Add QuitMode policy to GPUI (#42391)

Applications can select a policy for when the app quits using the new
function `Application::with_quit_mode`:
- Only on explicit calls to `App::quit`
- When the last window is closed
- Platform default (former on macOS, latter everywhere else) 

Release Notes:

- N/A

crates/gpui/src/app.rs

@@ -169,6 +169,13 @@ impl Application {
         self
     }
 
+    /// Configures when the application should automatically quit.
+    /// By default, [`QuitMode::Default`] is used.
+    pub fn with_quit_mode(self, mode: QuitMode) -> Self {
+        self.0.borrow_mut().quit_mode = mode;
+        self
+    }
+
     /// Start the application. The provided callback will be called once the
     /// app is fully launched.
     pub fn run<F>(self, on_finish_launching: F)
@@ -238,6 +245,18 @@ type WindowClosedHandler = Box<dyn FnMut(&mut App)>;
 type ReleaseListener = Box<dyn FnOnce(&mut dyn Any, &mut App) + 'static>;
 type NewEntityListener = Box<dyn FnMut(AnyEntity, &mut Option<&mut Window>, &mut App) + 'static>;
 
+/// Defines when the application should automatically quit.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum QuitMode {
+    /// Use [`QuitMode::Explicit`] on macOS and [`QuitMode::LastWindowClosed`] on other platforms.
+    #[default]
+    Default,
+    /// Quit automatically when the last window is closed.
+    LastWindowClosed,
+    /// Quit only when requested via [`App::quit`].
+    Explicit,
+}
+
 #[doc(hidden)]
 #[derive(Clone, PartialEq, Eq)]
 pub struct SystemWindowTab {
@@ -588,6 +607,7 @@ pub struct App {
     pub(crate) inspector_element_registry: InspectorElementRegistry,
     #[cfg(any(test, feature = "test-support", debug_assertions))]
     pub(crate) name: Option<&'static str>,
+    quit_mode: QuitMode,
     quitting: bool,
 }
 
@@ -659,6 +679,7 @@ impl App {
                 inspector_renderer: None,
                 #[cfg(any(feature = "inspector", debug_assertions))]
                 inspector_element_registry: InspectorElementRegistry::default(),
+                quit_mode: QuitMode::default(),
                 quitting: false,
 
                 #[cfg(any(test, feature = "test-support", debug_assertions))]
@@ -1172,6 +1193,12 @@ impl App {
         self.http_client = new_client;
     }
 
+    /// Configures when the application should automatically quit.
+    /// By default, [`QuitMode::Default`] is used.
+    pub fn set_quit_mode(&mut self, mode: QuitMode) {
+        self.quit_mode = mode;
+    }
+
     /// Returns the SVG renderer used by the application.
     pub fn svg_renderer(&self) -> SvgRenderer {
         self.svg_renderer.clone()
@@ -1379,6 +1406,16 @@ impl App {
                     callback(cx);
                     true
                 });
+
+                let quit_on_empty = match cx.quit_mode {
+                    QuitMode::Explicit => false,
+                    QuitMode::LastWindowClosed => true,
+                    QuitMode::Default => !cfg!(macos),
+                };
+
+                if quit_on_empty && cx.windows.is_empty() {
+                    cx.quit();
+                }
             } else {
                 cx.windows.get_mut(id)?.replace(window);
             }

crates/zed/src/main.rs

@@ -15,7 +15,7 @@ use extension::ExtensionHostProxy;
 use fs::{Fs, RealFs};
 use futures::{StreamExt, channel::oneshot, future};
 use git::GitHostingProviderRegistry;
-use gpui::{App, AppContext, Application, AsyncApp, Focusable as _, UpdateGlobal as _};
+use gpui::{App, AppContext, Application, AsyncApp, Focusable as _, QuitMode, UpdateGlobal as _};
 
 use gpui_tokio::Tokio;
 use language::LanguageRegistry;
@@ -87,31 +87,33 @@ fn files_not_created_on_launch(errors: HashMap<io::ErrorKind, Vec<&Path>>) {
         .collect::<Vec<_>>().join("\n\n");
 
     eprintln!("{message}: {error_details}");
-    Application::new().run(move |cx| {
-        if let Ok(window) = cx.open_window(gpui::WindowOptions::default(), |_, cx| {
-            cx.new(|_| gpui::Empty)
-        }) {
-            window
-                .update(cx, |_, window, cx| {
-                    let response = window.prompt(
-                        gpui::PromptLevel::Critical,
-                        message,
-                        Some(&error_details),
-                        &["Exit"],
-                        cx,
-                    );
+    Application::new()
+        .with_quit_mode(QuitMode::Explicit)
+        .run(move |cx| {
+            if let Ok(window) = cx.open_window(gpui::WindowOptions::default(), |_, cx| {
+                cx.new(|_| gpui::Empty)
+            }) {
+                window
+                    .update(cx, |_, window, cx| {
+                        let response = window.prompt(
+                            gpui::PromptLevel::Critical,
+                            message,
+                            Some(&error_details),
+                            &["Exit"],
+                            cx,
+                        );
 
-                    cx.spawn_in(window, async move |_, cx| {
-                        response.await?;
-                        cx.update(|_, cx| cx.quit())
+                        cx.spawn_in(window, async move |_, cx| {
+                            response.await?;
+                            cx.update(|_, cx| cx.quit())
+                        })
+                        .detach_and_log_err(cx);
                     })
-                    .detach_and_log_err(cx);
-                })
-                .log_err();
-        } else {
-            fail_to_open_window(anyhow::anyhow!("{message}: {error_details}"), cx)
-        }
-    })
+                    .log_err();
+            } else {
+                fail_to_open_window(anyhow::anyhow!("{message}: {error_details}"), cx)
+            }
+        })
 }
 
 fn fail_to_open_window_async(e: anyhow::Error, cx: &mut AsyncApp) {