Impl the FuelGauge text update logic

This does a linear search through all existing FuelGauges, but we're not
expecting that to be a problem. There should only be zero or one of them
on screen at any moment.

It also doesn't work because the rest of the game state isn't correct...

src/main.rs

@@ -86,5 +86,5 @@ fn spawn_machine_ui<M: SpawnUi>(
 ) {
   let _entity = event.target;
   // TODO: Hook up the Fuel component (and spawn one so that I can)
-  M::spawn_ui(commands, theme);
+  M::spawn_ui(commands, theme, event.target());
 }

src/widgets/fuel_gauge.rs

@@ -19,14 +19,26 @@ impl FuelGauge {
       },
       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()
-    ));
+  /// Observer system to update the [`FuelGauge`] widget when a machine emits a
+  /// [`FuelChanged`] event.
+  pub fn detect_fuel_change(
+    event: Trigger<FuelChanged>,
+    fuel_levels: Query<&Fuel>,
+    mut gauges: Query<(&FuelGauge, &mut Text)>,
+  ) {
+    // Find the FuelGauge that references the same Entity as the event target.
+    // That gauge's `Text` is the one to update.
+    let (_, mut label) = gauges
+      .iter_mut()
+      .find(|(gauge, _label)| gauge.0 == event.target())
+      .expect("Couldn't find any fuel gauges");
+
+    let fuel = fuel_levels.get(event.target()).expect(
+      "Logic error: a `FuelChanged` event was targetted at an entity with no `Fuel` component.",
+    );
+    label.0 = format!("Fuel: {}", fuel.0);
   }
 }

src/widgets/machines.rs

@@ -7,7 +7,7 @@ use crate::{
 };
 
 impl SpawnUi for CuttingMachine {
-  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
+  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>, machine_id: Entity) {
     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.
@@ -37,19 +37,10 @@ impl SpawnUi for CuttingMachine {
         ))
         .with_children(|cmds| {
           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);
-            },
-          );
+          button_cmds.observe(move |trigger: Trigger<Pointer<Click>>, mut com: Commands| {
+            dbg!("Cut button pressed. Triggering a FuelChanged event");
+            com.trigger_targets(FuelChanged, machine_id);
+          });
           // TODO: Attach on-press observer so this machine can do something
           // in response to that button being pressed
         });
@@ -58,7 +49,7 @@ impl SpawnUi for CuttingMachine {
 }
 
 impl SpawnUi for RotatingMachine {
-  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
+  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>, machine_id: Entity) {
     let base_entity = machine_ui_base(&mut commands, "Rotating Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
       commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));
@@ -93,7 +84,7 @@ impl SpawnUi for RotatingMachine {
 }
 
 impl SpawnUi for FlippingMachine {
-  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
+  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>, machine_id: Entity) {
     let base_entity = machine_ui_base(&mut commands, "Flipping Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
       commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));
@@ -128,7 +119,7 @@ impl SpawnUi for FlippingMachine {
 }
 
 impl SpawnUi for TransposingMachine {
-  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>) {
+  fn spawn_ui(mut commands: Commands, theme: Res<UiTheme>, machine_id: Entity) {
     let base_entity = machine_ui_base(&mut commands, "Transposing Machine", &theme);
     commands.entity(base_entity).with_children(|commands| {
       commands.spawn(FuelGauge::bundle(Entity::PLACEHOLDER));

src/widgets/mod.rs

@@ -30,12 +30,12 @@ impl Plugin for GameUiPlugin {
   }
 }
 
-/// Anything that can spawn a themed UI should impl this trait.
+/// Machines spawn their UI through this trait.
 ///
-/// This exists mainly so that I can write generic functions and have the *compiler*
+/// This exists mainly so that I can write generic functions and have the compiler
 /// expand the code for me, instead of doing it by hand.
 pub trait SpawnUi {
-  fn spawn_ui(commands: Commands, theme: Res<UiTheme>);
+  fn spawn_ui(commands: Commands, theme: Res<UiTheme>, machine_id: Entity);
 }
 
 /// The base panel for the machines that manipulate the room cards.