMoved(pos));
-            runner_lock
-                .input
-                .raw
-                .events
-                .push(egui::Event::PointerMoved(pos));
 
-            push_touches(&mut runner_lock, egui::TouchPhase::Move, &event);
+            push_touches(runner, egui::TouchPhase::Move, &event);
-            runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
             event.stop_propagation();
             event.prevent_default();
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&canvas, "touchend", |event: web_sys::TouchEvent, runner| {
-        &canvas,
+        if let Some(pos) = runner.input.latest_touch_pos {
-        "touchend",
+            let modifiers = runner.input.raw.modifiers;
-        |event: web_sys::TouchEvent, mut runner_lock| {
-            if let Some(pos) = runner_lock.input.latest_touch_pos {
-                let modifiers = runner_lock.input.raw.modifiers;
             // First release mouse to click:
-                runner_lock
+            runner.input.raw.events.push(egui::Event::PointerButton {
-                    .input
-                    .raw
-                    .events
-                    .push(egui::Event::PointerButton {
                 pos,
                 button: egui::PointerButton::Primary,
                 pressed: false,
                 modifiers,
             });
             // Then remove hover effect:
-                runner_lock.input.raw.events.push(egui::Event::PointerGone);
+            runner.input.raw.events.push(egui::Event::PointerGone);
 
-                push_touches(&mut runner_lock, egui::TouchPhase::End, &event);
+            push_touches(runner, egui::TouchPhase::End, &event);
-                runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
             event.stop_propagation();
             event.prevent_default();
         }
 
         // Finally, focus or blur text agent to toggle mobile keyboard:
-            text_agent::update_text_agent(runner_lock);
+        text_agent::update_text_agent(runner);
-        },
+    })?;
-    )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(
         &canvas,
         "touchcancel",
-        |event: web_sys::TouchEvent, mut runner_lock| {
+        |event: web_sys::TouchEvent, runner| {
-            push_touches(&mut runner_lock, egui::TouchPhase::Cancel, &event);
+            push_touches(runner, egui::TouchPhase::Cancel, &event);
             event.stop_propagation();
             event.prevent_default();
         },
     )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&canvas, "wheel", |event: web_sys::WheelEvent, runner| {
-        &canvas,
-        "wheel",
-        |event: web_sys::WheelEvent, mut runner_lock| {
         let unit = match event.delta_mode() {
             web_sys::WheelEvent::DOM_DELTA_PIXEL => egui::MouseWheelUnit::Point,
             web_sys::WheelEvent::DOM_DELTA_LINE => egui::MouseWheelUnit::Line,
@@ -472,16 +392,16 @@ pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Resul
         };
         // delta sign is flipped to match native (winit) convention.
         let delta = -egui::vec2(event.delta_x() as f32, event.delta_y() as f32);
-            let modifiers = runner_lock.input.raw.modifiers;
+        let modifiers = runner.input.raw.modifiers;
 
-            runner_lock.input.raw.events.push(egui::Event::MouseWheel {
+        runner.input.raw.events.push(egui::Event::MouseWheel {
             unit,
             delta,
             modifiers,
         });
 
         let scroll_multiplier = match unit {
-                egui::MouseWheelUnit::Page => canvas_size_in_points(runner_lock.canvas_id()).y,
+            egui::MouseWheelUnit::Page => canvas_size_in_points(runner.canvas_id()).y,
             egui::MouseWheelUnit::Line => {
                 #[allow(clippy::let_and_return)]
                 let points_per_scroll_line = 8.0; // Note that this is intentionally different from what we use in winit.
@@ -497,7 +417,7 @@ pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Resul
         // `modifiers_from_event()`, but we cannot directly use that fn for a [`WheelEvent`].
         if event.ctrl_key() || event.meta_key() {
             let factor = (delta.y / 200.0).exp();
-                runner_lock.input.raw.events.push(egui::Event::Zoom(factor));
+            runner.input.raw.events.push(egui::Event::Zoom(factor));
         } else {
             if event.shift_key() {
                 // Treat as horizontal scrolling.
@@ -505,60 +425,45 @@ pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Resul
                 delta = egui::vec2(delta.x + delta.y, 0.0);
             }
 
-                runner_lock
+            runner.input.raw.events.push(egui::Event::Scroll(delta));
-                    .input
-                    .raw
-                    .events
-                    .push(egui::Event::Scroll(delta));
         }
 
-            runner_lock.needs_repaint.repaint_asap();
+        runner.needs_repaint.repaint_asap();
         event.stop_propagation();
         event.prevent_default();
-        },
+    })?;
-    )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&canvas, "dragover", |event: web_sys::DragEvent, runner| {
-        &canvas,
-        "dragover",
-        |event: web_sys::DragEvent, mut runner_lock| {
         if let Some(data_transfer) = event.data_transfer() {
-                runner_lock.input.raw.hovered_files.clear();
+            runner.input.raw.hovered_files.clear();
             for i in 0..data_transfer.items().length() {
                 if let Some(item) = data_transfer.items().get(i) {
-                        runner_lock.input.raw.hovered_files.push(egui::HoveredFile {
+                    runner.input.raw.hovered_files.push(egui::HoveredFile {
                         mime: item.type_(),
                         ..Default::default()
                     });
                 }
             }
-                runner_lock.needs_repaint.repaint_asap();
+            runner.needs_repaint.repaint_asap();
             event.stop_propagation();
             event.prevent_default();
         }
-        },
+    })?;
-    )?;
 
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&canvas, "dragleave", |event: web_sys::DragEvent, runner| {
-        &canvas,
+        runner.input.raw.hovered_files.clear();
-        "dragleave",
+        runner.needs_repaint.repaint_asap();
-        |event: web_sys::DragEvent, mut runner_lock| {
-            runner_lock.input.raw.hovered_files.clear();
-            runner_lock.needs_repaint.repaint_asap();
         event.stop_propagation();
         event.prevent_default();
-        },
+    })?;
-    )?;
 
-    runner_container.add_event_listener(&canvas, "drop", {
+    runner_ref.add_event_listener(&canvas, "drop", {
-        let runner_ref = runner_container.runner.clone();
+        let runner_ref = runner_ref.clone();
 
-        move |event: web_sys::DragEvent, mut runner_lock| {
+        move |event: web_sys::DragEvent, runner| {
             if let Some(data_transfer) = event.data_transfer() {
-                runner_lock.input.raw.hovered_files.clear();
+                runner.input.raw.hovered_files.clear();
-                runner_lock.needs_repaint.repaint_asap();
+                runner.needs_repaint.repaint_asap();
-                // Unlock the runner so it can be locked after a future await point
-                drop(runner_lock);
 
                 if let Some(files) = data_transfer.files() {
                     for i in 0..files.length() {
@@ -578,8 +483,7 @@ pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Resul
                                         let bytes = js_sys::Uint8Array::new(&array_buffer).to_vec();
                                         log::debug!("Loaded {:?} ({} bytes).", name, bytes.len());
 
-                                        // Re-lock the mutex on the other side of the await point
+                                        if let Some(mut runner_lock) = runner_ref.try_lock() {
-                                        let mut runner_lock = runner_ref.lock();
                                             runner_lock.input.raw.dropped_files.push(
                                                 egui::DroppedFile {
                                                     name,
@@ -590,6 +494,7 @@ pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Resul
                                             );
                                             runner_lock.needs_repaint.repaint_asap();
                                         }
+                                    }
                                     Err(err) => {
                                         log::error!("Failed to read file: {:?}", err);
                                     }

crates/eframe/src/web/mod.rs

@@ -32,10 +32,7 @@ pub use events::*;
 pub use storage::*;
 
 use std::collections::BTreeMap;
-use std::sync::{
+use std::sync::Arc;
-    atomic::{AtomicBool, Ordering},
-    Arc,
-};
 
 use egui::Vec2;
 use wasm_bindgen::prelude::*;

crates/eframe/src/web/text_agent.rs

@@ -1,12 +1,12 @@
 //! The text agent is an `<input>` element used to trigger
 //! mobile keyboard and IME input.
+//!
+use std::{cell::Cell, rc::Rc};
 
-use super::{canvas_element, AppRunner, AppRunnerContainer};
-use egui::mutex::MutexGuard;
-use std::cell::Cell;
-use std::rc::Rc;
 use wasm_bindgen::prelude::*;
 
+use super::{canvas_element, AppRunner, AppRunnerRef};
+
 static AGENT_ID: &str = "egui_text_agent";
 
 pub fn text_agent() -> web_sys::HtmlInputElement {
@@ -21,7 +21,7 @@ pub fn text_agent() -> web_sys::HtmlInputElement {
 }
 
 /// Text event handler,
-pub fn install_text_agent(runner_container: &mut AppRunnerContainer) -> Result<(), JsValue> {
+pub fn install_text_agent(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
     let window = web_sys::window().unwrap();
     let document = window.document().unwrap();
     let body = document.body().expect("document should have a body");
@@ -44,60 +44,56 @@ pub fn install_text_agent(runner_container: &mut AppRunnerContainer) -> Result<(
     input.set_hidden(true);
 
     // When IME is off
-    runner_container.add_event_listener(&input, "input", {
+    runner_ref.add_event_listener(&input, "input", {
         let input_clone = input.clone();
         let is_composing = is_composing.clone();
 
-        move |_event: web_sys::InputEvent, mut runner_lock| {
+        move |_event: web_sys::InputEvent, runner| {
             let text = input_clone.value();
             if !text.is_empty() && !is_composing.get() {
                 input_clone.set_value("");
-                runner_lock.input.raw.events.push(egui::Event::Text(text));
+                runner.input.raw.events.push(egui::Event::Text(text));
-                runner_lock.needs_repaint.repaint_asap();
+                runner.needs_repaint.repaint_asap();
             }
         }
     })?;
 
     {
         // When IME is on, handle composition event
-        runner_container.add_event_listener(&input, "compositionstart", {
+        runner_ref.add_event_listener(&input, "compositionstart", {
             let input_clone = input.clone();
             let is_composing = is_composing.clone();
 
-            move |_event: web_sys::CompositionEvent, mut runner_lock: MutexGuard<'_, AppRunner>| {
+            move |_event: web_sys::CompositionEvent, runner: &mut AppRunner| {
                 is_composing.set(true);
                 input_clone.set_value("");
 
-                runner_lock
+                runner.input.raw.events.push(egui::Event::CompositionStart);
-                    .input
+                runner.needs_repaint.repaint_asap();
-                    .raw
-                    .events
-                    .push(egui::Event::CompositionStart);
-                runner_lock.needs_repaint.repaint_asap();
             }
         })?;
 
-        runner_container.add_event_listener(
+        runner_ref.add_event_listener(
             &input,
             "compositionupdate",
-            move |event: web_sys::CompositionEvent, mut runner_lock: MutexGuard<'_, AppRunner>| {
+            move |event: web_sys::CompositionEvent, runner: &mut AppRunner| {
                 if let Some(event) = event.data().map(egui::Event::CompositionUpdate) {
-                    runner_lock.input.raw.events.push(event);
+                    runner.input.raw.events.push(event);
-                    runner_lock.needs_repaint.repaint_asap();
+                    runner.needs_repaint.repaint_asap();
                 }
             },
         )?;
 
-        runner_container.add_event_listener(&input, "compositionend", {
+        runner_ref.add_event_listener(&input, "compositionend", {
             let input_clone = input.clone();
 
-            move |event: web_sys::CompositionEvent, mut runner_lock: MutexGuard<'_, AppRunner>| {
+            move |event: web_sys::CompositionEvent, runner: &mut AppRunner| {
                 is_composing.set(false);
                 input_clone.set_value("");
 
                 if let Some(event) = event.data().map(egui::Event::CompositionEnd) {
-                    runner_lock.input.raw.events.push(event);
+                    runner.input.raw.events.push(event);
-                    runner_lock.needs_repaint.repaint_asap();
+                    runner.needs_repaint.repaint_asap();
                 }
             }
         })?;
@@ -105,10 +101,7 @@ pub fn install_text_agent(runner_container: &mut AppRunnerContainer) -> Result<(
 
     // When input lost focus, focus on it again.
     // It is useful when user click somewhere outside canvas.
-    runner_container.add_event_listener(
+    runner_ref.add_event_listener(&input, "focusout", move |_event: web_sys::MouseEvent, _| {
-        &input,
-        "focusout",
-        move |_event: web_sys::MouseEvent, _| {
         // Delay 10 ms, and focus again.
         let func = js_sys::Function::new_no_args(&format!(
             "document.getElementById('{}').focus()",
@@ -117,8 +110,7 @@ pub fn install_text_agent(runner_container: &mut AppRunnerContainer) -> Result<(
         window
             .set_timeout_with_callback_and_timeout_and_arguments_0(&func, 10)
             .unwrap();
-        },
+    })?;
-    )?;
 
     body.append_child(&input)?;
 
@@ -126,7 +118,7 @@ pub fn install_text_agent(runner_container: &mut AppRunnerContainer) -> Result<(
 }
 
 /// Focus or blur text agent to toggle mobile keyboard.
-pub fn update_text_agent(runner: MutexGuard<'_, AppRunner>) -> Option<()> {
+pub fn update_text_agent(runner: &mut AppRunner) -> Option<()> {
     use web_sys::HtmlInputElement;
     let window = web_sys::window()?;
     let document = window.document()?;
@@ -166,9 +158,6 @@ pub fn update_text_agent(runner: MutexGuard<'_, AppRunner>) -> Option<()> {
             }
         }
     } else {
-        // Drop runner lock
-        drop(runner);
-
         // Holding the runner lock while calling input.blur() causes a panic.
         // This is most probably caused by the browser running the event handler
         // for the triggered blur event synchronously, meaning that the mutex
@@ -178,15 +167,33 @@ pub fn update_text_agent(runner: MutexGuard<'_, AppRunner>) -> Option<()> {
         // and this apparently is the fix for it
         //
         // ¯\_(ツ)_/¯ - @DusterTheFirst
-        input.blur().ok()?;
 
+        // So since we are inside a runner lock here, we just postpone the blur/hide:
+
+        call_after_delay(std::time::Duration::from_millis(0), move || {
+            input.blur().ok();
             input.set_hidden(true);
-        canvas_style.set_property("position", "absolute").ok()?;
+            canvas_style.set_property("position", "absolute").ok();
-        canvas_style.set_property("top", "0%").ok()?; // move back to normal position
+            canvas_style.set_property("top", "0%").ok(); // move back to normal position
+        });
     }
     Some(())
 }
 
+fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + '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
+}
+
 /// If context is running under mobile device?
 fn is_mobile() -> Option<bool> {
     const MOBILE_DEVICE: [&str; 6] = ["Android", "iPhone", "iPad", "iPod", "webOS", "BlackBerry"];

crates/egui_demo_app/src/backend_panel.rs

@@ -108,11 +108,10 @@ impl BackendPanel {
             ui.ctx().set_debug_on_hover(debug_on_hover);
         }
 
-        ui.separator();
-
         #[cfg(target_arch = "wasm32")]
         #[cfg(feature = "web_screen-reader")]
         {
+            ui.separator();
             let mut screen_reader = ui.ctx().options(|o| o.screen_reader);
             ui.checkbox(&mut screen_reader, "🔈 Screen reader").on_hover_text("Experimental feature: checking this will turn on the screen reader on supported platforms");
             ui.ctx().options_mut(|o| o.screen_reader = screen_reader);
@@ -125,6 +124,15 @@ impl BackendPanel {
                 frame.close();
             }
         }
+
+        if cfg!(debug_assertions) && cfg!(target_arch = "wasm32") {
+            ui.separator();
+            // For testing panic handling on web:
+            #[allow(clippy::manual_assert)]
+            if ui.button("panic!()").clicked() {
+                panic!("intentional panic!");
+            }
+        }
     }
 
     fn integration_ui(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame) {

crates/egui_demo_app/src/lib.rs

@@ -3,12 +3,9 @@
 
 mod apps;
 mod backend_panel;
-pub(crate) mod frame_history;
+mod frame_history;
 mod wrap_app;
 
-#[cfg(target_arch = "wasm32")]
-use eframe::web::AppRunnerRef;
-
 pub use wrap_app::WrapApp;
 
 /// Time of day as seconds since midnight. Used for clock in demo app.
@@ -21,60 +18,7 @@ pub(crate) fn seconds_since_midnight() -> f64 {
 // ----------------------------------------------------------------------------
 
 #[cfg(target_arch = "wasm32")]
-use eframe::wasm_bindgen::{self, prelude::*};
+mod web;
 
 #[cfg(target_arch = "wasm32")]
-#[wasm_bindgen]
+pub use web::*;
-pub struct WebHandle {
-    handle: AppRunnerRef,
-}
-
-#[cfg(target_arch = "wasm32")]
-#[wasm_bindgen]
-impl WebHandle {
-    #[wasm_bindgen]
-    pub fn stop_web(&self) -> Result<(), wasm_bindgen::JsValue> {
-        let mut app = self.handle.lock();
-        app.destroy()
-    }
-
-    #[wasm_bindgen]
-    pub fn set_some_content_from_javascript(&mut self, _some_data: &str) {
-        let _app = self.handle.lock().app_mut::<WrapApp>();
-        // _app.data = some_data;
-    }
-}
-
-#[cfg(target_arch = "wasm32")]
-#[wasm_bindgen]
-pub fn init_wasm_hooks() {
-    // Make sure panics are logged using `console.error`.
-    console_error_panic_hook::set_once();
-
-    // Redirect tracing to console.log and friends:
-    eframe::web::WebLogger::init(log::LevelFilter::Debug).ok();
-}
-
-#[cfg(target_arch = "wasm32")]
-#[wasm_bindgen]
-pub async fn start_separate(canvas_id: &str) -> Result<WebHandle, wasm_bindgen::JsValue> {
-    let web_options = eframe::WebOptions::default();
-    eframe::start_web(
-        canvas_id,
-        web_options,
-        Box::new(|cc| Box::new(WrapApp::new(cc))),
-    )
-    .await
-    .map(|handle| WebHandle { handle })
-}
-
-/// 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.
-#[cfg(target_arch = "wasm32")]
-#[wasm_bindgen]
-pub async fn start(canvas_id: &str) -> Result<WebHandle, wasm_bindgen::JsValue> {
-    init_wasm_hooks();
-    start_separate(canvas_id).await
-}

crates/egui_demo_app/src/web.rs

@@ -0,0 +1,57 @@
+use eframe::{
+    wasm_bindgen::{self, prelude::*},
+    web::AppRunnerRef,
+};
+
+use crate::WrapApp;
+
+#[wasm_bindgen]
+pub struct WebHandle {
+    runner: AppRunnerRef,
+}
+
+#[wasm_bindgen]
+impl WebHandle {
+    /// 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.
+    #[wasm_bindgen(constructor)]
+    pub async fn new(canvas_id: &str) -> Result<WebHandle, wasm_bindgen::JsValue> {
+        // Redirect tracing to console.log and friends:
+        eframe::web::WebLogger::init(log::LevelFilter::Debug).ok();
+
+        // Make sure panics are logged using `console.error`.
+        console_error_panic_hook::set_once();
+
+        let web_options = eframe::WebOptions::default();
+        let runner = eframe::start_web(
+            canvas_id,
+            web_options,
+            Box::new(|cc| Box::new(WrapApp::new(cc))),
+        )
+        .await?;
+
+        Ok(WebHandle { runner })
+    }
+
+    #[wasm_bindgen]
+    pub fn destroy(&self) {
+        self.runner.destroy();
+    }
+
+    #[wasm_bindgen]
+    pub fn has_panicked(&self) -> bool {
+        self.runner.panic_summary().is_some()
+    }
+
+    #[wasm_bindgen]
+    pub fn panic_message(&self) -> Option<String> {
+        self.runner.panic_summary().map(|s| s.message())
+    }
+
+    #[wasm_bindgen]
+    pub fn panic_callstack(&self) -> Option<String> {
+        self.runner.panic_summary().map(|s| s.callstack())
+    }
+}

docs/index.html

@@ -91,6 +91,7 @@
                 transform: rotate(360deg);
             }
         }
+
     </style>
 </head>
 
@@ -124,27 +125,58 @@
         // We'll defer our execution until the wasm is ready to go.
         // Here we tell bindgen the path to the wasm file so it can start
         // initialization and return to us a promise when it's done.
-        console.debug("loading wasm…");
+        console.debug("Loading wasm…");
         wasm_bindgen("./egui_demo_app_bg.wasm")
             .then(on_wasm_loaded)
-            .catch(on_wasm_error);
+            .catch(on_error);
 
         function on_wasm_loaded() {
-            console.debug("wasm loaded. starting app…");
+            console.debug("Wasm loaded. Starting app…");
 
             // This call installs a bunch of callbacks and then returns:
-            const handle = wasm_bindgen.start("the_canvas_id");
+            let handle = new wasm_bindgen.WebHandle("the_canvas_id");
-
+            handle.then(on_app_started).catch(on_error);
-            // call `handle.stop_web()` to stop
-            // uncomment to quick result
-            // setTimeout(() => {handle.stop_web(); handle.free())}, 2000)
-
-            console.debug("app started.");
-            document.getElementById("center_text").remove();
         }
 
-        function on_wasm_error(error) {
+        function on_app_started(handle) {
+            // Call `handle.destroy()` to stop. Uncomment to quick result:
+            // setTimeout(() => { handle.destroy(); handle.free()) }, 2000)
+
+            console.debug("App started.");
+            document.getElementById("center_text").innerHTML = '';
+
+            function check_for_panic() {
+                if (handle.has_panicked()) {
+                    console.error("The egui app has crashed");
+
+                    // The demo app already logs the panic message and callstack, but you
+                    // can access them like this if you want to show them in the html:
+                    // console.error(`${handle.panic_message()}`);
+                    // console.error(`${handle.panic_callstack()}`);
+
+                    document.getElementById("the_canvas_id").remove();
+                    document.getElementById("center_text").innerHTML = `
+                        <p>
+                            The egui app has crashed.
+                        </p>
+                        <p style="font-size:14px">
+                            See the console for details.
+                        </p>
+                        <p style="font-size:14px">
+                            Reload the page to try again.
+                        </p>`;
+                } else {
+                    let delay_ms = 1000;
+                    setTimeout(check_for_panic, delay_ms);
+                }
+            }
+
+            check_for_panic();
+        }
+
+        function on_error(error) {
             console.error("Failed to start: " + error);
+            document.getElementById("the_canvas_id").remove();
             document.getElementById("center_text").innerHTML = `
                 <p>
                     An error occurred during loading:

docs/multiple_apps.html

@@ -1,3 +1,4 @@
+<!-- TODO(emilk): make this example nicer. The layout is HORRIBLE -->
 <!DOCTYPE html>
 <html>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
@@ -52,7 +53,7 @@
             top: 0%;
             left: 50%;
             transform: translate(-50%, 0%); */
-            width:90%;
+            width: 45%;
             height: 90%;
         }
 
@@ -99,6 +100,7 @@
                 transform: rotate(360deg);
             }
         }
+
     </style>
 </head>
 
@@ -112,15 +114,13 @@
     </button>
 
     <div class="canvas_wrap one">
-        <canvas id="the_canvas_id_one"></canvas>
+        <canvas id="canvas_id_one"></canvas>
     </div>
 
     <div class="canvas_wrap two">
-        <canvas id="the_canvas_id_two"></canvas>
+        <canvas id="canvas_id_two"></canvas>
     </div>
 
-
-
     <div class="centered" id="center_text">
         <p style="font-size:16px">
             Loading…
@@ -148,38 +148,41 @@
         // We'll defer our execution until the wasm is ready to go.
         // Here we tell bindgen the path to the wasm file so it can start
         // initialization and return to us a promise when it's done.
-        console.debug("loading wasm…");
+        console.debug("Loading wasm…");
         wasm_bindgen("./egui_demo_app_bg.wasm")
             .then(on_wasm_loaded)
-            .catch(on_wasm_error);
+            .catch(on_error);
 
         function on_wasm_loaded() {
-            console.debug("wasm loaded. starting app…");
+            console.debug("Wasm loaded. Starting apps…");
 
             // This call installs a bunch of callbacks and then returns:
 
-            wasm_bindgen.init_wasm_hooks()
+            const handle_one = new wasm_bindgen.WebHandle("canvas_id_one");
+            const handle_two = new wasm_bindgen.WebHandle("canvas_id_two");
 
-            const handle_one = wasm_bindgen.start_separate("the_canvas_id_one");
+            Promise.all([handle_one, handle_two]).then((handles) => {
-            const handle_two = wasm_bindgen.start_separate("the_canvas_id_two");
+                on_apps_started(handles[0], handles[1])
+            }).catch(on_error);
+        }
 
+        function on_apps_started(handle_one, handle_two) {
             const button = document.getElementsByClassName("stop_one")[0]
 
             button.addEventListener("click", () => {
-                handle_one.stop_web()
+                document.getElementById("canvas_id_one").remove()
+                handle_one.destroy()
                 handle_one.free()
             });
 
+            // Call `handle.destroy()` to stop. Uncomment to quick result:
+            // setTimeout(() => { handle.destroy() }, 2000)
 
-            // call `handle.stop_web()` to stop
+            console.debug("Apps started.");
-            // uncomment to quick result
-            // setTimeout(() => {handle.stop_web()}, 2000)
-
-            console.debug("app started.");
             document.getElementById("center_text").remove();
         }
 
-        function on_wasm_error(error) {
+        function on_error(error) {
             console.error("Failed to start: " + error);
             document.getElementById("center_text").innerHTML = `
                 <p>