Support returning errors when creating the app (#4565)

The closure passed to `eframe::run_native` now returns a `Result`,
allowing you to return an error during app creation, which will be
returned to the caller of `run_native`.

This means you need to wrap your `Box::new(MyApp::new(…))` in an
`Ok(…)`.

* Closes https://github.com/emilk/egui/issues/4474

crates/eframe/src/epi.rs

@@ -41,10 +41,12 @@ pub type EventLoopBuilderHook = Box<dyn FnOnce(&mut EventLoopBuilder<UserEvent>)
 #[cfg(any(feature = "glow", feature = "wgpu"))]
 pub type WindowBuilderHook = Box<dyn FnOnce(egui::ViewportBuilder) -> egui::ViewportBuilder>;
 
+type DynError = Box<dyn std::error::Error>;
+
 /// This is how your app is created.
 ///
 /// You can use the [`CreationContext`] to setup egui, restore state, setup OpenGL things, etc.
-pub type AppCreator = Box<dyn FnOnce(&CreationContext<'_>) -> Box<dyn App>>;
+pub type AppCreator = Box<dyn FnOnce(&CreationContext<'_>) -> Result<Box<dyn App>, DynError>>;
 
 /// Data that is passed to [`AppCreator`] that can be used to setup and initialize your app.
 pub struct CreationContext<'s> {

crates/eframe/src/lib.rs

@@ -30,7 +30,7 @@
 //!
 //! fn main() {
 //!     let native_options = eframe::NativeOptions::default();
-//!     eframe::run_native("My egui App", native_options, Box::new(|cc| Box::new(MyEguiApp::new(cc))));
+//!     eframe::run_native("My egui App", native_options, Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc)))));
 //! }
 //!
 //! #[derive(Default)]
@@ -90,7 +90,7 @@
 //!             .start(
 //!                 canvas_id,
 //!                 eframe::WebOptions::default(),
-//!                 Box::new(|cc| Box::new(MyEguiApp::new(cc))),
+//!                 Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc))),)
 //!             )
 //!             .await
 //!     }
@@ -199,7 +199,7 @@ pub mod icon_data;
 ///
 /// fn main() -> eframe::Result<()> {
 ///     let native_options = eframe::NativeOptions::default();
-///     eframe::run_native("MyApp", native_options, Box::new(|cc| Box::new(MyEguiApp::new(cc))))
+///     eframe::run_native("MyApp", native_options, Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc)))))
 /// }
 ///
 /// #[derive(Default)]
@@ -324,7 +324,7 @@ pub fn run_simple_native(
     run_native(
         app_name,
         native_options,
-        Box::new(|_cc| Box::new(SimpleApp { update_fun })),
+        Box::new(|_cc| Ok(Box::new(SimpleApp { update_fun }))),
     )
 }
 
@@ -333,6 +333,9 @@ pub fn run_simple_native(
 /// The different problems that can occur when trying to run `eframe`.
 #[derive(Debug)]
 pub enum Error {
+    /// Something went wrong in user code when creating the app.
+    AppCreation(Box<dyn std::error::Error>),
+
     /// An error from [`winit`].
     #[cfg(not(target_arch = "wasm32"))]
     Winit(winit::error::OsError),
@@ -403,6 +406,8 @@ impl From<egui_wgpu::WgpuError> for Error {
 impl std::fmt::Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
+            Self::AppCreation(err) => write!(f, "app creation error: {err}"),
+
             #[cfg(not(target_arch = "wasm32"))]
             Self::Winit(err) => {
                 write!(f, "winit error: {err}")

crates/eframe/src/native/glow_integration.rs

@@ -313,7 +313,7 @@ impl GlowWinitApp {
                 raw_window_handle: window.window_handle().map(|h| h.as_raw()),
             };
             crate::profile_scope!("app_creator");
-            app_creator(&cc)
+            app_creator(&cc).map_err(crate::Error::AppCreation)?
         };
 
         let glutin = Rc::new(RefCell::new(glutin));

crates/eframe/src/native/wgpu_integration.rs

@@ -182,7 +182,7 @@ impl WgpuWinitApp {
         storage: Option<Box<dyn Storage>>,
         window: Window,
         builder: ViewportBuilder,
-    ) -> Result<&mut WgpuWinitRunning, egui_wgpu::WgpuError> {
+    ) -> crate::Result<&mut WgpuWinitRunning> {
         crate::profile_function!();
 
         #[allow(unsafe_code, unused_mut, unused_unsafe)]
@@ -272,7 +272,7 @@ impl WgpuWinitApp {
         };
         let app = {
             crate::profile_scope!("user_app_creator");
-            app_creator(&cc)
+            app_creator(&cc).map_err(crate::Error::AppCreation)?
         };
 
         let mut viewport_from_window = HashMap::default();

crates/eframe/src/web/app_runner.rs

@@ -30,7 +30,7 @@ impl Drop for AppRunner {
 
 impl AppRunner {
     /// # Errors
-    /// Failure to initialize WebGL renderer.
+    /// Failure to initialize WebGL renderer, or failure to create app.
     pub async fn new(
         canvas_id: &str,
         web_options: crate::WebOptions,
@@ -71,7 +71,7 @@ impl AppRunner {
         let theme = system_theme.unwrap_or(web_options.default_theme);
         egui_ctx.set_visuals(theme.egui_visuals());
 
-        let app = app_creator(&epi::CreationContext {
+        let cc = epi::CreationContext {
             egui_ctx: egui_ctx.clone(),
             integration_info: info.clone(),
             storage: Some(&storage),
@@ -86,7 +86,8 @@ impl AppRunner {
             wgpu_render_state: painter.render_state(),
             #[cfg(all(feature = "wgpu", feature = "glow"))]
             wgpu_render_state: None,
-        });
+        };
+        let app = app_creator(&cc).map_err(|err| err.to_string())?;
 
         let frame = epi::Frame {
             info,

crates/eframe/src/web/web_runner.rs

@@ -54,7 +54,7 @@ impl WebRunner {
     /// Create the application, install callbacks, and start running the app.
     ///
     /// # Errors
-    /// Failing to initialize graphics.
+    /// Failing to initialize graphics, or failure to create app.
     pub async fn start(
         &self,
         canvas_id: &str,