3 files changed, 146 insertions, 0 deletions

diff --git a/ecs/examples/extension.rs b/ecs/examples/extension.rs
new file mode 100644
index 0000000..2022b05
--- /dev/null
+++ b/ecs/examples/extension.rs
@@ -0,0 +1,77 @@
+use ecs::actions::Actions;
+use ecs::event::Event;
+use ecs::extension::{Collector as ExtensionCollector, Extension};
+use ecs::{Component, Query, World};
+
+#[derive(Debug, Component)]
+struct Position
+{
+    x: u32,
+    y: u32,
+}
+
+#[derive(Debug, Component)]
+struct EnemySpawnSource;
+
+#[derive(Debug, Component)]
+enum EvilnessLevel
+{
+    Medium,
+}
+
+#[derive(Debug)]
+struct Update;
+
+impl Event for Update {}
+
+fn spawn_enemies(
+    spawner_query: Query<(EnemySpawnSource, Position)>,
+    enemies_query: Query<(EvilnessLevel,)>,
+    mut actions: Actions,
+)
+{
+    let Some((_, enemy_spawner_position)) = spawner_query.iter().next() else {
+        return;
+    };
+
+    let enemy_cnt = enemies_query.iter().count();
+
+    if enemy_cnt > 3 {
+        return;
+    }
+
+    actions.spawn((
+        EvilnessLevel::Medium,
+        Position {
+            x: enemy_spawner_position.x * enemy_cnt as u32,
+            y: enemy_spawner_position.y,
+        },
+    ));
+
+    println!("Spawned enemy with medium evilness and 45 strength");
+}
+
+struct EnemySpawningExtension;
+
+impl Extension for EnemySpawningExtension
+{
+    fn collect(self, mut collector: ExtensionCollector<'_>)
+    {
+        collector.add_system(Update, spawn_enemies);
+
+        collector.add_entity((Position { x: 187, y: 30 }, EnemySpawnSource));
+    }
+}
+
+fn main()
+{
+    let mut world = World::new();
+
+    world.add_extension(EnemySpawningExtension);
+
+    for _ in 0..7 {
+        world.emit(Update);
+
+        world.perform_queued_actions();
+    }
+}
diff --git a/ecs/src/extension.rs b/ecs/src/extension.rs
new file mode 100644
index 0000000..fc5a345
--- /dev/null
+++ b/ecs/src/extension.rs
@@ -0,0 +1,56 @@
+use crate::component::Sequence as ComponentSequence;
+use crate::event::Event;
+use crate::sole::Sole;
+use crate::system::System;
+use crate::{SoleAlreadyExistsError, World};
+
+/// A collection of systems, entities & soles that can be added to a [`World`].
+pub trait Extension
+{
+    fn collect(self, collector: Collector<'_>);
+}
+
+/// Passed to a [`Extension`] to collects it's systems, entities & soles.
+pub struct Collector<'world>
+{
+    world: &'world mut World,
+}
+
+impl<'world> Collector<'world>
+{
+    /// Returns a new `Collector` for the given [`World`].
+    pub fn new(world: &'world mut World) -> Self
+    {
+        Self { world }
+    }
+
+    /// Adds a system to the [`World`].
+    pub fn add_system<'this, EventT, SystemImpl>(
+        &'this mut self,
+        event: EventT,
+        system: impl System<'this, SystemImpl>,
+    ) where
+        EventT: Event,
+    {
+        self.world.register_system(event, system);
+    }
+
+    /// Adds a entity to the [`World`].
+    pub fn add_entity<Comps>(&mut self, components: Comps)
+    where
+        Comps: ComponentSequence,
+    {
+        self.world.create_entity(components);
+    }
+
+    /// Adds a globally shared singleton value to the [`World`].
+    ///
+    /// # Errors
+    /// Returns `Err` if this [`Sole`] has already been added.
+    pub fn add_sole<SoleT>(&mut self, sole: SoleT) -> Result<(), SoleAlreadyExistsError>
+    where
+        SoleT: Sole,
+    {
+        self.world.add_sole(sole)
+    }
+}
diff --git a/ecs/src/lib.rs b/ecs/src/lib.rs
index 285f3b3..009ff21 100644
--- a/ecs/src/lib.rs
+++ b/ecs/src/lib.rs
@@ -12,6 +12,7 @@ use std::vec::Drain;
 use crate::actions::Action;
 use crate::component::{Component, Sequence as ComponentSequence};
 use crate::event::{Event, Id as EventId, Ids, Sequence as EventSequence};
+use crate::extension::{Collector as ExtensionCollector, Extension};
 use crate::lock::Lock;
 use crate::sole::Sole;
 use crate::system::{System, TypeErased as TypeErasedSystem};
@@ -20,6 +21,7 @@ use crate::type_name::TypeName;
 pub mod actions;
 pub mod component;
 pub mod event;
+pub mod extension;
 pub mod lock;
 pub mod query;
 pub mod sole;
@@ -120,6 +122,17 @@ impl World
         drop(event);
     }
 
+    /// Adds a extensions.
+    ///
+    /// # Panics
+    /// Will panic if mutable internal lock cannot be acquired.
+    pub fn add_extension(&mut self, extension: impl Extension)
+    {
+        let extension_collector = ExtensionCollector::new(self);
+
+        extension.collect(extension_collector);
+    }
+
     /// Emits a event, running all systems listening to the event for each compatible
     /// entity.
     ///