diff --git a/src/engine.rs b/src/engine.rs
index 4a960c0..bf360ac 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -1,5 +1,5 @@
 use std::any::{Any, TypeId};
-use std::collections::HashMap;
+use std::rc::Rc;
 use std::time::Duration;
 
 use sdl2::{
@@ -40,33 +40,34 @@ macro_rules! impl_from_T_for_Error {
     };
 }
 
-impl_from_T_for_Error!(String, WindowBuildError, IntegerOrSdlError);
+impl_from_T_for_Error!(String, WindowBuildError, IntegerOrSdlError, &str);
 
-pub struct Entity(u64, Vec<Box<dyn Component>>);
+pub type Id = u64;
+
+pub struct Entity(Id, Vec<Box<dyn Component>>);
 
 pub trait Component {
     fn inner_type_id(&self) -> TypeId;
     fn as_any(&mut self) -> &mut dyn Any;
 }
 
-pub struct Sprite {
-    path: std::path::PathBuf,
-}
+#[derive(Clone, Copy)]
+pub struct Sprite(Id);
 
 pub struct Context<'context, 'game>
 where
     'game: 'context,
 {
-    id_counter: &'context mut u64,
+    id_counter: &'context mut Id,
     canvas: &'context mut Canvas<Window>,
     texture_creator: *const TextureCreator<WindowContext>,
     entities: &'context mut Vec<Entity>,
-    systems: &'context mut Vec<Box<dyn System>>,
-    textures: &'context mut HashMap<std::path::PathBuf, Texture<'game>>,
+    systems: &'context mut Vec<Rc<dyn System>>,
+    textures: &'context mut Vec<(Id, Texture<'game>)>,
 }
 
 impl<'context, 'game> Context<'context, 'game> {
-    pub fn entities_with_component<T: 'static>(&self) -> Vec<u64> {
+    pub fn entities_with_component<T: 'static + Component>(&self) -> Vec<u64> {
         let entity_type_id = TypeId::of::<T>();
         self.entities
             .iter()
@@ -83,7 +84,7 @@ impl<'context, 'game> Context<'context, 'game> {
             .collect()
     }
 
-    pub fn entity_component<T: 'static>(&mut self, entity_id: u64) -> &mut T {
+    pub fn entity_component<T: 'static + Component>(&mut self, entity_id: u64) -> &mut T {
         let entity_type_id = TypeId::of::<T>();
         let Entity(_id, components) = self
             .entities
@@ -111,12 +112,19 @@ impl<'context, 'game> Context<'context, 'game> {
     {
         let path = path.as_ref().to_path_buf();
         let texture: Texture<'game> = unsafe { (*self.texture_creator).load_texture(&path)? };
-        self.textures.insert(path.clone(), texture);
-        Ok(Sprite { path })
+        let id = *self.id_counter;
+        *self.id_counter += 1;
+
+        self.textures.push((id, texture));
+        Ok(Sprite(id))
     }
 
     pub fn draw_sprite(&mut self, sprite: &Sprite, x: i32, y: i32) -> Result<(), Error> {
-        let texture = &self.textures[&sprite.path];
+        let texture = self
+            .textures
+            .iter()
+            .find_map(|v| if v.0 == sprite.0 { Some(&v.1) } else { None })
+            .ok_or_else(|| "invalid sprite id")?;
         self.canvas.copy(
             texture,
             None,
@@ -136,7 +144,7 @@ impl<'context, 'game> Context<'context, 'game> {
         self.entities.push(Entity(id, components));
     }
 
-    pub fn add_system(&mut self, system: Box<dyn System>) {
+    pub fn add_system(&mut self, system: Rc<dyn System>) {
         system.on_add(self);
         self.systems.push(system)
     }
@@ -144,7 +152,9 @@ impl<'context, 'game> Context<'context, 'game> {
 
 pub trait System {
     fn on_add(&self, _ctx: &mut Context) {}
-    fn on_update(&self, _ctx: &mut Context) {}
+    fn on_update(&self, _ctx: &mut Context) -> Result<(), Error> {
+        Ok(())
+    }
 }
 
 pub struct Game<'a> {
@@ -157,8 +167,8 @@ pub struct Game<'a> {
     event_pump: sdl2::EventPump,
     entities: Vec<Entity>,
     components: Vec<(u64, Box<dyn Component>)>,
-    systems: Vec<Box<dyn System>>,
-    textures: HashMap<std::path::PathBuf, Texture<'a>>,
+    systems: Vec<Rc<dyn System>>,
+    textures: Vec<(Id, Texture<'a>)>,
 }
 
 impl<'game> Game<'game> {
@@ -189,11 +199,11 @@ impl<'game> Game<'game> {
             entities: vec![],
             components: vec![],
             systems: vec![],
-            textures: HashMap::new(),
+            textures: vec![],
         })
     }
 
-    pub fn run<F: Fn(&mut Context)>(&mut self, f: F) {
+    pub fn run(&mut self) {
         'running: loop {
             for event in self.event_pump.poll_iter() {
                 match event {
@@ -207,11 +217,15 @@ impl<'game> Game<'game> {
             }
             self.canvas.set_draw_color(Color::RGB(60, 180, 180));
             self.canvas.clear();
-            f(&mut self.context());
+            for system in self.systems.clone() {
+                let Err(err) = system.on_update(&mut self.context()) else {
+                    continue;
+                };
+                println!("error occcurred updating system: {err}");
+            }
             self.canvas.present();
             std::thread::sleep(Duration::new(0, 1_000_000_000u32 / 60))
         }
-        self.canvas.present();
     }
 
     pub fn context<'context>(&'context mut self) -> Context<'context, 'game>
diff --git a/src/main.rs b/src/main.rs
index 73c98cc..6a8a098 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,32 +1,59 @@
 #![allow(dead_code)]
 
 use component_macro::Component;
-use engine::{Component, Sprite, System};
+use engine::{Sprite, System};
 use std::any::{Any, TypeId};
+use std::rc::Rc;
 mod engine;
 
+use engine::Component;
+
 #[derive(Component)]
 struct Player {
     sprite: Option<Sprite>,
 }
 
+#[derive(Component)]
+struct Position(i32, i32);
+
+struct Gravity;
+
+impl System for Gravity {
+    fn on_update(&self, ctx: &mut engine::Context) -> Result<(), engine::Error> {
+        for id in ctx.entities_with_component::<Position>() {
+            let Position(_, y) = ctx.entity_component::<Position>(id);
+            *y += 1;
+        }
+        Ok(())
+    }
+}
+
 struct PlayerRenderer;
 
 impl System for PlayerRenderer {
-    fn on_add(&self, _ctx: &mut engine::Context) {}
-
-    fn on_update(&self, _ctx: &mut engine::Context) {}
+    fn on_update(&self, ctx: &mut engine::Context) -> Result<(), engine::Error> {
+        for id in ctx.entities_with_component::<Player>() {
+            let &mut Position(x, y) = ctx.entity_component::<Position>(id);
+            let player = ctx.entity_component::<Player>(id);
+            let sprite = player.sprite.clone().unwrap();
+            ctx.draw_sprite(&sprite, x, y)?;
+        }
+        Ok(())
+    }
 }
 
 fn main() {
     let mut game = engine::Game::new().unwrap();
 
     let mut context = game.context();
-    context.add_system(Box::new(PlayerRenderer));
-    context.spawn(vec![Box::new(Player { sprite: None })]);
-
-    game.run(|context| {
-        let sprite = context.load_sprite("textures/player.png").unwrap();
-        context.draw_sprite(&sprite, 16, 16).unwrap();
-    });
+    context.add_system(Rc::new(PlayerRenderer));
+    context.add_system(Rc::new(Gravity));
+    let sprite = context.load_sprite("textures/player.png").unwrap();
+    context.spawn(vec![
+        Box::new(Player {
+            sprite: Some(sprite),
+        }),
+        Box::new(Position(16, 0)),
+    ]);
+    game.run();
 }