UI picking example

Cargo.toml

@@ -3764,7 +3764,17 @@ name = "Sprite Picking"
 description = "Demonstrates picking sprites and sprite atlases"
 category = "Picking"
 wasm = true
-required-features = ["bevy_sprite_picking_backend"]
+
+[[example]]
+name = "ui_picking"
+path = "examples/picking/ui_picking.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.ui_picking]
+name = "UI Picking"
+description = "Demonstrates picking in a UI context"
+category = "Picking"
+wasm = true
 
 [[example]]
 name = "animation_masks"

examples/picking/ui_picking.rs

@@ -1,37 +1,53 @@
-//! Demonstrates the use of picking in Bevy's UI.
+//! Demonstrates the use of picking in Bevy's UI. Also shows use of one-shot systems to spawn
+//! boilerplate UI on-demand.
 //!
-//! This example displays a simple inventory system and an animated sprite. Items can be dragged
-//! and dropped within the inventory, or onto the sprite with differing effects depending on the
-//! item.
-//!
-//! The implementation is not intended for serious use as a comprehensive inventory system!
+//! This example displays a simple inventory. Items can be dragged and dropped within the
+//! inventory.
 
-use bevy::prelude::*;
-use std::fmt::Debug;
+use bevy::{ecs::system::RunSystemOnce, prelude::*, window::PrimaryWindow};
 
 fn main() {
     App::new()
+        .init_resource::<DragDetails>()
         .add_plugins(DefaultPlugins.set(ImagePlugin::default_nearest()))
         .add_systems(Startup, (setup, spawn_text))
+        .add_systems(Update, drag_inventory_items)
         .run();
 }
 
-/// Set up some UI to manipulate.
-fn setup(asset_server: Res<AssetServer>, mut commands: Commands) {
-    // We need a simple camera to display our UI.
-    commands.spawn((Name::new("Camera"), Camera2d));
+#[derive(Resource, Copy, Clone, Default)]
+struct DragDetails {
+    pub drag_origin: Option<Entity>,
+    pub drop_target: Option<Entity>,
+}
 
-    let inventory_node = Node {
-        margin: UiRect::all(Val::Px(5.)),
-        height: Val::Px(50.),
-        width: Val::Px(50.),
-        padding: UiRect::all(Val::Px(5.)),
-        ..default()
-    };
-    // Spawn some arbitrarily-positioned inventory slots as UI nodes.
-    commands
-        .spawn((
+/// Marker component for the root inventory UI `Node`.
+#[derive(Component)]
+struct Inventory;
+
+/// Marker component for an individual slot within the Inventory.
+#[derive(Component)]
+struct InventorySlot;
+
+/// Marker component for entities which can be dragged.
+#[derive(Component)]
+struct Dragable;
+
+/// Marker component for entities which are currently being dragged, and should follow the pointer.
+#[derive(Component)]
+struct Dragging;
+
+/// Set up some UI to manipulate.
+///
+/// Systems that accept a &mut World argument implement the `Command` trait. We use this rather
+/// than a system accepting `Commands`, because we want access to one-shot systems below.
+fn setup(world: &mut World) {
+    // We need a simple camera to display our UI.
+    world.spawn((Name::new("Camera"), Camera2d));
+
+    world.spawn((
         Name::new("Inventory"),
+        Inventory,
         // This first node acts like a container. You can think of it as the box in which
         // inventory slots are arranged. It's useful to remember that picking events can
         // "bubble" up through layers of UI, so if required this parent node can act on events
@@ -46,60 +62,176 @@ fn setup(asset_server: Res<AssetServer>, mut commands: Commands) {
             ..default()
         },
         BackgroundColor(Color::WHITE.with_alpha(0.1)),
-        ))
-        .observe(|t: Trigger<Pointer<Over>>| {
-            dbg!(t);
-        })
-        .with_children(|parent| {
-            parent
-                .spawn((
-                    Name::new("Inventory Slot A"),
-                    inventory_node.clone(),
-                    BackgroundColor(Color::WHITE.with_alpha(0.5)),
-                ))
-                .with_children(|parent| {
-                    parent.spawn(ImageNode::new(
-                        asset_server.load("textures/rpg/props/generic-rpg-loot01.png"),
     ));
-                })
-                .observe(drag_handler::<Pointer<DragEnd>>());
 
+    // To reduce boilerplate, let's spawn the inventory slots using one-shot systems. The slots
+    // are each identical, and the UI Layout engine takes care of arranging them for us in the
+    // above container. The `.unwrap()` is for simplicity in this example, whereas you might
+    // use `.expect()`.
+    world.run_system_once(spawn_inventory_slot).unwrap();
+    world.run_system_once(spawn_inventory_slot).unwrap();
+    world.run_system_once(spawn_inventory_slot).unwrap();
+    world.run_system_once(spawn_inventory_slot).unwrap();
+
+    // Now we need to add some items to the inventory.
+    world.run_system_once(add_items_to_inventory).unwrap();
+}
+
+/// Spawn individual inventory slots, using the `Inventory` root UI node as its parent.
+fn spawn_inventory_slot(mut commands: Commands, inventory: Single<Entity, With<Inventory>>) {
+    commands.entity(*inventory).with_children(|parent| {
         parent
             .spawn((
-                    Name::new("Inventory Slot B"),
-                    inventory_node.clone(),
+                Name::new("Inventory Slot"),
+                Node {
+                    margin: UiRect::all(Val::Px(5.)),
+                    height: Val::Px(50.),
+                    width: Val::Px(50.),
+                    padding: UiRect::all(Val::Px(5.)),
+                    ..default()
+                },
+                InventorySlot,
                 BackgroundColor(Color::WHITE.with_alpha(0.5)),
             ))
-                .observe(drag_handler::<Pointer<DragStart>>())
-                .with_children(|parent| {
-                    parent.spawn((ImageNode::new(
-                        asset_server.load("textures/rpg/props/generic-rpg-loot02.png"),
-                    ),));
+            // Update the `DragDetails` as the UI `Node`s receive picking events.
+            .observe(set_drop_target())
+            .observe(clear_drop_target());
     });
+}
 
+/// This observer detects the beginning of a drag-and-drop motion and takes care of tasks such as
+/// detaching the target from its parent and allowing the node to be moved.
+fn drag_start() -> impl Fn(
+    Trigger<Pointer<DragStart>>,
+    Commands,
+    Query<(Entity, &mut Node, &mut Transform, &Parent), With<Dragable>>,
+    ResMut<DragDetails>,
+) {
+    move |ev, mut commands, mut dragable, mut drag_details| {
+        let Ok((item, mut item_node, mut transform, parent)) = dragable.get_mut(ev.target) else {
+            return;
+        };
+
+        // Record which slot we started from, in case we need to drop the item back again.
+        drag_details.drag_origin = Some(parent.get());
+
+        // Absolutely-positioned items can be made to follow the cursor.
+        item_node.position_type = PositionType::Absolute;
+
+        // TODO: is there a better solution for the scaling problem? Without this, and without a
+        // parent element, the asset will be un-scaled (and appear tiny). Copying its current
+        // scaling doesn't seem to be helpful.
+        transform.scale *= 4.;
+
+        // Add a marker to tell our systems that the node is being dragged, and remove it from the
+        // list of children on its current parent entity.
+        commands.entity(item).insert(Dragging).remove_parent();
+    }
+}
+
+/// This observer detects the end of our drag-and-drop, and checks the `DragDetails` resource to
+/// decide where to place the node: either in a new `InventorySlot`, or back in the one it came
+/// from.
+fn drag_end() -> impl Fn(
+    Trigger<Pointer<DragEnd>>,
+    Commands,
+    Query<(Entity, &mut Node, &mut Transform), With<Dragging>>,
+    ResMut<DragDetails>,
+) {
+    move |_ev, mut commands, mut dragging_item, drag_details| {
+        let Ok((item, mut item_node, mut transform)) = dragging_item.get_single_mut() else {
+            return;
+        };
+
+        // Not dragging anymore!
+        commands.entity(item).remove::<Dragging>();
+
+        // We either want the new location, or the original one. In theory, both being `None`
+        // shouldn't happen.
+        let Some(drop_target) = drag_details.drop_target.or(drag_details.drag_origin) else {
+            return;
+        };
+
+        // Whichever slot we pick, add the `Node` as a child entity.
+        commands.entity(drop_target).add_child(item);
+
+        // Re-integrate the `Node` into the relatively-positioned layout.
+        item_node.position_type = PositionType::Relative;
+        item_node.left = Val::Auto;
+        item_node.top = Val::Auto;
+        transform.scale /= 4.;
+    }
+}
+
+/// This observer updates our record of which inventory slot is the current target. We
+/// should also set the `drop_target` to `None` if the pointer is hovering over empty
+/// space, for example. Even though all of our hovered `Node`s receive picking events, we don't
+/// need to check the type of the `ev.target` because we've only attached this observer to
+/// `InventorySlot`s.
+fn set_drop_target() -> impl Fn(Trigger<Pointer<Over>>, ResMut<DragDetails>) {
+    move |ev, mut details| {
+        details.drop_target = Some(ev.target);
+    }
+}
+
+/// This observer ensures that even if we'd previously hovered over an `InventorySlot`, we clear
+/// the drop target again once the pointer leaves it.
+fn clear_drop_target() -> impl Fn(Trigger<Pointer<Out>>, ResMut<DragDetails>) {
+    move |_ev, mut details| {
+        details.drop_target = None;
+    }
+}
+
+/// Update the transform of the dragged item to follow the cursor.
+fn drag_inventory_items(
+    mut dragging: Query<&mut Node, With<Dragging>>,
+    primary_window: Single<&Window, With<PrimaryWindow>>,
+) {
+    let Ok(mut item_node) = dragging.get_single_mut() else {
+        return;
+    };
+
+    let Some(cursor_pos) = primary_window.cursor_position() else {
+        return;
+    };
+    // We can't assign exact values with a `PositionType::Absolute` UI node. However, we can use
+    // distance from top of viewport and distance from left of viewport to accomplish the same
+    // thing.
+    item_node.left = Val::Px(cursor_pos.x);
+    item_node.top = Val::Px(cursor_pos.y);
+}
+
+/// Spawn two arbitrary inventory items. Here we'll just pick the first two slots.
+fn add_items_to_inventory(
+    asset_server: Res<AssetServer>,
+    mut commands: Commands,
+    slots: Query<Entity, With<InventorySlot>>,
+) {
+    let paths = [
+        "textures/rpg/props/generic-rpg-loot01.png",
+        "textures/rpg/props/generic-rpg-loot02.png",
+    ];
+    for (i, slot) in slots.iter().enumerate().take(2) {
+        commands.entity(slot).with_children(|parent| {
             parent
                 .spawn((
-                    Name::new("Inventory Slot C"),
-                    inventory_node.clone(),
-                    BackgroundColor(Color::WHITE.with_alpha(0.5)),
+                    Dragable,
+                    // This node serves solely to be dragged! It provides the transform, using required
+                    // components.
+                    Node::default(),
+                    ImageNode::new(asset_server.load(paths[i])),
                 ))
-                .observe(drag_handler::<Pointer<DragEnd>>());
-
-            parent
-                .spawn((
-                    Name::new("Inventory Slot C"),
-                    inventory_node.clone(),
-                    BackgroundColor(Color::WHITE.with_alpha(0.5)),
-                ))
-                .observe(drag_handler::<Pointer<DragStart>>());
+                .observe(drag_start())
+                .observe(drag_end());
         });
+    }
 }
 
 /// Display instructions.
 fn spawn_text(mut commands: Commands) {
     commands.spawn((
         Name::new("Instructions"),
-        Text::new("Drag and drop birds within the inventory slots."),
+        Text::new("Drag and drop items within the inventory slots."),
         Node {
             position_type: PositionType::Absolute,
             top: Val::Px(12.),
@@ -108,13 +240,3 @@ fn spawn_text(mut commands: Commands) {
         },
     ));
 }
-
-fn drag_handler<E: Debug + Clone + Reflect>() -> impl Fn(Trigger<E>, Query<&mut Sprite>) {
-    move |ev, _sprites| {
-        dbg!(ev);
-    }
-}
-//
-// fn drop_handler<E: Debug + Clone + Reflect>() -> impl Fn(Trigger<E>) {
-//     move |ev| {}
-// }