Proof-of-concept for fuel change event handling

Added a `FuelChanged` event which is to be used to trigger Observers
watching for a machine's fuel level change.

I've written a slapdash observer to emit these events when the cutting
machine's big red button is pressed. That observer *must* be replaced
because of how it feeds the machine ID into the UI (it only works when
there is exactly 1 CuttingMachine, which won't be the case).

The dummy machines are missing their machine components. I've added the
`CuttingMachine` component to the cutting machine just to test the event
passing.

src/game/mod.rs

@@ -34,4 +34,7 @@ pub mod consumables {
 
   #[derive(Component)]
   pub struct Fuel(u32);
+
+  #[derive(Event)]
+  pub struct FuelChanged;
 }

src/main.rs

@@ -46,6 +46,7 @@ fn dummy_machines(mut commands: Commands) {
   // Spawn a dummy Cutting Machine
   commands
     .spawn((
+      CuttingMachine,
       Sprite::from_color(RED, Vec2::splat(40.0)),
       Transform::from_translation(Vec3::new(-40.0, 40.0, 0.0)),
       Pickable::default(),

src/widgets/fuel_gauge.rs

@@ -0,0 +1,32 @@
+use bevy::{color::palettes::css::*, prelude::*, ui::Val::*};
+
+use crate::game::consumables::{Fuel, FuelChanged};
+
+#[derive(Component)]
+#[require(Label)]
+pub struct FuelGauge(Entity);
+
+impl FuelGauge {
+  /// Default bundle for a machine's fuel gauge widget. Give it the ID of the
+  /// machine whose info it is displaying.
+  pub fn bundle(target: Entity) -> impl Bundle {
+    (
+      FuelGauge(target),
+      Node {
+        min_width: Px(20.0),
+        min_height: Px(10.0),
+        ..default()
+      },
+      BackgroundColor(GREEN.into()),
+      Text::new("Fuel: "),
+      children![TextSpan::new("<n>"),],
+    )
+  }
+
+  pub fn detect_fuel_change(event: Trigger<FuelChanged>) {
+    dbg!(format!(
+      "Detected fuel change on entity ID: {}",
+      event.target()
+    ));
+  }
+}

src/widgets/machines.rs

@@ -1,9 +1,9 @@
-use bevy::{color::palettes::css::*, prelude::*, ui::Val::*};
+use bevy::{color::palettes::css::*, prelude::*};
 
 use crate::{
-  game::machines::*,
+  game::{consumables::FuelChanged, machines::*},
   resources::UiTheme,
-  widgets::{BigRedButton, SpawnUi, machine_ui_base},
+  widgets::{BigRedButton, SpawnUi, fuel_gauge::FuelGauge, machine_ui_base},
 };
 
 impl SpawnUi for CuttingMachine {
@@ -11,7 +11,8 @@ impl SpawnUi for CuttingMachine {
     let base_entity = machine_ui_base(&mut commands, "Cutting Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
       // Left panel. For fuel or machine stats or whatever.
-      commands.spawn(fuel_gauge_placeholder());
+      // TODO: Pass along target machine, not the UI's root entity.
+      commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));
 
       // Center panel (placeholder for the Card view)
       commands.spawn((
@@ -35,7 +36,20 @@ impl SpawnUi for CuttingMachine {
           Pickable::default(),
         ))
         .with_children(|cmds| {
-          let _button_cmds = cmds.spawn(BigRedButton::bundle("CUT"));
+          let mut button_cmds = cmds.spawn(BigRedButton::bundle("CUT"));
+          button_cmds.observe(
+            |trigger: Trigger<Pointer<Click>>,
+             mut com: Commands,
+             q: Single<(Entity, &CuttingMachine)>| {
+              dbg!("Cut button pressed. Triggering a FuelChanged event");
+
+              // Feed through the target machine, once the SpawnUi trait is updated
+              // to require it gets here in the first place.
+
+              let cutting_machine = q.0;
+              com.trigger_targets(FuelChanged, cutting_machine);
+            },
+          );
           // TODO: Attach on-press observer so this machine can do something
           // in response to that button being pressed
         });
@@ -47,7 +61,7 @@ impl SpawnUi for RotatingMachine {
   fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
     let base_entity = machine_ui_base(&mut commands, "Rotating Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
-      commands.spawn(fuel_gauge_placeholder());
+      commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));
 
       // Center panel (placeholder for input-output rotation)
       commands.spawn((
@@ -82,7 +96,7 @@ impl SpawnUi for FlippingMachine {
   fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
     let base_entity = machine_ui_base(&mut commands, "Flipping Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
-      commands.spawn(fuel_gauge_placeholder());
+      commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));
 
       // Center panel (placeholder)
       commands.spawn((
@@ -117,7 +131,7 @@ impl SpawnUi for TransposingMachine {
   fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
     let base_entity = machine_ui_base(&mut commands, "Transposing Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
-      commands.spawn(fuel_gauge_placeholder());
+      commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));
 
       // Center panel (placeholder)
       commands.spawn((
@@ -147,15 +161,3 @@ impl SpawnUi for TransposingMachine {
     });
   }
 }
-
-fn fuel_gauge_placeholder() -> impl Bundle {
-  (
-    Node {
-      padding: UiRect::all(Px(10.0)),
-      ..default()
-    },
-    BackgroundColor(GREEN.into()),
-    Pickable::default(),
-    children![(Text::new("Uses: <n>"), TextColor(BLACK.into()),)],
-  )
-}

src/widgets/mod.rs

@@ -1,10 +1,11 @@
 //! Catch-all location for UI bits
 
+mod fuel_gauge;
 pub mod machines;
 
 use bevy::{color::palettes::css::*, prelude::*, ui::Val::*};
 
-use crate::resources::UiTheme;
+use crate::{resources::UiTheme, widgets::fuel_gauge::FuelGauge};
 
 /// Plugin to set up systems & resources for the custom UI elements
 ///
@@ -24,7 +25,8 @@ impl Plugin for GameUiPlugin {
       .add_observer(BigRedButton::button_hover_start)
       .add_observer(BigRedButton::button_hover_stop)
       .add_observer(BigRedButton::button_press_start)
-      .add_observer(BigRedButton::button_press_stop);
+      .add_observer(BigRedButton::button_press_stop)
+      .add_observer(FuelGauge::detect_fuel_change);
   }
 }