Add option to center a window (bevyengine#4999)

# Objective
- Fixes bevyengine#4993 

## Solution

- ~~Add `centered` property to `WindowDescriptor`~~
- Add `WindowPosition` enum
- `WindowDescriptor.position` is now `WindowPosition` instead of `Option<Vec2>`
- Add `center_window` function to `Window`

## Migration Guide
- If using `WindowDescriptor`, replace `position: None` with `position: WindowPosition::Default` and `position: Some(vec2)`  with `WindowPosition::At(vec2)`.

I'm not sure if this is the best approach, so feel free to give any feedback.
Also I'm not sure how `Option`s should be handled in `bevy_winit/src/lib.rs:161`.

Also, on window creation we can't (or at least I couldn't) get `outer_size`, so this doesn't include decorations in calculations.

crates/bevy_window/src/lib.rs

@@ -16,8 +16,8 @@ pub use windows::*;
 pub mod prelude {
     #[doc(hidden)]
     pub use crate::{
-        CursorEntered, CursorIcon, CursorLeft, CursorMoved, FileDragAndDrop, ReceivedCharacter,
-        Window, WindowDescriptor, WindowMoved, Windows,
+        CursorEntered, CursorIcon, CursorLeft, CursorMoved, FileDragAndDrop, MonitorSelection,
+        ReceivedCharacter, Window, WindowDescriptor, WindowMoved, WindowPosition, Windows,
     };
 }
 

crates/bevy_window/src/window.rs

@@ -259,6 +259,8 @@ pub enum WindowCommand {
     SetPosition {
         position: IVec2,
     },
+    /// Modifies the position of the window to be in the center of the current monitor
+    Center(MonitorSelection),
     /// Set the window's [`WindowResizeConstraints`]
     SetResizeConstraints {
         resize_constraints: WindowResizeConstraints,
@@ -416,6 +418,17 @@ impl Window {
             .push(WindowCommand::SetPosition { position });
     }
 
+    /// Modifies the position of the window to be in the center of the current monitor
+    ///
+    /// # Platform-specific
+    /// - iOS: Can only be called on the main thread.
+    /// - Web / Android / Wayland: Unsupported.
+    #[inline]
+    pub fn center_window(&mut self, monitor_selection: MonitorSelection) {
+        self.command_queue
+            .push(WindowCommand::Center(monitor_selection));
+    }
+
     /// Modifies the minimum and maximum window bounds for resizing in logical pixels.
     #[inline]
     pub fn set_resize_constraints(&mut self, resize_constraints: WindowResizeConstraints) {
@@ -714,6 +727,32 @@ impl Window {
     }
 }
 
+/// Defines where window should be placed at on creation.
+#[derive(Debug, Clone, Copy)]
+pub enum WindowPosition {
+    /// Position will be set by the window manager
+    Automatic,
+    /// Window will be centered on the selected monitor
+    ///
+    /// Note that this does not account for window decorations.
+    Centered(MonitorSelection),
+    /// The window's top-left corner will be placed at the specified position (in pixels)
+    ///
+    /// (0,0) represents top-left corner of screen space.
+    At(Vec2),
+}
+
+/// Defines which monitor to use.
+#[derive(Debug, Clone, Copy)]
+pub enum MonitorSelection {
+    /// Uses current monitor of the window.
+    Current,
+    /// Uses primary monitor of the system.
+    Primary,
+    /// Uses monitor with the specified index.
+    Number(usize),
+}
+
 /// Describes the information needed for creating a window.
 ///
 /// This should be set up before adding the [`WindowPlugin`](crate::WindowPlugin).
@@ -732,10 +771,8 @@ pub struct WindowDescriptor {
     ///
     /// May vary from the physical height due to different pixel density on different monitors.
     pub height: f32,
-    /// The position on the screen that the window will be centered at.
-    ///
-    /// If set to `None`, some platform-specific position will be chosen.
-    pub position: Option<Vec2>,
+    /// The position on the screen that the window will be placed at.
+    pub position: WindowPosition,
     /// Sets minimum and maximum resize limits.
     pub resize_constraints: WindowResizeConstraints,
     /// Overrides the window's ratio of physical pixels to logical pixels.
@@ -799,7 +836,7 @@ impl Default for WindowDescriptor {
             title: "app".to_string(),
             width: 1280.,
             height: 720.,
-            position: None,
+            position: WindowPosition::Automatic,
             resize_constraints: WindowResizeConstraints::default(),
             scale_factor_override: None,
             present_mode: PresentMode::Fifo,

crates/bevy_winit/src/lib.rs

@@ -162,6 +162,31 @@ fn change_window(
                         y: position[1],
                     });
                 }
+                bevy_window::WindowCommand::Center(monitor_selection) => {
+                    let window = winit_windows.get_window(id).unwrap();
+
+                    use bevy_window::MonitorSelection::*;
+                    let maybe_monitor = match monitor_selection {
+                        Current => window.current_monitor(),
+                        Primary => window.primary_monitor(),
+                        Number(n) => window.available_monitors().nth(n),
+                    };
+
+                    if let Some(monitor) = maybe_monitor {
+                        let screen_size = monitor.size();
+
+                        let window_size = window.outer_size();
+
+                        window.set_outer_position(PhysicalPosition {
+                            x: screen_size.width.saturating_sub(window_size.width) as f64 / 2.
+                                + monitor.position().x as f64,
+                            y: screen_size.height.saturating_sub(window_size.height) as f64 / 2.
+                                + monitor.position().y as f64,
+                        });
+                    } else {
+                        warn!("Couldn't get monitor selected with: {monitor_selection:?}");
+                    }
+                }
                 bevy_window::WindowCommand::SetResizeConstraints { resize_constraints } => {
                     let window = winit_windows.get_window(id).unwrap();
                     let constraints = resize_constraints.check_constraints();

crates/bevy_winit/src/winit_windows.rs

@@ -1,8 +1,8 @@
 use bevy_math::IVec2;
-use bevy_utils::HashMap;
+use bevy_utils::{tracing::warn, HashMap};
 use bevy_window::{Window, WindowDescriptor, WindowId, WindowMode};
 use raw_window_handle::HasRawWindowHandle;
-use winit::dpi::LogicalSize;
+use winit::dpi::{LogicalPosition, LogicalSize, PhysicalPosition};
 
 #[derive(Debug, Default)]
 pub struct WinitWindows {
@@ -49,30 +49,61 @@ impl WinitWindows {
                     ..
                 } = window_descriptor;
 
-                if let Some(position) = position {
-                    if let Some(sf) = scale_factor_override {
-                        winit_window_builder = winit_window_builder.with_position(
-                            winit::dpi::LogicalPosition::new(
-                                position[0] as f64,
-                                position[1] as f64,
-                            )
-                            .to_physical::<f64>(*sf),
-                        );
-                    } else {
-                        winit_window_builder =
-                            winit_window_builder.with_position(winit::dpi::LogicalPosition::new(
-                                position[0] as f64,
-                                position[1] as f64,
-                            ));
+                use bevy_window::WindowPosition::*;
+                match position {
+                    Automatic => { /* Window manager will handle position */ }
+                    Centered(monitor_selection) => {
+                        use bevy_window::MonitorSelection::*;
+                        let maybe_monitor = match monitor_selection {
+                            Current => {
+                                warn!("Can't select current monitor on window creation!");
+                                None
+                            }
+                            Primary => event_loop.primary_monitor(),
+                            Number(n) => event_loop.available_monitors().nth(*n),
+                        };
+
+                        if let Some(monitor) = maybe_monitor {
+                            let screen_size = monitor.size();
+
+                            let scale_factor = scale_factor_override.unwrap_or(1.0);
+
+                            // Logical to physical window size
+                            let (width, height): (u32, u32) = LogicalSize::new(*width, *height)
+                                .to_physical::<u32>(scale_factor)
+                                .into();
+
+                            let position = PhysicalPosition {
+                                x: screen_size.width.saturating_sub(width) as f64 / 2.
+                                    + monitor.position().x as f64,
+                                y: screen_size.height.saturating_sub(height) as f64 / 2.
+                                    + monitor.position().y as f64,
+                            };
+
+                            winit_window_builder = winit_window_builder.with_position(position);
+                        } else {
+                            warn!("Couldn't get monitor selected with: {monitor_selection:?}");
+                        }
+                    }
+                    At(position) => {
+                        if let Some(sf) = scale_factor_override {
+                            winit_window_builder = winit_window_builder.with_position(
+                                LogicalPosition::new(position[0] as f64, position[1] as f64)
+                                    .to_physical::<f64>(*sf),
+                            );
+                        } else {
+                            winit_window_builder = winit_window_builder.with_position(
+                                LogicalPosition::new(position[0] as f64, position[1] as f64),
+                            );
+                        }
                     }
                 }
+
                 if let Some(sf) = scale_factor_override {
-                    winit_window_builder.with_inner_size(
-                        winit::dpi::LogicalSize::new(*width, *height).to_physical::<f64>(*sf),
-                    )
-                } else {
                     winit_window_builder
-                        .with_inner_size(winit::dpi::LogicalSize::new(*width, *height))
+                        .with_inner_size(LogicalSize::new(*width, *height).to_physical::<f64>(*sf))
+                } else {
+                    winit_window_builder.with_inner_size(LogicalSize::new(*width, *height))
                 }
             }
             .with_resizable(window_descriptor.resizable)