Add user control of UI cameras

Add fields to UiCameraConfig to control the UI camera scale and
position.

Fixes #5242. It is again possible to manipulate the ui camera.

An alternative design was considered, where instead of having a
component that controls all of the UI camera settings, the component
would only hold an Entity referencing another camera.

That design was abandoned in favor of the current one because the
viewport is tightly bound to the "actual" camera the UI camera is
attached to. So it would be awkward to maintain independently two
different cameras.

crates/bevy_ui/src/entity.rs

@@ -2,10 +2,12 @@
 
 use crate::{
     widget::{Button, ImageMode},
-    CalculatedSize, FocusPolicy, Interaction, Node, Style, UiColor, UiImage,
+    CalculatedSize, FocusPolicy, Interaction, Node, Style, UiColor, UiImage, UI_CAMERA_FAR,
 };
 use bevy_ecs::{bundle::Bundle, prelude::Component};
+use bevy_math::Vec2;
 use bevy_render::{
+    camera::{DepthCalculation, OrthographicProjection, WindowOrigin},
     prelude::ComputedVisibility,
     view::{RenderLayers, Visibility},
 };
@@ -186,13 +188,42 @@ pub struct UiCameraConfig {
     pub show_ui: bool,
     /// The ui camera layers this camera can see.
     pub ui_render_layers: RenderLayers,
+    /// The position of the UI camera in UI space.
+    pub position: Vec2,
+    /// The projection data for the UI camera.
+    ///
+    /// The code relies on this not being set,
+    /// please use [`UiCameraConfig::scale_mut`] and [`UiCameraConfig::projection`]
+    /// instead.
+    /// This is only public so it is possible to use the struct update syntax.
+    #[doc(hidden)]
+    pub projection: OrthographicProjection,
+}
+
+impl UiCameraConfig {
+    /// Get mutably the scale of the UI camera, useful for zoom effects.
+    pub fn scale_mut(&mut self) -> &mut f32 {
+        &mut self.projection.scale
+    }
+
+    /// The projection data for the UI camera.
+    pub fn projection(&self) -> &OrthographicProjection {
+        &self.projection
+    }
 }
 
 impl Default for UiCameraConfig {
     fn default() -> Self {
         Self {
             show_ui: true,
             ui_render_layers: Default::default(),
+            position: Vec2::ZERO,
+            projection: OrthographicProjection {
+                far: UI_CAMERA_FAR,
+                window_origin: WindowOrigin::BottomLeft,
+                depth_calculation: DepthCalculation::ZDifference,
+                ..Default::default()
+            },
         }
     }
 }

crates/bevy_ui/src/lib.rs

@@ -30,7 +30,9 @@ use bevy_input::InputSystem;
 use bevy_render::view::VisibilitySystems;
 use bevy_transform::TransformSystem;
 use bevy_window::ModifiesWindows;
-use update::{ui_z_system, update_clipping_system, update_layer_visibility};
+use update::{
+    ui_z_system, update_clipping_system, update_layer_visibility, update_ui_camera_perspective,
+};
 
 /// The basic plugin for Bevy UI
 #[derive(Default)]
@@ -48,6 +50,8 @@ pub enum UiSystem {
     ///
     /// [`ComputedVisibility`]: bevy_render::view::ComputedVisibility
     LayerVisibility,
+    /// Update Ui camera perspective to fit new viewport logical size.
+    UpdateUiCameraPerspective,
 }
 
 impl Plugin for UiPlugin {
@@ -93,6 +97,10 @@ impl Plugin for UiPlugin {
                 CoreStage::PostUpdate,
                 widget::image_node_system.before(UiSystem::Flex),
             )
+            .add_system_to_stage(
+                CoreStage::Last,
+                update_ui_camera_perspective.label(UiSystem::UpdateUiCameraPerspective),
+            )
             .add_system_to_stage(
                 CoreStage::PostUpdate,
                 flex_node_system

crates/bevy_ui/src/render/mod.rs

@@ -12,7 +12,7 @@ use bevy_ecs::prelude::*;
 use bevy_math::{Mat4, Vec2, Vec3, Vec4Swizzles};
 use bevy_reflect::TypeUuid;
 use bevy_render::{
-    camera::{Camera, CameraProjection, DepthCalculation, OrthographicProjection, WindowOrigin},
+    camera::{Camera, CameraProjection},
     color::Color,
     render_asset::RenderAssets,
     render_graph::{RenderGraph, RunGraphOnViewNode, SlotInfo, SlotType},
@@ -219,7 +219,7 @@ pub fn extract_uinodes(
 /// as ui elements are "stacked on top of each other", they are within the camera's view
 /// and have room to grow.
 // TODO: Consider computing this value at runtime based on the maximum z-value.
-const UI_CAMERA_FAR: f32 = 1000.0;
+pub(crate) const UI_CAMERA_FAR: f32 = 1000.0;
 
 // This value is subtracted from the far distance for the camera's z-position to ensure nodes at z == 0.0 are rendered
 // TODO: Evaluate if we still need this.
@@ -251,26 +251,14 @@ pub fn extract_default_ui_camera_view<T: Component>(
         if !ui_config.show_ui {
             continue;
         }
-        let logical_size = if let Some(logical_size) = camera.logical_viewport_size() {
-            logical_size
-        } else {
-            continue;
-        };
-        let mut projection = OrthographicProjection {
-            far: UI_CAMERA_FAR,
-            window_origin: WindowOrigin::BottomLeft,
-            depth_calculation: DepthCalculation::ZDifference,
-            ..Default::default()
-        };
-        projection.update(logical_size.x, logical_size.y);
         if let Some(physical_size) = camera.physical_viewport_size() {
             let ui_camera = commands
                 .spawn()
                 .insert(ExtractedView {
-                    projection: projection.get_projection_matrix(),
+                    projection: ui_config.projection.get_projection_matrix(),
                     transform: GlobalTransform::from_xyz(
-                        0.0,
-                        0.0,
+                        ui_config.position.x,
+                        ui_config.position.y,
                         UI_CAMERA_FAR + UI_CAMERA_TRANSFORM_OFFSET,
                     ),
                     width: physical_size.x,

crates/bevy_ui/src/update.rs

@@ -5,12 +5,16 @@ use crate::{entity::UiCameraConfig, CalculatedClip, Overflow, Style};
 use super::Node;
 use bevy_ecs::{
     entity::Entity,
+    prelude::{Changed, Or},
     query::{With, Without},
     system::{Commands, Query},
 };
 use bevy_hierarchy::{Children, Parent};
 use bevy_math::Vec2;
-use bevy_render::view::{ComputedVisibility, RenderLayers};
+use bevy_render::{
+    camera::{Camera, CameraProjection},
+    view::{ComputedVisibility, RenderLayers},
+};
 use bevy_sprite::Rect;
 use bevy_transform::components::{GlobalTransform, Transform};
 
@@ -107,6 +111,19 @@ pub fn update_clipping_system(
     }
 }
 
+pub fn update_ui_camera_perspective(
+    mut query: Query<
+        (&Camera, &mut UiCameraConfig),
+        Or<(Changed<Camera>, Changed<UiCameraConfig>)>,
+    >,
+) {
+    for (camera, mut ui_config) in query.iter_mut() {
+        if let Some(logical_size) = camera.logical_viewport_size() {
+            ui_config.projection.update(logical_size.x, logical_size.y);
+        }
+    }
+}
+
 fn update_clipping(
     commands: &mut Commands,
     children_query: &Query<&Children>,