Allow closing windows at runtime (#3575)

# Objective

Fixes #3180, builds from #2898

## Solution

Support requesting a window to be closed and closing a window in `bevy_window`, and handle this in `bevy_winit`.

This is a stopgap until we move to windows as entites, which I'm sure I'll get around to eventually.

## Changelog

### Added

- `Window::close` to allow closing windows.
- `WindowClosed` to allow reacting to windows being closed.

### Changed

Replaced `bevy::system::exit_on_esc_system` with `bevy::window::close_on_esc`.

## Fixed

The app no longer exits when any window is closed. This difference is only observable when there are multiple windows. 

## Migration Guide

`bevy::system::exit_on_esc_system` has been removed. Use `bevy::window::close_on_esc` instead.
`CloseWindow` has been removed. Use `Window::close` instead.
The `Close` variant has been added to `WindowCommand`. Handle this by closing the relevant window.

crates/bevy_input/src/lib.rs

@@ -3,7 +3,6 @@ pub mod gamepad;
 mod input;
 pub mod keyboard;
 pub mod mouse;
-pub mod system;
 pub mod touch;
 
 pub use axis::*;

crates/bevy_render/src/view/window.rs

@@ -7,7 +7,7 @@ use crate::{
 use bevy_app::{App, Plugin};
 use bevy_ecs::prelude::*;
 use bevy_utils::{tracing::debug, HashMap, HashSet};
-use bevy_window::{PresentMode, RawWindowHandleWrapper, WindowId, Windows};
+use bevy_window::{PresentMode, RawWindowHandleWrapper, WindowClosed, WindowId, Windows};
 use std::ops::{Deref, DerefMut};
 use wgpu::TextureFormat;
 
@@ -67,8 +67,12 @@ impl DerefMut for ExtractedWindows {
     }
 }
 
-fn extract_windows(mut render_world: ResMut<RenderWorld>, windows: Res<Windows>) {
-    let mut extracted_windows = render_world.resource_mut::<ExtractedWindows>();
+fn extract_windows(
+    mut render_world: ResMut<RenderWorld>,
+    mut closed: EventReader<WindowClosed>,
+    windows: Res<Windows>,
+) {
+    let mut extracted_windows = render_world.get_resource_mut::<ExtractedWindows>().unwrap();
     for window in windows.iter() {
         let (new_width, new_height) = (
             window.physical_width().max(1),
@@ -105,6 +109,9 @@ fn extract_windows(mut render_world: ResMut<RenderWorld>, windows: Res<Windows>)
             extracted_window.physical_height = new_height;
         }
     }
+    for closed_window in closed.iter() {
+        extracted_windows.remove(&closed_window.id);
+    }
 }
 
 #[derive(Default)]

crates/bevy_window/Cargo.toml

@@ -14,6 +14,8 @@ bevy_app = { path = "../bevy_app", version = "0.8.0-dev" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.8.0-dev" }
 bevy_math = { path = "../bevy_math", version = "0.8.0-dev" }
 bevy_utils = { path = "../bevy_utils", version = "0.8.0-dev" }
+# Used for close_on_esc
+bevy_input = { path = "../bevy_input", version = "0.8.0-dev" }
 raw-window-handle = "0.4.2"
 
 # other

crates/bevy_window/src/event.rs

@@ -25,22 +25,37 @@ pub struct CreateWindow {
 #[derive(Debug, Clone)]
 pub struct RequestRedraw;
 
-/// An event that indicates a window should be closed.
+/// An event that is sent whenever a new window is created.
+///
+/// To create a new window, send a [`CreateWindow`] event - this
+/// event will be sent in the handler for that event.
 #[derive(Debug, Clone)]
-pub struct CloseWindow {
+pub struct WindowCreated {
     pub id: WindowId,
 }
 
-/// An event that is sent whenever a new window is created.
+/// An event that is sent whenever the operating systems requests that a window
+/// be closed. This will be sent when the close button of the window is pressed.
+///
+/// If the default [`WindowPlugin`] is used, these events are handled
+/// by [closing] the corresponding [`Window`].  
+/// To disable this behaviour, set `close_when_requested` on the [`WindowPlugin`]
+/// to `false`.
+///
+/// [`WindowPlugin`]: crate::WindowPlugin
+/// [`Window`]: crate::Window
+/// [closing]: crate::Window::close
 #[derive(Debug, Clone)]
-pub struct WindowCreated {
+pub struct WindowCloseRequested {
     pub id: WindowId,
 }
 
-/// An event that is sent whenever a close was requested for a window. For example: when the "close"
-/// button is pressed on a window.
+/// An event that is sent whenever a window is closed. This will be sent by the
+/// handler for [`Window::close`].
+///
+/// [`Window::close`]: crate::Window::close
 #[derive(Debug, Clone)]
-pub struct WindowCloseRequested {
+pub struct WindowClosed {
     pub id: WindowId,
 }
 

crates/bevy_window/src/lib.rs

@@ -24,15 +24,34 @@ use bevy_app::prelude::*;
 use bevy_ecs::{event::Events, schedule::SystemLabel};
 
 pub struct WindowPlugin {
+    /// Whether to create a window when added.
+    ///
+    /// Note that if there are no windows, by default the App will exit,
+    /// due to [`exit_on_all_closed`].
     pub add_primary_window: bool,
-    pub exit_on_close: bool,
+    /// Whether to exit the app when there are no open windows.
+    /// If disabling this, ensure that you send the [`bevy_app::AppExit`]
+    /// event when the app should exit. If this does not occur, you will
+    /// create 'headless' processes (processes without windows), which may
+    /// surprise your users. It is recommended to leave this setting as `true`.
+    ///
+    /// If true, this plugin will add [`exit_on_all_closed`] to [`CoreStage::Update`].
+    pub exit_on_all_closed: bool,
+    /// Whether to close windows when they are requested to be closed (i.e.
+    /// when the close button is pressed)
+    ///
+    /// If true, this plugin will add [`close_when_requested`] to [`CoreStage::Update`].
+    /// If this system (or a replacement) is not running, the close button will have no effect.
+    /// This may surprise your users. It is recommended to leave this setting as `true`.
+    pub close_when_requested: bool,
 }
 
 impl Default for WindowPlugin {
     fn default() -> Self {
         WindowPlugin {
             add_primary_window: true,
-            exit_on_close: true,
+            exit_on_all_closed: true,
+            close_when_requested: true,
         }
     }
 }
@@ -42,9 +61,9 @@ impl Plugin for WindowPlugin {
         app.add_event::<WindowResized>()
             .add_event::<CreateWindow>()
             .add_event::<WindowCreated>()
+            .add_event::<WindowClosed>()
             .add_event::<WindowCloseRequested>()
             .add_event::<RequestRedraw>()
-            .add_event::<CloseWindow>()
             .add_event::<CursorMoved>()
             .add_event::<CursorEntered>()
             .add_event::<CursorLeft>()
@@ -69,8 +88,11 @@ impl Plugin for WindowPlugin {
             });
         }
 
-        if self.exit_on_close {
-            app.add_system(exit_on_window_close_system);
+        if self.exit_on_all_closed {
+            app.add_system(exit_on_all_closed);
+        }
+        if self.close_when_requested {
+            app.add_system(close_when_requested);
         }
     }
 }

crates/bevy_window/src/system.rs

@@ -1,12 +1,57 @@
-use crate::WindowCloseRequested;
+use crate::{Window, WindowCloseRequested, WindowFocused, WindowId, Windows};
+
 use bevy_app::AppExit;
-use bevy_ecs::event::{EventReader, EventWriter};
+use bevy_ecs::prelude::*;
+use bevy_input::{keyboard::KeyCode, Input};
 
-pub fn exit_on_window_close_system(
-    mut app_exit_events: EventWriter<AppExit>,
-    mut window_close_requested_events: EventReader<WindowCloseRequested>,
-) {
-    if window_close_requested_events.iter().next().is_some() {
+/// Exit the application when there are no open windows.
+///
+/// This system is added by the [`WindowPlugin`] in the default configuration.
+/// To disable this behaviour, set `close_when_requested` (on the [`WindowPlugin`]) to `false`.
+/// Ensure that you read the caveats documented on that field if doing so.
+///
+/// [`WindowPlugin`]: crate::WindowPlugin
+pub fn exit_on_all_closed(mut app_exit_events: EventWriter<AppExit>, windows: Res<Windows>) {
+    if windows.iter().count() == 0 {
         app_exit_events.send(AppExit);
     }
 }
+
+/// Close windows in response to [`WindowCloseRequested`] (e.g.  when the close button is pressed).
+///
+/// This system is added by the [`WindowPlugin`] in the default configuration.
+/// To disable this behaviour, set `close_when_requested` (on the [`WindowPlugin`]) to `false`.
+/// Ensure that you read the caveats documented on that field if doing so.
+///
+/// [`WindowPlugin`]: crate::WindowPlugin
+pub fn close_when_requested(
+    mut windows: ResMut<Windows>,
+    mut closed: EventReader<WindowCloseRequested>,
+) {
+    for event in closed.iter() {
+        windows.get_mut(event.id).map(Window::close);
+    }
+}
+
+/// Close the focused window whenever the escape key (<kbd>Esc</kbd>) is pressed
+///
+/// This is useful for examples or prototyping.
+pub fn close_on_esc(
+    mut focused: Local<Option<WindowId>>,
+    mut focused_events: EventReader<WindowFocused>,
+    mut windows: ResMut<Windows>,
+    input: Res<Input<KeyCode>>,
+) {
+    // TODO: Track this in e.g. a resource to ensure consistent behaviour across similar systems
+    for event in focused_events.iter() {
+        *focused = event.focused.then(|| event.id);
+    }
+
+    if let Some(focused) = &*focused {
+        if input.just_pressed(KeyCode::Escape) {
+            if let Some(window) = windows.get_mut(*focused) {
+                window.close();
+            }
+        }
+    }
+}

crates/bevy_window/src/window.rs

@@ -219,6 +219,7 @@ pub enum WindowCommand {
     SetResizeConstraints {
         resize_constraints: WindowResizeConstraints,
     },
+    Close,
 }
 
 /// Defines the way a window is displayed
@@ -571,6 +572,21 @@ impl Window {
         });
     }
 
+    /// Close the operating system window corresponding to this [`Window`].  
+    /// This will also lead to this [`Window`] being removed from the
+    /// [`Windows`] resource.
+    ///
+    /// If the default [`WindowPlugin`] is used, when no windows are
+    /// open, the [app will exit](bevy_app::AppExit).  
+    /// To disable this behaviour, set `exit_on_all_closed` on the [`WindowPlugin`]
+    /// to `false`
+    ///
+    /// [`Windows`]: crate::Windows
+    /// [`WindowPlugin`]: crate::WindowPlugin
+    pub fn close(&mut self) {
+        self.command_queue.push(WindowCommand::Close);
+    }
+
     #[inline]
     pub fn drain_commands(&mut self) -> impl Iterator<Item = WindowCommand> + '_ {
         self.command_queue.drain(..)

crates/bevy_window/src/windows.rs

@@ -70,4 +70,8 @@ impl Windows {
     pub fn iter_mut(&mut self) -> impl Iterator<Item = &mut Window> {
         self.windows.values_mut()
     }
+
+    pub fn remove(&mut self, id: WindowId) -> Option<Window> {
+        self.windows.remove(&id)
+    }
 }

crates/bevy_winit/src/lib.rs

@@ -2,39 +2,38 @@ mod converters;
 mod winit_config;
 mod winit_windows;
 
-use bevy_input::{
-    keyboard::KeyboardInput,
-    mouse::{MouseButtonInput, MouseMotion, MouseScrollUnit, MouseWheel},
-    touch::TouchInput,
-};
 pub use winit_config::*;
 pub use winit_windows::*;
 
 use bevy_app::{App, AppExit, CoreStage, Plugin};
+use bevy_ecs::prelude::*;
 use bevy_ecs::{
-    event::{EventWriter, Events, ManualEventReader},
-    schedule::ParallelSystemDescriptorCoercion,
-    system::{NonSend, ResMut},
+    event::{Events, ManualEventReader},
     world::World,
 };
+use bevy_input::{
+    keyboard::KeyboardInput,
+    mouse::{MouseButtonInput, MouseMotion, MouseScrollUnit, MouseWheel},
+    touch::TouchInput,
+};
 use bevy_math::{ivec2, DVec2, Vec2};
 use bevy_utils::{
-    tracing::{error, trace, warn},
+    tracing::{error, info, trace, warn},
     Instant,
 };
 use bevy_window::{
     CreateWindow, CursorEntered, CursorLeft, CursorMoved, FileDragAndDrop, ModifiesWindows,
     ReceivedCharacter, RequestRedraw, WindowBackendScaleFactorChanged, WindowCloseRequested,
-    WindowCreated, WindowFocused, WindowMoved, WindowResized, WindowScaleFactorChanged, Windows,
+    WindowClosed, WindowCreated, WindowFocused, WindowMoved, WindowResized,
+    WindowScaleFactorChanged, Windows,
 };
+
 use winit::{
-    dpi::PhysicalPosition,
+    dpi::{LogicalSize, PhysicalPosition},
     event::{self, DeviceEvent, Event, StartCause, WindowEvent},
     event_loop::{ControlFlow, EventLoop, EventLoopWindowTarget},
 };
 
-use winit::dpi::LogicalSize;
-
 #[derive(Default)]
 pub struct WinitPlugin;
 
@@ -51,10 +50,12 @@ impl Plugin for WinitPlugin {
 }
 
 fn change_window(
-    winit_windows: NonSend<WinitWindows>,
+    mut winit_windows: NonSendMut<WinitWindows>,
     mut windows: ResMut<Windows>,
     mut window_dpi_changed_events: EventWriter<WindowScaleFactorChanged>,
+    mut window_close_events: EventWriter<WindowClosed>,
 ) {
+    let mut removed_windows = vec![];
     for bevy_window in windows.iter_mut() {
         let id = bevy_window.id();
         for command in bevy_window.drain_commands() {
@@ -166,9 +167,25 @@ fn change_window(
                         window.set_max_inner_size(Some(max_inner_size));
                     }
                 }
+                bevy_window::WindowCommand::Close => {
+                    // Since we have borrowed `windows` to iterate through them, we can't remove the window from it.
+                    // Add the removal requests to a queue to solve this
+                    removed_windows.push(id);
+                    // No need to run any further commands - this drops the rest of the commands, although the `bevy_window::Window` will be dropped later anyway
+                    break;
+                }
             }
         }
     }
+    if !removed_windows.is_empty() {
+        for id in removed_windows {
+            // Close the OS window. (The `Drop` impl actually closes the window)
+            let _ = winit_windows.remove_window(id);
+            // Clean up our own data structures
+            windows.remove(id);
+            window_close_events.send(WindowClosed { id });
+        }
+    }
 }
 
 fn run<F>(event_loop: EventLoop<()>, event_handler: F) -> !
@@ -316,7 +333,8 @@ pub fn winit_runner_with(mut app: App) {
                 let window = if let Some(window) = windows.get_mut(window_id) {
                     window
                 } else {
-                    warn!("Skipped event for unknown Window Id {:?}", winit_window_id);
+                    // If we're here, this window was previously opened
+                    info!("Skipped event for closed window: {:?}", window_id);
                     return;
                 };
                 winit_state.low_power_event = true;

crates/bevy_winit/src/winit_windows.rs

@@ -181,6 +181,12 @@ impl WinitWindows {
     pub fn get_window_id(&self, id: winit::window::WindowId) -> Option<WindowId> {
         self.winit_to_window_id.get(&id).cloned()
     }
+
+    pub fn remove_window(&mut self, id: WindowId) -> Option<winit::window::Window> {
+        let winit_id = self.window_id_to_winit.remove(&id)?;
+        // Don't remove from winit_to_window_id, to track that we used to know about this winit window
+        self.windows.remove(&winit_id)
+    }
 }
 
 pub fn get_fitting_videomode(