Add basic template macro

Cargo.toml

@@ -11,3 +11,6 @@ tags = ["bevy", "ecs"]
 [dependencies]
 bevy_ecs = { version = "0.15", default-features = false }
 bevy_hierarchy = { version = "0.15", default-features = false }
+
+[dev-dependencies]
+bevy = { version = "0.15" }

examples/super-sheep-counter-2000.rs

@@ -0,0 +1,100 @@
+//! Super Sheep-Counter 2000
+//!
+//! An all-in-one numerical ruminant package.
+
+use i_cant_believe_its_not_bsn::*;
+
+use bevy::color::palettes::css;
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_plugins(sheep_plugin)
+        .run();
+}
+
+fn sheep_plugin(app: &mut App) {
+    app.add_systems(Startup, setup)
+        .add_systems(Update, sheep_system)
+        .add_observer(observe_buttons);
+}
+
+fn setup(mut commands: Commands) {
+    commands.spawn(Camera2d);
+}
+
+#[derive(Component)]
+struct Sheep;
+
+#[derive(Component)]
+enum Button {
+    Increment,
+    Decrement,
+}
+
+// A query that pulls data from the ecs and then updates it using a template.
+fn sheep_system(mut commands: Commands, sheep: Query<&Sheep>) {
+    let num_sheep = sheep.iter().len();
+
+    let template = template!(
+        root: {
+            Node {
+                position_type: PositionType::Absolute,
+                bottom: Val::Px(5.0),
+                right: Val::Px(5.0),
+                ..default()
+            }
+        } [
+            { counter(num_sheep, "sheep", Button::Increment, Button::Decrement) };
+        ];
+    );
+
+    commands.build(template);
+}
+
+// A function that returns an ecs template.
+fn counter<T: Component>(num: usize, name: &str, inc: T, dec: T) -> Template {
+    template! {
+        header: { Text::new("You have ") } [
+            number: { TextSpan::new(format!("{num}")) };
+            sheep: { TextSpan::new(format!(" {name}!")) };
+        ];
+        increase: {(
+            Button, Text::new("Increase"), TextColor(css::GREEN.into()), inc, visible_if(num < 100)
+        )};
+        decrease: {(
+            Button, Text::new("Decrease"), TextColor(css::RED.into()), dec, visible_if(num > 0)
+        )};
+    }
+}
+
+// A component helper function for computing visibility.
+fn visible_if(condition: bool) -> Visibility {
+    if condition {
+        Visibility::Visible
+    } else {
+        Visibility::Hidden
+    }
+}
+
+// A global observer which responds to button clicks.
+fn observe_buttons(
+    mut trigger: Trigger<Pointer<Up>>,
+    buttons: Query<&Button>,
+    sheep: Query<Entity, With<Sheep>>,
+    mut commands: Commands,
+) {
+    match buttons.get(trigger.target).ok() {
+        Some(Button::Increment) => {
+            commands.spawn(Sheep);
+        }
+        Some(Button::Decrement) => {
+            if let Some(sheep) = sheep.iter().next() {
+                commands.entity(sheep).despawn_recursive();
+            }
+        }
+        _ => {}
+    }
+    trigger.propagate(false);
+}

src/lib.rs

@@ -5,3 +5,6 @@ pub use hierarchy::*;
 
 mod maybe;
 pub use maybe::*;
+
+mod template;
+pub use template::*;