3 files changed, 123 insertions, 2 deletions

diff --git a/ecs/src/component.rs b/ecs/src/component.rs
index 4829050..8119e93 100644
--- a/ecs/src/component.rs
+++ b/ecs/src/component.rs
@@ -1,5 +1,6 @@
 use std::any::{Any, TypeId};
 use std::fmt::Debug;
+use std::ops::{Deref, DerefMut};
 
 use seq_macro::seq;
 
@@ -112,3 +113,36 @@ macro_rules! inner {
 seq!(C in 0..=64 {
     inner!(C);
 });
+
+/// Holds a component which is local to a single system.
+#[derive(Debug)]
+pub struct Local<'world, Value>
+{
+    value: &'world mut Value,
+}
+
+impl<'world, Value> Local<'world, Value>
+{
+    pub(crate) fn new(value: &'world mut Value) -> Self
+    {
+        Self { value }
+    }
+}
+
+impl<'world, Value> Deref for Local<'world, Value>
+{
+    type Target = Value;
+
+    fn deref(&self) -> &Self::Target
+    {
+        self.value
+    }
+}
+
+impl<'world, Value> DerefMut for Local<'world, Value>
+{
+    fn deref_mut(&mut self) -> &mut Self::Target
+    {
+        &mut self.value
+    }
+}
diff --git a/ecs/src/system.rs b/ecs/src/system.rs
index ecf1885..8f4d0b0 100644
--- a/ecs/src/system.rs
+++ b/ecs/src/system.rs
@@ -1,14 +1,21 @@
 use std::any::Any;
+use std::convert::Infallible;
 use std::fmt::Debug;
 
 use crate::component::Sequence as ComponentSequence;
 use crate::{ComponentStorage, Query};
 
+pub mod stateful;
+
 pub trait System<Impl>: 'static
 {
     type Query<'a>;
 
-    fn run(&self, component_storage: &mut ComponentStorage);
+    type Input;
+
+    fn initialize(self, input: Self::Input) -> Self;
+
+    fn run(&mut self, component_storage: &mut ComponentStorage);
 
     fn into_type_erased(self) -> TypeErased;
 }
@@ -18,9 +25,15 @@ where
     Func: Fn(Query<Comps>) + 'static,
     Comps: ComponentSequence,
 {
+    type Input = Infallible;
     type Query<'a> = Query<'a, Comps>;
 
-    fn run(&self, component_storage: &mut ComponentStorage)
+    fn initialize(self, _input: Self::Input) -> Self
+    {
+        self
+    }
+
+    fn run(&mut self, component_storage: &mut ComponentStorage)
     {
         self(Query::new(component_storage));
     }
@@ -38,6 +51,13 @@ where
     }
 }
 
+pub trait Into<Impl>
+{
+    type System;
+
+    fn into_system(self) -> Self::System;
+}
+
 pub struct TypeErased
 {
     data: Box<dyn Any>,
diff --git a/ecs/src/system/stateful.rs b/ecs/src/system/stateful.rs
new file mode 100644
index 0000000..b641cf2
--- /dev/null
+++ b/ecs/src/system/stateful.rs
@@ -0,0 +1,67 @@
+use std::marker::PhantomData;
+
+use crate::component::Local;
+use crate::system::{System, TypeErased};
+use crate::{ComponentStorage, Query};
+
+/// A stateful system.
+pub struct Stateful<Func, LocalComponent, Comps>
+{
+    func: Func,
+    local_component: Option<LocalComponent>,
+    _comps_pd: PhantomData<Comps>,
+}
+
+impl<Func: 'static, Comps: 'static, LocalComponent: 'static>
+    System<fn(Query<Comps>, Local<LocalComponent>)>
+    for Stateful<Func, LocalComponent, Comps>
+where
+    Func: Fn(Query<Comps>, Local<LocalComponent>),
+{
+    type Input = LocalComponent;
+    type Query<'a> = Query<'a, Comps>;
+
+    fn initialize(mut self, input: Self::Input) -> Self
+    {
+        self.local_component = Some(input);
+
+        self
+    }
+
+    fn run(&mut self, component_storage: &mut ComponentStorage)
+    {
+        (self.func)(
+            Query::new(component_storage),
+            Local::new(self.local_component.as_mut().unwrap()),
+        );
+    }
+
+    fn into_type_erased(self) -> TypeErased
+    {
+        TypeErased {
+            data: Box::new(self),
+            func: Box::new(move |data, component_storage| {
+                let this = data.downcast_mut::<Self>().unwrap();
+
+                this.run(component_storage);
+            }),
+        }
+    }
+}
+
+impl<Func: 'static, Comps, LocalComponent: 'static>
+    crate::system::Into<fn(Query<Comps>, Local<LocalComponent>)> for Func
+where
+    Func: Fn(Query<Comps>, Local<LocalComponent>),
+{
+    type System = Stateful<Func, LocalComponent, Comps>;
+
+    fn into_system(self) -> Self::System
+    {
+        Self::System {
+            func: self,
+            local_component: None,
+            _comps_pd: PhantomData,
+        }
+    }
+}