Add docs and common helper functions to Windows (#4107)

# Objective

- Improve documentation.
- Provide helper functions for common uses of `Windows` relating to getting the primary `Window`.
- Reduce repeated `Window` code.

# Solution

- Adds infallible `primary()` and `primary_mut()` functions with standard error text. This replaces the commonly used `get_primary().unwrap()` seen throughout bevy which has inconsistent or nonexistent error messages.
- Adds `scale_factor(WindowId)` to replace repeated code blocks throughout.

# Considerations

- The added functions can panic if the primary window does not exist.
    - It is very uncommon for the primary window to not exist, as seen by the regular use of `get_primary().unwrap()`. Most users will have a single window and will need to reference the primary window in their code multiple times.
    - The panic provides a consistent error message to make this class of error easy to spot from the panic text.
    - This follows the established standard of short names for infallible-but-unlikely-to-panic functions in bevy.
- Removes line noise for common usage of `Windows`.

crates/bevy_text/src/text2d.rs

@@ -9,7 +9,7 @@ use bevy_math::{Size, Vec3};
 use bevy_render::{texture::Image, view::Visibility, RenderWorld};
 use bevy_sprite::{ExtractedSprite, ExtractedSprites, TextureAtlas};
 use bevy_transform::prelude::{GlobalTransform, Transform};
-use bevy_window::Windows;
+use bevy_window::{WindowId, Windows};
 
 use crate::{
     DefaultTextPipeline, Font, FontAtlasSet, HorizontalAlign, Text, Text2dSize, TextError,
@@ -50,11 +50,7 @@ pub fn extract_text2d_sprite(
 ) {
     let mut extracted_sprites = render_world.resource_mut::<ExtractedSprites>();
 
-    let scale_factor = if let Some(window) = windows.get_primary() {
-        window.scale_factor() as f32
-    } else {
-        1.
-    };
+    let scale_factor = windows.scale_factor(WindowId::primary()) as f32;
 
     for (entity, visibility, text, transform, calculated_size) in text2d_query.iter() {
         if !visibility.is_visible {
@@ -139,11 +135,7 @@ pub fn text2d_system(
         return;
     }
 
-    let scale_factor = if let Some(window) = windows.get_primary() {
-        window.scale_factor()
-    } else {
-        1.
-    };
+    let scale_factor = windows.scale_factor(WindowId::primary());
 
     // Computes all text in the local queue
     let mut new_queue = Vec::new();

crates/bevy_ui/src/flex/mod.rs

@@ -218,11 +218,7 @@ pub fn flex_node_system(
     }
 
     // assume one window for time being...
-    let logical_to_physical_factor = if let Some(primary_window) = windows.get_primary() {
-        primary_window.scale_factor()
-    } else {
-        1.
-    };
+    let logical_to_physical_factor = windows.scale_factor(WindowId::primary());
 
     if scale_factor_events.iter().next_back().is_some() {
         update_changed(

crates/bevy_ui/src/render/mod.rs

@@ -30,7 +30,7 @@ use bevy_sprite::{Rect, SpriteAssetEvents, TextureAtlas};
 use bevy_text::{DefaultTextPipeline, Text};
 use bevy_transform::components::GlobalTransform;
 use bevy_utils::HashMap;
-use bevy_window::Windows;
+use bevy_window::{WindowId, Windows};
 
 use bytemuck::{Pod, Zeroable};
 
@@ -186,11 +186,7 @@ pub fn extract_text_uinodes(
 ) {
     let mut extracted_uinodes = render_world.resource_mut::<ExtractedUiNodes>();
 
-    let scale_factor = if let Some(window) = windows.get_primary() {
-        window.scale_factor() as f32
-    } else {
-        1.
-    };
+    let scale_factor = windows.scale_factor(WindowId::primary()) as f32;
 
     for (entity, uinode, transform, text, visibility, clip) in uinode_query.iter() {
         if !visibility.is_visible {

crates/bevy_ui/src/widget/text.rs

@@ -10,7 +10,7 @@ use bevy_math::Size;
 use bevy_render::texture::Image;
 use bevy_sprite::TextureAtlas;
 use bevy_text::{DefaultTextPipeline, Font, FontAtlasSet, Text, TextError};
-use bevy_window::Windows;
+use bevy_window::{WindowId, Windows};
 
 #[derive(Debug, Default)]
 pub struct QueuedText {
@@ -52,11 +52,7 @@ pub fn text_system(
         QueryState<(&Text, &Style, &mut CalculatedSize)>,
     )>,
 ) {
-    let scale_factor = if let Some(window) = windows.get_primary() {
-        window.scale_factor()
-    } else {
-        1.
-    };
+    let scale_factor = windows.scale_factor(WindowId::primary());
 
     let inv_scale_factor = 1. / scale_factor;
 

crates/bevy_window/src/windows.rs

@@ -1,36 +1,72 @@
 use super::{Window, WindowId};
 use bevy_utils::HashMap;
 
+/// A collection of [`Window`]s with unique [`WindowId`]s.
 #[derive(Debug, Default)]
 pub struct Windows {
     windows: HashMap<WindowId, Window>,
 }
 
 impl Windows {
+    /// Add the provided [`Window`] to the [`Windows`] resource.
     pub fn add(&mut self, window: Window) {
         self.windows.insert(window.id(), window);
     }
 
+    /// Get a reference to the [`Window`] of `id`
     pub fn get(&self, id: WindowId) -> Option<&Window> {
         self.windows.get(&id)
     }
 
+    /// Get a mutable reference to the provided [`WindowId`].
     pub fn get_mut(&mut self, id: WindowId) -> Option<&mut Window> {
         self.windows.get_mut(&id)
     }
 
+    /// Get a reference to the primary [`Window`].
     pub fn get_primary(&self) -> Option<&Window> {
         self.get(WindowId::primary())
     }
 
+    /// Get a reference to the primary [`Window`].
+    ///
+    /// # Panics
+    ///
+    /// Panics if the primary window does not exist in [`Windows`]
+    pub fn primary(&self) -> &Window {
+        self.get_primary().expect("Primary window does not exist")
+    }
+
+    /// Get a mutable reference to the primary [`Window`].
     pub fn get_primary_mut(&mut self) -> Option<&mut Window> {
         self.get_mut(WindowId::primary())
     }
 
+    /// Get a mutable reference to the primary [`Window`].
+    ///
+    /// # Panics
+    ///
+    /// Panics if the primary window does not exist in [`Windows`]
+    pub fn primary_mut(&mut self) -> &mut Window {
+        self.get_primary_mut()
+            .expect("Primary window does not exist")
+    }
+
+    /// Returns the scale factor for the [`Window`] of `id`, or `1.0` if the window does not exist.
+    pub fn scale_factor(&self, id: WindowId) -> f64 {
+        if let Some(window) = self.get(id) {
+            window.scale_factor()
+        } else {
+            1.0
+        }
+    }
+
+    /// An iterator over all registered [`Window`]s
     pub fn iter(&self) -> impl Iterator<Item = &Window> {
         self.windows.values()
     }
 
+    /// A mutable iterator over all registered [`Window`]s
     pub fn iter_mut(&mut self) -> impl Iterator<Item = &mut Window> {
         self.windows.values_mut()
     }

crates/bevy_winit/src/lib.rs

@@ -412,7 +412,7 @@ pub fn winit_runner_with(mut app: App) {
                         // On a mobile window, the start is from the top while on PC/Linux/OSX from
                         // bottom
                         if cfg!(target_os = "android") || cfg!(target_os = "ios") {
-                            let window_height = windows.get_primary().unwrap().height();
+                            let window_height = windows.primary().height();
                             location.y = window_height - location.y;
                         }
                         touch_input_events.send(converters::convert_touch_input(touch, location));

examples/2d/contributors.rs

@@ -245,7 +245,7 @@ fn collision_system(
 ) {
     let mut rnd = rand::thread_rng();
 
-    let window = windows.get_primary().unwrap();
+    let window = windows.primary();
 
     let ceiling = window.height() / 2.;
     let ground = -(window.height() / 2.);

examples/ecs/parallel_query.rs

@@ -41,7 +41,7 @@ fn bounce_system(
     windows: Res<Windows>,
     mut sprites: Query<(&Transform, &mut Velocity)>,
 ) {
-    let window = windows.get_primary().expect("No primary window.");
+    let window = windows.primary();
     let width = window.width();
     let height = window.height();
     let left = width / -2.0;

examples/ios/src/lib.rs

@@ -28,7 +28,7 @@ fn touch_camera(
             *last_position = None;
         }
         if let Some(last_position) = *last_position {
-            let window = windows.get_primary().unwrap();
+            let window = windows.primary();
             let mut transform = camera.single_mut();
             *transform = Transform::from_xyz(
                 transform.translation.x

examples/tools/bevymark.rs

@@ -189,7 +189,7 @@ fn spawn_birds(
     spawn_count: usize,
     texture: Handle<Image>,
 ) {
-    let window = windows.get_primary().unwrap();
+    let window = windows.primary();
     let bird_x = (window.width() as f32 / -2.) + HALF_BIRD_SIZE;
     let bird_y = (window.height() as f32 / 2.) - HALF_BIRD_SIZE;
     let mut rng = thread_rng();
@@ -230,7 +230,7 @@ fn movement_system(time: Res<Time>, mut bird_query: Query<(&mut Bird, &mut Trans
 }
 
 fn collision_system(windows: Res<Windows>, mut bird_query: Query<(&mut Bird, &Transform)>) {
-    let window = windows.get_primary().unwrap();
+    let window = windows.primary();
     let half_width = window.width() as f32 * 0.5;
     let half_height = window.height() as f32 * 0.5;
 

examples/window/scale_factor_override.rs

@@ -64,7 +64,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
 
 /// This system toggles scale factor overrides when enter is pressed
 fn toggle_override(input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
-    let window = windows.get_primary_mut().unwrap();
+    let window = windows.primary_mut();
     if input.just_pressed(KeyCode::Return) {
         window.set_scale_factor_override(window.scale_factor_override().xor(Some(1.)));
     }
@@ -72,7 +72,7 @@ fn toggle_override(input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
 
 /// This system changes the scale factor override when up or down is pressed
 fn change_scale_factor(input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
-    let window = windows.get_primary_mut().unwrap();
+    let window = windows.primary_mut();
     if input.just_pressed(KeyCode::Up) {
         window.set_scale_factor_override(window.scale_factor_override().map(|n| n + 1.));
     } else if input.just_pressed(KeyCode::Down) {

examples/window/window_settings.rs

@@ -19,7 +19,7 @@ fn main() {
 
 /// This system will then change the title during execution
 fn change_title(time: Res<Time>, mut windows: ResMut<Windows>) {
-    let window = windows.get_primary_mut().unwrap();
+    let window = windows.primary_mut();
     window.set_title(format!(
         "Seconds since startup: {}",
         time.seconds_since_startup().round()
@@ -28,7 +28,7 @@ fn change_title(time: Res<Time>, mut windows: ResMut<Windows>) {
 
 /// This system toggles the cursor's visibility when the space bar is pressed
 fn toggle_cursor(input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
-    let window = windows.get_primary_mut().unwrap();
+    let window = windows.primary_mut();
     if input.just_pressed(KeyCode::Space) {
         window.set_cursor_lock_mode(!window.cursor_locked());
         window.set_cursor_visibility(!window.cursor_visible());
@@ -48,7 +48,7 @@ fn cycle_cursor_icon(
         CursorIcon::Text,
         CursorIcon::Copy,
     ];
-    let window = windows.get_primary_mut().unwrap();
+    let window = windows.primary_mut();
     if input.just_pressed(MouseButton::Left) {
         *index = (*index + 1) % ICONS.len();
         window.set_cursor_icon(ICONS[*index]);