eframe web: Don't throw away frames on click/copy/cut

crates/eframe/src/web/app_runner.rs

@@ -1,5 +1,4 @@
 use egui::TexturesDelta;
-use wasm_bindgen::JsValue;
 
 use crate::{epi, App};
 
@@ -17,7 +16,10 @@ pub struct AppRunner {
     screen_reader: super::screen_reader::ScreenReader,
     pub(crate) text_cursor_pos: Option<egui::Pos2>,
     pub(crate) mutable_text_under_cursor: bool,
+
+    // Output for the last run:
     textures_delta: TexturesDelta,
+    clipped_primitives: Option<Vec<egui::ClippedPrimitive>>,
 }
 
 impl Drop for AppRunner {
@@ -115,6 +117,7 @@ impl AppRunner {
             text_cursor_pos: None,
             mutable_text_under_cursor: false,
             textures_delta: Default::default(),
+            clipped_primitives: None,
         };
 
         runner.input.raw.max_texture_side = Some(runner.painter.max_texture_side());
@@ -170,8 +173,26 @@ impl AppRunner {
         self.painter.destroy();
     }
 
-    /// Call [`Self::paint`] later to paint
-    pub fn logic(&mut self) -> Vec<egui::ClippedPrimitive> {
+    /// Runs the user code and paints the UI.
+    ///
+    /// If there is already an outstanding frame of output,
+    /// that is painted instead.
+    pub fn run_and_paint(&mut self) {
+        if self.clipped_primitives.is_none() {
+            // Run user code, and paint the results:
+            self.logic();
+            self.paint();
+        } else {
+            // We have already run the logic, e.g. in an on-click event,
+            // so let's only present the results:
+            self.paint();
+        }
+    }
+
+    /// Runs the logic, but doesn't paint the result.
+    ///
+    /// The result can be painted later with a call to [`Self::run_and_paint`] or [`Self::paint`].
+    pub fn logic(&mut self) {
         let frame_start = now_sec();
 
         super::resize_canvas_to_screen_size(self.canvas_id(), self.web_options.max_size_points);
@@ -203,25 +224,26 @@ impl AppRunner {
 
         self.handle_platform_output(platform_output);
         self.textures_delta.append(textures_delta);
-        let clipped_primitives = self.egui_ctx.tessellate(shapes, pixels_per_point);
+        self.clipped_primitives = Some(self.egui_ctx.tessellate(shapes, pixels_per_point));
 
         self.frame.info.cpu_usage = Some((now_sec() - frame_start) as f32);
-
-        clipped_primitives
     }
 
     /// Paint the results of the last call to [`Self::logic`].
-    pub fn paint(&mut self, clipped_primitives: &[egui::ClippedPrimitive]) -> Result<(), JsValue> {
+    pub fn paint(&mut self) {
         let textures_delta = std::mem::take(&mut self.textures_delta);
-
-        self.painter.paint_and_update_textures(
-            self.app.clear_color(&self.egui_ctx.style().visuals),
-            clipped_primitives,
-            self.egui_ctx.pixels_per_point(),
-            &textures_delta,
-        )?;
-
-        Ok(())
+        let clipped_primitives = std::mem::take(&mut self.clipped_primitives);
+
+        if let Some(clipped_primitives) = clipped_primitives {
+            if let Err(err) = self.painter.paint_and_update_textures(
+                self.app.clear_color(&self.egui_ctx.style().visuals),
+                &clipped_primitives,
+                self.egui_ctx.pixels_per_point(),
+                &textures_delta,
+            ) {
+                log::error!("Failed to paint: {}", super::string_from_js_value(&err));
+            }
+        }
     }
 
     fn handle_platform_output(&mut self, platform_output: egui::PlatformOutput) {

crates/eframe/src/web/backend.rs

@@ -73,6 +73,10 @@ impl NeedRepaint {
         *repaint_time = repaint_time.min(super::now_sec() + num_seconds);
     }
 
+    pub fn needs_repaint(&self) -> bool {
+        self.when_to_repaint() <= super::now_sec()
+    }
+
     pub fn repaint_asap(&self) {
         *self.0.lock() = f64::NEG_INFINITY;
     }

crates/eframe/src/web/events.rs

@@ -8,22 +8,19 @@ use super::*;
 fn paint_and_schedule(runner_ref: &WebRunner) -> Result<(), JsValue> {
     // Only paint and schedule if there has been no panic
     if let Some(mut runner_lock) = runner_ref.try_lock() {
-        paint_if_needed(&mut runner_lock)?;
+        paint_if_needed(&mut runner_lock);
         drop(runner_lock);
         request_animation_frame(runner_ref.clone())?;
     }
-
     Ok(())
 }
 
-fn paint_if_needed(runner: &mut AppRunner) -> Result<(), JsValue> {
-    if runner.needs_repaint.when_to_repaint() <= now_sec() {
+fn paint_if_needed(runner: &mut AppRunner) {
+    if runner.needs_repaint.needs_repaint() {
         runner.needs_repaint.clear();
-        let clipped_primitives = runner.logic();
-        runner.paint(&clipped_primitives)?;
-        runner.auto_save_if_needed();
+        runner.run_and_paint();
     }
-    Ok(())
+    runner.auto_save_if_needed();
 }
 
 pub(crate) fn request_animation_frame(runner_ref: WebRunner) -> Result<(), JsValue> {
@@ -177,10 +174,14 @@ pub(crate) fn install_document_events(runner_ref: &WebRunner) -> Result<(), JsVa
         "cut",
         |event: web_sys::ClipboardEvent, runner| {
             runner.input.raw.events.push(egui::Event::Cut);
+
             // In Safari we are only allowed to write to the clipboard during the
             // event callback, which is why we run the app logic here and now:
-            runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+            runner.logic();
+
+            // Make sure we paint the output of the above logic call asap:
             runner.needs_repaint.repaint_asap();
+
             event.stop_propagation();
             event.prevent_default();
         },
@@ -192,10 +193,14 @@ pub(crate) fn install_document_events(runner_ref: &WebRunner) -> Result<(), JsVa
         "copy",
         |event: web_sys::ClipboardEvent, runner| {
             runner.input.raw.events.push(egui::Event::Copy);
+
             // In Safari we are only allowed to write to the clipboard during the
             // event callback, which is why we run the app logic here and now:
-            runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+            runner.logic();
+
+            // Make sure we paint the output of the above logic call asap:
             runner.needs_repaint.repaint_asap();
+
             event.stop_propagation();
             event.prevent_default();
         },
@@ -281,9 +286,12 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
                     pressed: true,
                     modifiers,
                 });
+
                 // In Safari we are only allowed to write to the clipboard during the
                 // event callback, which is why we run the app logic here and now:
-                runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+                runner.logic();
+
+                // Make sure we paint the output of the above logic call asap:
                 runner.needs_repaint.repaint_asap();
             }
             event.stop_propagation();
@@ -313,9 +321,12 @@ pub(crate) fn install_canvas_events(runner_ref: &WebRunner) -> Result<(), JsValu
                 pressed: false,
                 modifiers,
             });
+
             // In Safari we are only allowed to write to the clipboard during the
             // event callback, which is why we run the app logic here and now:
-            runner.logic(); // we ignore the returned triangles, but schedule a repaint right after
+            runner.logic();
+
+            // Make sure we paint the output of the above logic call asap:
             runner.needs_repaint.repaint_asap();
 
             text_agent::update_text_agent(runner);

crates/eframe/src/web/mod.rs

@@ -49,6 +49,10 @@ use crate::Theme;
 
 // ----------------------------------------------------------------------------
 
+pub(crate) fn string_from_js_value(value: &JsValue) -> String {
+    value.as_string().unwrap_or_else(|| format!("{value:#?}"))
+}
+
 /// Current time in seconds (since undefined point in time).
 ///
 /// Monotonically increasing.
@@ -196,7 +200,7 @@ fn set_clipboard_text(s: &str) {
             let future = wasm_bindgen_futures::JsFuture::from(promise);
             let future = async move {
                 if let Err(err) = future.await {
-                    log::error!("Copy/cut action failed: {err:?}");
+                    log::error!("Copy/cut action failed: {}", string_from_js_value(&err));
                 }
             };
             wasm_bindgen_futures::spawn_local(future);

crates/eframe/src/web/web_runner.rs

@@ -95,7 +95,10 @@ impl WebRunner {
             log::debug!("Unsubscribing from {} events", events_to_unsubscribe.len());
             for x in events_to_unsubscribe {
                 if let Err(err) = x.unsubscribe() {
-                    log::warn!("Failed to unsubscribe from event: {err:?}");
+                    log::warn!(
+                        "Failed to unsubscribe from event: {}",
+                        super::string_from_js_value(&err)
+                    );
                 }
             }
         }