diff --git a/src/core/sense.rs b/src/core/sense.rs
index c598b04..d1789f9 100644
--- a/src/core/sense.rs
+++ b/src/core/sense.rs
@@ -1,34 +1,57 @@
-use std::marker::PhantomData;
+use crate::{
+    Painter, Sense, SenseFn, SenseShape, SenseTrigger, Ui, Widget, WidgetFn, WidgetId, WidgetLike,
+};
 
-use crate::{Painter, Widget, WidgetFn, WidgetId, WidgetLike};
-
-pub struct Sensor<F: SenseFn<Ctx>, Ctx> {
+pub struct Sensor<Ctx> {
     inner: WidgetId,
-    f: F,
-    _pd: PhantomData<Ctx>,
+    sense: Sense,
+    f: Box<dyn SenseFn<Ctx>>,
 }
 
-impl<F: SenseFn<Ctx>, Ctx: 'static> Widget<Ctx> for Sensor<F, Ctx> {
+impl<Ctx: 'static> Widget<Ctx> for Sensor<Ctx> {
     fn draw(&self, painter: &mut Painter<Ctx>) {
-        (self.f)(painter.ctx_mut());
-        painter.draw(&self.inner);
+        painter.sense(
+            SenseTrigger {
+                shape: painter.region,
+                sense: self.sense,
+            },
+            self.f.box_clone(),
+        );
+        painter.draw(self.inner.as_any());
     }
 }
 
-pub trait SenseFn<Ctx> = Fn(&mut Ctx) + 'static;
-
-pub trait Sensable<Ctx: 'static, Tag> {
-    fn sense<F: SenseFn<Ctx>>(self, f: F) -> impl WidgetFn<Sensor<F, Ctx>, Ctx>;
+pub trait SenseCtx: 'static {
+    fn active(&mut self, trigger: &SenseTrigger) -> bool;
 }
 
-impl<W: WidgetLike<Ctx, Tag>, Ctx: 'static, Tag> Sensable<Ctx, Tag> for W {
-    fn sense<F: SenseFn<Ctx>>(self, f: F) -> impl WidgetFn<Sensor<F, Ctx>, Ctx> {
-        |ui| {
-            let inner = self.add(ui).erase_type();
+pub trait WidgetSenseFn<W: Widget<Ctx>, Ctx> = Fn(&WidgetId<W>, &mut Ui<Ctx>, &mut Ctx) + 'static;
+
+pub trait Sensable<W: Widget<Ctx>, Ctx: 'static, Tag> {
+    // copied here so LSP can at least get the UI and id
+    fn sense<F: Fn(&WidgetId<W>, &mut Ui<Ctx>, &mut Ctx) + 'static + Clone>(
+        self,
+        sense: Sense,
+        f: F,
+    ) -> impl WidgetFn<Sensor<Ctx>, Ctx>;
+}
+
+impl<W: WidgetLike<Ctx, Tag>, Ctx: SenseCtx, Tag> Sensable<W::Widget, Ctx, Tag> for W
+where
+    W::Widget: Widget<Ctx>,
+{
+    fn sense<F: WidgetSenseFn<W::Widget, Ctx> + Clone>(
+        self,
+        sense: Sense,
+        f: F,
+    ) -> impl WidgetFn<Sensor<Ctx>, Ctx> {
+        move |ui| {
+            let inner_arg = self.add(ui);
+            let inner = inner_arg.clone().erase_type();
             Sensor {
                 inner,
-                f,
-                _pd: PhantomData,
+                sense,
+                f: Box::new(move |ui, ctx| (f)(&inner_arg, ui, ctx)),
             }
         }
     }
diff --git a/src/layout/painter.rs b/src/layout/painter.rs
index 38bf6ab..e92e05f 100644
--- a/src/layout/painter.rs
+++ b/src/layout/painter.rs
@@ -1,20 +1,22 @@
 use crate::{
-    UiRegion, WidgetId, Widgets,
+    SenseFn, SenseTrigger, Sensors, UiRegion, WidgetId, Widgets,
     primitive::{PrimitiveData, PrimitiveInstance, Primitives},
 };
 
 pub struct Painter<'a, Ctx> {
     nodes: &'a Widgets<Ctx>,
     ctx: &'a mut Ctx,
+    sensors: &'a mut Sensors<Ctx>,
     primitives: Primitives,
     pub region: UiRegion,
 }
 
 impl<'a, Ctx> Painter<'a, Ctx> {
-    pub fn new(nodes: &'a Widgets<Ctx>, ctx: &'a mut Ctx) -> Self {
+    pub fn new(nodes: &'a Widgets<Ctx>, ctx: &'a mut Ctx, sensors: &'a mut Sensors<Ctx>) -> Self {
         Self {
             nodes,
             ctx,
+            sensors,
             primitives: Primitives::default(),
             region: UiRegion::full(),
         }
@@ -31,17 +33,27 @@ impl<'a, Ctx> Painter<'a, Ctx> {
             .extend_from_slice(bytemuck::cast_slice::<_, u32>(&[data]));
     }
 
-    pub fn draw(&mut self, id: &WidgetId) where Ctx: 'static {
+    pub fn draw(&mut self, id: &WidgetId)
+    where
+        Ctx: 'static,
+    {
         self.nodes.get_dyn(id).draw(self);
     }
 
-    pub fn draw_within(&mut self, node: &WidgetId, region: UiRegion) where Ctx: 'static {
+    pub fn draw_within(&mut self, node: &WidgetId, region: UiRegion)
+    where
+        Ctx: 'static,
+    {
         let old = self.region;
         self.region.select(&region);
         self.draw(node);
         self.region = old;
     }
 
+    pub fn sense(&mut self, trigger: SenseTrigger, f: Box<dyn SenseFn<Ctx>>) {
+        self.sensors.push((trigger, f));
+    }
+
     pub fn finish(self) -> Primitives {
         self.primitives
     }
diff --git a/src/layout/region.rs b/src/layout/region.rs
index bc61169..bc0e275 100644
--- a/src/layout/region.rs
+++ b/src/layout/region.rs
@@ -149,6 +149,26 @@ impl UiRegion {
         self.bot_right.flip();
         std::mem::swap(&mut self.top_left, &mut self.bot_right);
     }
+
+    pub fn to_screen(&self, size: Vec2) -> ScreenRect {
+        ScreenRect {
+            top_left: self.top_left.anchor * size + self.top_left.offset,
+            bot_right: self.bot_right.anchor * size + self.bot_right.offset,
+        }
+    }
+}
+
+pub struct ScreenRect {
+    top_left: Vec2,
+    bot_right: Vec2,
+}
+impl ScreenRect {
+    pub fn contains(&self, pos: Vec2) -> bool {
+        pos.x >= self.top_left.x
+            && pos.x <= self.bot_right.x
+            && pos.y >= self.top_left.y
+            && pos.y <= self.bot_right.y
+    }
 }
 
 pub struct UIRegionAxisView<'a> {
diff --git a/src/layout/ui.rs b/src/layout/ui.rs
index 1dc1a82..250e7d7 100644
--- a/src/layout/ui.rs
+++ b/src/layout/ui.rs
@@ -1,5 +1,5 @@
 use crate::{
-    HashMap, Painter, Widget, WidgetId, WidgetLike,
+    HashMap, Painter, SenseCtx, UiRegion, Widget, WidgetId, WidgetLike,
     primitive::Primitives,
     util::{IDTracker, Id},
 };
@@ -13,6 +13,7 @@ pub struct Ui<Ctx> {
     base: Option<WidgetId>,
     widgets: Widgets<Ctx>,
     updates: Vec<WidgetId>,
+    sensors: Sensors<Ctx>,
     primitives: Primitives,
     full_redraw: bool,
 }
@@ -20,6 +21,27 @@ pub struct Ui<Ctx> {
 #[derive(Default)]
 pub struct Widgets<Ctx>(HashMap<Id, Box<dyn Widget<Ctx>>>);
 
+#[derive(Clone, Copy)]
+pub enum Sense {
+    Click,
+    Hover,
+}
+pub type Sensors<Ctx> = Vec<(SenseTrigger, Box<dyn SenseFn<Ctx>>)>;
+pub trait SenseFn_<Ctx> = Fn(&mut Ui<Ctx>, &mut Ctx) + 'static;
+pub type SenseShape = UiRegion;
+pub struct SenseTrigger {
+    pub shape: SenseShape,
+    pub sense: Sense,
+}
+pub trait SenseFn<Ctx>: SenseFn_<Ctx> {
+    fn box_clone(&self) -> Box<dyn SenseFn<Ctx>>;
+}
+impl<F: SenseFn_<Ctx> + Clone, Ctx> SenseFn<Ctx> for F {
+    fn box_clone(&self) -> Box<dyn SenseFn<Ctx>> {
+        Box::new(self.clone())
+    }
+}
+
 impl<Ctx> Ui<Ctx> {
     pub fn add<W: Widget<Ctx>, Tag>(
         &mut self,
@@ -67,13 +89,27 @@ impl<Ctx> Ui<Ctx> {
     where
         Ctx: 'static,
     {
-        let mut painter = Painter::new(&self.widgets, ctx);
+        self.sensors.clear();
+        let mut painter = Painter::new(&self.widgets, ctx, &mut self.sensors);
         if let Some(base) = &self.base {
             painter.draw(base);
         }
         self.primitives = painter.finish();
     }
 
+    pub fn run_sensors(&mut self, ctx: &mut Ctx)
+    where
+        Ctx: SenseCtx,
+    {
+        for (t, f) in self.sensors.iter().rev() {
+            if ctx.active(t) {
+                let f = f.as_ref().box_clone();
+                f(self, ctx);
+                return;
+            }
+        }
+    }
+
     pub fn update(&mut self, ctx: &mut Ctx) -> Option<&Primitives>
     where
         Ctx: 'static,
@@ -94,8 +130,6 @@ impl<Ctx> Ui<Ctx> {
     pub fn needs_redraw(&self) -> bool {
         self.full_redraw || !self.updates.is_empty()
     }
-
-    pub fn set_mouse_pos(&mut self) {}
 }
 
 impl<W: Widget<Ctx>, Ctx> Index<&WidgetId<W>> for Ui<Ctx> {
@@ -157,7 +191,8 @@ impl<Ctx> Default for Ui<Ctx> {
             widgets: Widgets::new(),
             updates: Default::default(),
             primitives: Default::default(),
-            full_redraw: Default::default(),
+            full_redraw: false,
+            sensors: Default::default(),
         }
     }
 }
diff --git a/src/layout/widget.rs b/src/layout/widget.rs
index f8d3df4..81ccc90 100644
--- a/src/layout/widget.rs
+++ b/src/layout/widget.rs
@@ -16,6 +16,7 @@ pub struct AnyWidget;
 ///
 /// W does not need to implement widget so that AnyWidget is valid;
 /// Instead, add generic bounds on methods that take an ID if they need specific data.
+#[repr(C)]
 #[derive(Eq, Hash, PartialEq, Debug)]
 pub struct WidgetId<W = AnyWidget> {
     pub(super) ty: TypeId,
@@ -38,10 +39,16 @@ impl<W> WidgetId<W> {
             _pd: PhantomData,
         }
     }
+
     pub fn erase_type(self) -> WidgetId<AnyWidget> {
         self.cast_type()
     }
 
+    pub fn as_any(&self) -> &WidgetId<AnyWidget> {
+        // safety: self is repr(C) and generic only used for phantom data
+        unsafe { std::mem::transmute(self) }
+    }
+
     fn cast_type<W2>(self) -> WidgetId<W2> {
         WidgetId {
             ty: self.ty,
diff --git a/src/testing/app.rs b/src/testing/app.rs
index 5d097d1..a738825 100644
--- a/src/testing/app.rs
+++ b/src/testing/app.rs
@@ -1,3 +1,4 @@
+use gui::Ui;
 use winit::{
     application::ApplicationHandler,
     event::WindowEvent,
@@ -9,7 +10,7 @@ use super::Client;
 
 #[derive(Default)]
 pub struct App {
-    client: Option<Client>,
+    client: Option<(Client, Ui<Client>)>,
 }
 
 impl App {
@@ -25,12 +26,14 @@ impl ApplicationHandler for App {
             let window = event_loop
                 .create_window(Window::default_attributes())
                 .unwrap();
-            let client = Client::new(window.into());
-            self.client = Some(client);
+            let (ui, ids) = Client::create_ui();
+            let client = Client::new(window.into(), ids);
+            self.client = Some((client, ui));
         }
     }
 
     fn window_event(&mut self, event_loop: &ActiveEventLoop, _id: WindowId, event: WindowEvent) {
-        self.client.as_mut().unwrap().event(event, event_loop);
+        let (client, ui) = self.client.as_mut().unwrap();
+        client.event(event, event_loop, ui);
     }
 }
diff --git a/src/testing/input.rs b/src/testing/input.rs
new file mode 100644
index 0000000..da40e31
--- /dev/null
+++ b/src/testing/input.rs
@@ -0,0 +1,58 @@
+use gui::{SenseCtx, SenseTrigger, Vec2};
+use winit::event::WindowEvent;
+
+use crate::testing::Client;
+
+#[derive(Default)]
+pub struct Input {
+    size: Vec2,
+    mouse_pos: Vec2,
+    mouse_pressed: bool,
+    mouse_just_pressed: bool,
+}
+
+impl Input {
+    pub fn event(&mut self, event: &WindowEvent) {
+        self.mouse_just_pressed = false;
+        match event {
+            WindowEvent::Resized(size) => {
+                self.size = Vec2::new(size.width as f32, size.height as f32)
+            }
+            WindowEvent::CursorMoved { position, .. } => {
+                self.mouse_pos = Vec2::new(position.x as f32, position.y as f32)
+            }
+            WindowEvent::MouseInput { state, button, .. } => match button {
+                winit::event::MouseButton::Left => {
+                    if state.is_pressed() {
+                        if !self.mouse_pressed {
+                            self.mouse_just_pressed = true;
+                        } else {
+                            self.mouse_just_pressed = false;
+                        }
+                        self.mouse_pressed = true;
+                    } else {
+                        self.mouse_pressed = false;
+                    }
+                }
+                _ => (),
+            },
+            _ => (),
+        }
+    }
+    fn active(&mut self, trigger: &SenseTrigger) -> bool {
+        let region = trigger.shape.to_screen(self.size);
+        if !region.contains(self.mouse_pos) {
+            return false;
+        }
+        match trigger.sense {
+            gui::Sense::Click => self.mouse_just_pressed,
+            gui::Sense::Hover => true,
+        }
+    }
+}
+
+impl SenseCtx for Client {
+    fn active(&mut self, trigger: &SenseTrigger) -> bool {
+        self.input.active(trigger)
+    }
+}
diff --git a/src/testing/mod.rs b/src/testing/mod.rs
index a530518..759d832 100644
--- a/src/testing/mod.rs
+++ b/src/testing/mod.rs
@@ -5,39 +5,44 @@ use gui::*;
 use render::Renderer;
 use winit::{event::WindowEvent, event_loop::ActiveEventLoop, window::Window};
 
+use crate::testing::input::Input;
+
 mod app;
+mod input;
 mod render;
 
 pub fn main() {
     App::run();
 }
 
-struct Data {
-    x: u32,
-}
-
 pub struct Client {
     renderer: Renderer,
-    ui: Ui<Data>,
+    input: Input,
+    ui: UiIds,
+}
+
+pub struct UiIds {
     test: WidgetId<Span>,
 }
 
 impl Client {
-    pub fn new(window: Arc<Window>) -> Self {
-        let renderer = Renderer::new(window);
+    pub fn create_ui() -> (Ui<Self>, UiIds) {
+        let mut ui = Ui::new();
+        let test = ui.id();
         let rect = Rect {
             color: UiColor::WHITE,
             radius: 20.0,
             thickness: 0.0,
             inner_radius: 0.0,
         };
-        let mut ui = Ui::new();
-        let test = ui.id();
         ui.set_base(
             (
                 (
                     rect.color(UiColor::BLUE)
-                        .sense(|d: &mut Data| println!("{}", d.x)),
+                        .sense(Sense::Click, |id, ui, client| {
+                            println!("hello!");
+                            ui[id].color.a -= 1;
+                        }),
                     (
                         rect.color(UiColor::RED).center((100.0, 100.0)),
                         (
@@ -75,26 +80,33 @@ impl Client {
                 .span(Dir::DOWN, [3, 1, 1])
                 .pad(10),
         );
-        Self { renderer, ui, test }
+        (ui, UiIds { test })
     }
 
-    pub fn event(&mut self, event: WindowEvent, event_loop: &ActiveEventLoop) {
+    pub fn new(window: Arc<Window>, ui: UiIds) -> Self {
+        let renderer = Renderer::new(window);
+        Self {
+            renderer,
+            ui,
+            input: Input::default(),
+        }
+    }
+
+    pub fn event(&mut self, event: WindowEvent, event_loop: &ActiveEventLoop, ui: &mut Ui<Self>) {
+        self.input.event(&event);
+        ui.run_sensors(self);
         match event {
             WindowEvent::CloseRequested => event_loop.exit(),
             WindowEvent::RedrawRequested => {
-                let primitives = self.ui.update(&mut Data { x: 39 });
+                let primitives = ui.update(self);
                 self.renderer.update(primitives);
                 self.renderer.draw()
             }
             WindowEvent::Resized(size) => self.renderer.resize(&size),
-            WindowEvent::KeyboardInput { event, .. } => {
-                if event.state.is_pressed() {
-                    let child = self.ui.add(Rect::new(Color::YELLOW)).erase_type();
-                    self.ui[&self.test].children.push((child, fixed(20.0)));
-                    self.renderer.window().request_redraw();
-                }
-            }
             _ => (),
         }
+        if ui.needs_redraw() {
+            self.renderer.window().request_redraw();
+        }
     }
 }