Re-work event loop run() API so it can return a Result

This re-works the portable `run()` API that consumes the `EventLoop` and
runs the loop on the calling thread until the app exits.

This can be supported across _all_ platforms and compared to the
previous `run() -> !` API is now able to return a `Result` status on all
platforms except iOS and Web. Fixes: #2709

By moving away from `run() -> !` we stop calling `std::process::exit()`
internally as a means to kill the process without returning which means
it's possible to return an exit status and applications can return from
their `main()` function normally.

This also fixes Android support where an Activity runs in a thread but
we can't assume to have full ownership of the process (other services
could be running in separate threads).

Additionally all examples have generally been updated so that `main()`
returns a `Result` from `run()`

Fixes: #2709

examples/child_window.rs

@@ -3,7 +3,7 @@
 mod fill;
 
 #[cfg(any(x11_platform, macos_platform, windows_platform))]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     use std::collections::HashMap;
 
     use raw_window_handle::HasRawWindowHandle;

examples/control_flow.rs

@@ -27,7 +27,7 @@ enum Mode {
 const WAIT_TIME: time::Duration = time::Duration::from_millis(100);
 const POLL_SLEEP_TIME: time::Duration = time::Duration::from_millis(100);
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
 
     println!("Press '1' to switch to Wait mode.");
@@ -122,5 +122,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/cursor.rs

@@ -10,7 +10,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -54,7 +54,7 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }
 
 const CURSORS: &[CursorIcon] = &[

examples/cursor_grab.rs

@@ -11,7 +11,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -73,5 +73,5 @@ fn main() {
             Event::RedrawRequested(_) => fill::fill_window(&window),
             _ => (),
         }
-    });
+    })
 }

examples/custom_events.rs

@@ -1,7 +1,7 @@
 #![allow(clippy::single_match)]
 
 #[cfg(not(wasm_platform))]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     use simple_logger::SimpleLogger;
     use winit::{
         event::{Event, WindowEvent},
@@ -52,7 +52,7 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }
 
 #[cfg(wasm_platform)]

examples/drag_window.rs

@@ -11,7 +11,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -69,7 +69,7 @@ fn main() {
             }
         }
         _ => (),
-    });
+    })
 }
 
 fn name_windows(window_id: WindowId, switched: bool, window_1: &Window, window_2: &Window) {

examples/fullscreen.rs

@@ -12,7 +12,7 @@ use winit::platform::macos::WindowExtMacOS;
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -131,5 +131,5 @@ fn main() {
             }
             _ => {}
         }
-    });
+    })
 }

examples/handling_close.rs

@@ -11,7 +11,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -87,5 +87,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/ime.rs

@@ -13,7 +13,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new()
         .with_level(LevelFilter::Trace)
         .init()
@@ -105,5 +105,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/key_binding.rs

@@ -17,7 +17,7 @@ fn main() {
 }
 
 #[cfg(any(target_os = "macos", target_os = "windows", target_os = "linux"))]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     #[path = "util/fill.rs"]
     mod fill;
 
@@ -61,5 +61,5 @@ fn main() {
             }
             _ => (),
         };
-    });
+    })
 }

examples/mouse_wheel.rs

@@ -10,7 +10,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -64,5 +64,5 @@ In other words, the deltas indicate the direction in which to move the content (
             }
             _ => (),
         }
-    });
+    })
 }

examples/multithreaded.rs

@@ -1,7 +1,7 @@
 #![allow(clippy::single_match)]
 
 #[cfg(not(wasm_platform))]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     use std::{collections::HashMap, sync::mpsc, thread, time::Duration};
 
     use simple_logger::SimpleLogger;

examples/multiwindow.rs

@@ -13,7 +13,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 

examples/request_redraw.rs

@@ -10,7 +10,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -41,5 +41,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/request_redraw_threaded.rs

@@ -1,7 +1,7 @@
 #![allow(clippy::single_match)]
 
 #[cfg(not(wasm_platform))]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     use std::{sync::Arc, thread, time};
 
     use simple_logger::SimpleLogger;
@@ -49,7 +49,7 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }
 
 #[cfg(wasm_platform)]

examples/resizable.rs

@@ -12,7 +12,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -53,5 +53,5 @@ fn main() {
             }
             _ => (),
         };
-    });
+    })
 }

examples/startup_notification.rs

@@ -1,9 +1,5 @@
 //! Demonstrates the use of startup notifications on Linux.
 
-fn main() {
-    example::main();
-}
-
 #[cfg(any(x11_platform, wayland_platform))]
 #[path = "./util/fill.rs"]
 mod fill;
@@ -21,7 +17,7 @@ mod example {
     };
     use winit::window::{Window, WindowBuilder, WindowId};
 
-    pub(super) fn main() {
+    pub(super) fn main() -> Result<(), impl std::error::Error> {
         // Create the event loop and get the activation token.
         let event_loop = EventLoop::new();
         let mut current_token = match event_loop.read_token_from_env() {
@@ -115,13 +111,16 @@ mod example {
             }
 
             flow.set_wait();
-        });
+        })
     }
 }
 
-#[cfg(not(any(x11_platform, wayland_platform)))]
-mod example {
-    pub(super) fn main() {
-        println!("This example is only supported on X11 and Wayland platforms.");
-    }
+#[cfg(any(x11_platform, wayland_platform))]
+fn main() -> Result<(), impl std::error::Error> {
+    example::main()
+}
+
+#[cfg(not(any(x11_platform, wayland_platform)))]
+fn main() {
+    println!("This example is only supported on X11 and Wayland platforms.");
 }

examples/theme.rs

@@ -11,7 +11,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -75,5 +75,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/timer.rs

@@ -16,7 +16,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -47,5 +47,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/touchpad_gestures.rs

@@ -8,7 +8,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -47,5 +47,5 @@ fn main() {
         } else if let Event::RedrawRequested(_) = event {
             fill::fill_window(&window);
         }
-    });
+    })
 }

examples/transparent.rs

@@ -10,7 +10,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -36,5 +36,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/web.rs

@@ -7,7 +7,7 @@ use winit::{
     window::{Fullscreen, WindowBuilder},
 };
 
-pub fn main() {
+pub fn main() -> Result<(), impl std::error::Error> {
     let event_loop = EventLoop::new();
 
     let builder = WindowBuilder::new().with_title("A fantastic window!");
@@ -56,7 +56,7 @@ pub fn main() {
             }
             _ => (),
         }
-    });
+    })
 }
 
 #[cfg(wasm_platform)]
@@ -72,7 +72,7 @@ mod wasm {
         console_log::init_with_level(log::Level::Debug).expect("error initializing logger");
 
         #[allow(clippy::main_recursion)]
-        super::main();
+        let _ = super::main();
     }
 
     pub fn insert_canvas_and_create_log_list(window: &Window) -> web_sys::Element {

examples/web_aspect_ratio.rs

@@ -47,7 +47,7 @@ This example demonstrates the desired future functionality which will possibly b
         // Render once with the size info we currently have
         render_circle(&canvas, window.inner_size());
 
-        event_loop.run(move |event, _, control_flow| {
+        let _ = event_loop.run(move |event, _, control_flow| {
             *control_flow = ControlFlow::Wait;
 
             match event {

examples/window.rs

@@ -10,7 +10,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -37,5 +37,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/window_buttons.rs

@@ -14,7 +14,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -71,5 +71,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/window_debug.rs

@@ -14,7 +14,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -139,5 +139,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/window_drag_resize.rs

@@ -13,7 +13,7 @@ const BORDER: f64 = 8.0;
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -74,7 +74,7 @@ fn main() {
             fill::fill_window(&window);
         }
         _ => (),
-    });
+    })
 }
 
 fn cursor_direction_icon(resize_direction: Option<ResizeDirection>) -> CursorIcon {

examples/window_icon.rs

@@ -12,7 +12,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
 
     // You'll have to choose an icon size at your own discretion. On X11, the desired size varies
@@ -48,7 +48,7 @@ fn main() {
         } else if let Event::RedrawRequested(_) = event {
             fill::fill_window(&window);
         }
-    });
+    })
 }
 
 fn load_icon(path: &Path) -> Icon {

examples/window_option_as_alt.rs

@@ -18,7 +18,7 @@ mod fill;
 /// Prints the keyboard events characters received when option_is_alt is true versus false.
 /// A left mouse click will toggle option_is_alt.
 #[cfg(target_os = "macos")]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     let event_loop = EventLoop::new();
 
     let window = WindowBuilder::new()
@@ -66,7 +66,7 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }
 
 #[cfg(not(target_os = "macos"))]

examples/window_resize_increments.rs

@@ -11,7 +11,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -60,5 +60,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }

examples/window_tabbing.rs

@@ -19,7 +19,7 @@ use winit::{
 mod fill;
 
 #[cfg(target_os = "macos")]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 

src/event_loop.rs

@@ -18,6 +18,7 @@ use std::time::{Duration, Instant};
 #[cfg(wasm_platform)]
 use web_time::{Duration, Instant};
 
+use crate::error::RunLoopError;
 use crate::{event::Event, monitor::MonitorHandle, platform_impl};
 
 /// Provides a way to retrieve events from the system and from the windows that were registered to
@@ -285,23 +286,33 @@ impl<T> EventLoop<T> {
         EventLoopBuilder::<T>::with_user_event().build()
     }
 
-    /// Hijacks the calling thread and initializes the winit event loop with the provided
-    /// closure. Since the closure is `'static`, it must be a `move` closure if it needs to
+    /// Runs the event loop in the calling thread and calls the given `event_handler` closure
+    /// to dispatch any pending events.
+    ///
+    /// Since the closure is `'static`, it must be a `move` closure if it needs to
     /// access any data from the calling context.
     ///
     /// See the [`ControlFlow`] docs for information on how changes to `&mut ControlFlow` impact the
     /// event loop's behavior.
     ///
-    /// Any values not passed to this function will *not* be dropped.
-    ///
     /// ## Platform-specific
     ///
     /// - **X11 / Wayland:** The program terminates with exit code 1 if the display server
     ///   disconnects.
+    /// - **iOS:** Will never return to the caller and so values not passed to this function will
+    ///   *not* be dropped before the process exits.
+    /// - **Web:** Will _act_ as if it never returns to the caller by throwing a Javascript exception
+    ///   (that Rust doesn't see) that will also mean that the rest of the function is never executed
+    ///   and any values not passed to this function will *not* be dropped.
+    ///
+    ///   Web applications are recommended to use `spawn()` instead of `run()` to avoid the need
+    ///   for the Javascript exception trick, and to make it clearer that the event loop runs
+    ///   asynchronously (via the browser's own, internal, event loop) and doesn't block the
+    ///   current thread of execution like it does on other platforms.
     ///
     /// [`ControlFlow`]: crate::event_loop::ControlFlow
     #[inline]
-    pub fn run<F>(self, event_handler: F) -> !
+    pub fn run<F>(self, event_handler: F) -> Result<(), RunLoopError>
     where
         F: 'static + FnMut(Event<'_, T>, &EventLoopWindowTarget<T>, &mut ControlFlow),
     {

src/platform_impl/android/mod.rs

@@ -526,17 +526,12 @@ impl<T: 'static> EventLoop<T> {
         self.pending_redraw = pending_redraw;
     }
 
-    pub fn run<F>(mut self, event_handler: F) -> !
+    pub fn run<F>(mut self, event_handler: F) -> Result<(), RunLoopError>
     where
         F: 'static
             + FnMut(event::Event<'_, T>, &event_loop::EventLoopWindowTarget<T>, &mut ControlFlow),
     {
-        let exit_code = match self.run_ondemand(event_handler) {
-            Err(RunLoopError::ExitFailure(code)) => code,
-            Err(_err) => 1,
-            Ok(_) => 0,
-        };
-        ::std::process::exit(exit_code);
+        self.run_ondemand(event_handler)
     }
 
     pub fn run_ondemand<F>(&mut self, mut event_handler: F) -> Result<(), RunLoopError>

src/platform_impl/linux/mod.rs

@@ -830,11 +830,11 @@ impl<T: 'static> EventLoop<T> {
         x11_or_wayland!(match self; EventLoop(evlp) => evlp.create_proxy(); as EventLoopProxy)
     }
 
-    pub fn run<F>(self, callback: F) -> !
+    pub fn run<F>(mut self, callback: F) -> Result<(), RunLoopError>
     where
-        F: 'static + FnMut(crate::event::Event<'_, T>, &RootELW<T>, &mut ControlFlow),
+        F: FnMut(crate::event::Event<'_, T>, &RootELW<T>, &mut ControlFlow),
     {
-        x11_or_wayland!(match self; EventLoop(evlp) => evlp.run(callback))
+        self.run_ondemand(callback)
     }
 
     pub fn run_ondemand<F>(&mut self, callback: F) -> Result<(), RunLoopError>

src/platform_impl/linux/wayland/event_loop/mod.rs

@@ -145,18 +145,6 @@ impl<T: 'static> EventLoop<T> {
         Ok(event_loop)
     }
 
-    pub fn run<F>(mut self, callback: F) -> !
-    where
-        F: FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow) + 'static,
-    {
-        let exit_code = match self.run_ondemand(callback) {
-            Err(RunLoopError::ExitFailure(code)) => code,
-            Err(_err) => 1,
-            Ok(_) => 0,
-        };
-        ::std::process::exit(exit_code)
-    }
-
     pub fn run_ondemand<F>(&mut self, mut event_handler: F) -> Result<(), RunLoopError>
     where
         F: FnMut(Event<'_, T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),

src/platform_impl/linux/x11/mod.rs

@@ -432,18 +432,6 @@ impl<T: 'static> EventLoop<T> {
         &self.target
     }
 
-    pub fn run<F>(mut self, callback: F) -> !
-    where
-        F: FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow) + 'static,
-    {
-        let exit_code = match self.run_ondemand(callback) {
-            Err(RunLoopError::ExitFailure(code)) => code,
-            Err(_err) => 1,
-            Ok(_) => 0,
-        };
-        ::std::process::exit(exit_code)
-    }
-
     pub fn run_ondemand<F>(&mut self, mut event_handler: F) -> Result<(), RunLoopError>
     where
         F: FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),

src/platform_impl/macos/event_loop.rs

@@ -6,7 +6,7 @@ use std::{
     mem,
     os::raw::c_void,
     panic::{catch_unwind, resume_unwind, AssertUnwindSafe, RefUnwindSafe, UnwindSafe},
-    process, ptr,
+    ptr,
     rc::{Rc, Weak},
     sync::mpsc,
 };
@@ -192,16 +192,11 @@ impl<T> EventLoop<T> {
         &self.window_target
     }
 
-    pub fn run<F>(mut self, callback: F) -> !
+    pub fn run<F>(mut self, callback: F) -> Result<(), RunLoopError>
     where
         F: 'static + FnMut(Event<'_, T>, &RootWindowTarget<T>, &mut ControlFlow),
     {
-        let exit_code = match self.run_ondemand(callback) {
-            Err(RunLoopError::ExitFailure(code)) => code,
-            Err(_err) => 1,
-            Ok(_) => 0,
-        };
-        process::exit(exit_code);
+        self.run_ondemand(callback)
     }
 
     // NB: we don't base this on `pump_events` because for `MacOs` we can't support

src/platform_impl/orbital/event_loop.rs

@@ -12,6 +12,7 @@ use orbclient::{
 use raw_window_handle::{OrbitalDisplayHandle, RawDisplayHandle};
 
 use crate::{
+    error::RunLoopError,
     event::{self, Ime, Modifiers, StartCause},
     event_loop::{self, ControlFlow},
     keyboard::{
@@ -442,7 +443,7 @@ impl<T: 'static> EventLoop<T> {
         }
     }
 
-    pub fn run<F>(mut self, event_handler: F) -> !
+    pub fn run<F>(mut self, mut event_handler_inner: F) -> Result<(), RunLoopError>
     where
         F: 'static
             + FnMut(event::Event<'_, T>, &event_loop::EventLoopWindowTarget<T>, &mut ControlFlow),
@@ -688,7 +689,11 @@ impl<T: 'static> EventLoop<T> {
             &mut control_flow,
         );
 
-        ::std::process::exit(code);
+        if code == 0 {
+            Ok(())
+        } else {
+            Err(RunLoopError::ExitFailure(code))
+        }
     }
 
     pub fn window_target(&self) -> &event_loop::EventLoopWindowTarget<T> {

src/platform_impl/windows/event_loop.rs

@@ -243,16 +243,11 @@ impl<T: 'static> EventLoop<T> {
         &self.window_target
     }
 
-    pub fn run<F>(mut self, event_handler: F) -> !
+    pub fn run<F>(mut self, event_handler: F) -> Result<(), RunLoopError>
     where
         F: 'static + FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
     {
-        let exit_code = match self.run_ondemand(event_handler) {
-            Err(RunLoopError::ExitFailure(code)) => code,
-            Err(_err) => 1,
-            Ok(_) => 0,
-        };
-        ::std::process::exit(exit_code);
+        self.run_ondemand(event_handler)
     }
 
     pub fn run_ondemand<F>(&mut self, mut event_handler: F) -> Result<(), RunLoopError>