From 15813ebb6c302a0a287c080eac341a660e7be3eb Mon Sep 17 00:00:00 2001
From: Will Temple <will@wtemple.net>
Date: Fri, 20 Mar 2026 13:27:38 -0400
Subject: [PATCH] Phase 3: event/effect

---
 lib/reactivity/src/effect.rs | 180 ++++++++++++++++++++++++
 lib/reactivity/src/event.rs  | 236 +++++++++++++++++++++++++++++++
 lib/reactivity/src/lib.rs    |   6 +-
 lib/reactivity/src/thunk.rs  | 264 ++++++++++++++++++++++++++++++++++-
 4 files changed, 682 insertions(+), 4 deletions(-)
 create mode 100644 lib/reactivity/src/effect.rs
 create mode 100644 lib/reactivity/src/event.rs

diff --git a/lib/reactivity/src/effect.rs b/lib/reactivity/src/effect.rs
new file mode 100644
index 0000000..9157cc2
--- /dev/null
+++ b/lib/reactivity/src/effect.rs
@@ -0,0 +1,180 @@
+use std::cell::{Cell, RefCell};
+use std::rc::{Rc, Weak};
+
+use crate::reactor::ObserverHook;
+use crate::{NodeId, Reactor, current};
+
+/// Creates an effect in the current thread's default reactor.
+pub fn effect(f: impl Fn() + 'static) -> EffectHandle {
+    current().effect(f)
+}
+
+/// Creates an effect associated with `reactor`.
+pub fn effect_in(reactor: &Reactor, f: impl Fn() + 'static) -> EffectHandle {
+    reactor.effect(f)
+}
+
+/// Disposable handle for a reactive effect.
+#[derive(Clone)]
+pub struct EffectHandle {
+    inner: Rc<EffectInner>,
+}
+
+impl Reactor {
+    /// Creates an effect associated with this reactor.
+    ///
+    /// The effect is scheduled immediately and then re-scheduled whenever one of its dependencies
+    /// changes.
+    pub fn effect(&self, f: impl Fn() + 'static) -> EffectHandle {
+        EffectHandle::new(self.clone(), f)
+    }
+}
+
+impl EffectHandle {
+    fn new(reactor: Reactor, effect: impl Fn() + 'static) -> Self {
+        let id = reactor.allocate_node();
+        let inner = Rc::new(EffectInner {
+            reactor: reactor.clone(),
+            id,
+            effect: Box::new(effect),
+            scheduled: Cell::new(false),
+            disposed: Cell::new(false),
+            self_ref: RefCell::new(Weak::new()),
+        });
+        *inner.self_ref.borrow_mut() = Rc::downgrade(&inner);
+
+        let observer: Rc<dyn ObserverHook> = inner.clone();
+        reactor.register_observer(id, observer);
+        inner.schedule();
+        Self { inner }
+    }
+
+    /// Disposes the effect immediately.
+    pub fn dispose(&self) {
+        self.inner.dispose();
+    }
+
+    /// Returns `true` if the effect has been disposed.
+    pub fn is_disposed(&self) -> bool {
+        self.inner.disposed.get()
+    }
+}
+
+struct EffectInner {
+    reactor: Reactor,
+    id: NodeId,
+    effect: Box<dyn Fn() + 'static>,
+    scheduled: Cell<bool>,
+    disposed: Cell<bool>,
+    self_ref: RefCell<Weak<EffectInner>>,
+}
+
+impl EffectInner {
+    fn schedule(&self) {
+        if self.disposed.get() || self.scheduled.replace(true) {
+            return;
+        }
+
+        let weak = self.self_ref.borrow().clone();
+        let reactor = self.reactor.clone();
+        reactor.schedule(move || {
+            let Some(inner) = weak.upgrade() else {
+                return;
+            };
+            if inner.disposed.get() {
+                inner.scheduled.set(false);
+                return;
+            }
+
+            inner.scheduled.set(false);
+            inner.reactor.run_in_context(inner.id, || (inner.effect)());
+        });
+    }
+
+    fn dispose(&self) {
+        if self.disposed.replace(true) {
+            return;
+        }
+
+        self.reactor.unregister_observer(self.id);
+        self.reactor.dispose(self.id);
+    }
+}
+
+impl ObserverHook for EffectInner {
+    fn notify(&self) {
+        self.schedule();
+    }
+}
+
+impl Drop for EffectInner {
+    fn drop(&mut self) {
+        self.dispose();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::cell::{Cell as Counter, RefCell};
+    use std::rc::Rc;
+
+    use ruin_runtime::{queue_task, run};
+
+    use crate::{Reactor, cell_in};
+
+    use super::EffectHandle;
+
+    #[test]
+    fn effects_flush_through_microtasks_and_coalesce() {
+        let seen = Rc::new(RefCell::new(Vec::new()));
+        let handle_slot = Rc::new(RefCell::new(None::<EffectHandle>));
+
+        queue_task({
+            let seen = Rc::clone(&seen);
+            let handle_slot = Rc::clone(&handle_slot);
+            move || {
+                let reactor = Reactor::new();
+                let source = cell_in(&reactor, 0usize);
+                let effect = reactor.effect({
+                    let seen = Rc::clone(&seen);
+                    let source = source.clone();
+                    move || seen.borrow_mut().push(source.get())
+                });
+
+                source.set(1);
+                source.set(2);
+                assert!(seen.borrow().is_empty(), "effect should not run inline");
+
+                *handle_slot.borrow_mut() = Some(effect);
+            }
+        });
+
+        run();
+        assert_eq!(&*seen.borrow(), &[2]);
+
+        let reruns = Rc::new(Counter::new(0usize));
+        queue_task({
+            let reruns = Rc::clone(&reruns);
+            let seen = Rc::clone(&seen);
+            let handle_slot = Rc::clone(&handle_slot);
+            move || {
+                let reactor = Reactor::new();
+                let source = cell_in(&reactor, 2usize);
+                let effect = reactor.effect({
+                    let reruns = Rc::clone(&reruns);
+                    let seen = Rc::clone(&seen);
+                    let source = source.clone();
+                    move || {
+                        reruns.set(reruns.get() + 1);
+                        seen.borrow_mut().push(source.get());
+                    }
+                });
+                source.set(3);
+                source.set(4);
+                *handle_slot.borrow_mut() = Some(effect);
+            }
+        });
+        run();
+        assert_eq!(reruns.get(), 1);
+    }
+}
diff --git a/lib/reactivity/src/event.rs b/lib/reactivity/src/event.rs
new file mode 100644
index 0000000..bc807dd
--- /dev/null
+++ b/lib/reactivity/src/event.rs
@@ -0,0 +1,236 @@
+use std::cell::{Cell, RefCell};
+use std::collections::BTreeMap;
+use std::rc::Rc;
+
+use crate::{NodeId, Reactor, current};
+
+type SubscriberFn<T> = dyn Fn(&T) + 'static;
+
+/// Creates an event in the current thread's default reactor.
+pub fn event<T: 'static>() -> Event<T> {
+    current().event()
+}
+
+/// Creates an event associated with `reactor`.
+pub fn event_in<T: 'static>(reactor: &Reactor) -> Event<T> {
+    reactor.event()
+}
+
+/// Creates a reactive draining subscription for `event` in the current reactor.
+pub fn on<T: Clone + 'static>(event: &Event<T>, handler: impl Fn(&T) + 'static) -> Subscription {
+    current().on(event, handler)
+}
+
+/// Creates a reactive draining subscription for `event` associated with `reactor`.
+pub fn on_in<T: Clone + 'static>(
+    reactor: &Reactor,
+    event: &Event<T>,
+    handler: impl Fn(&T) + 'static,
+) -> Subscription {
+    reactor.on(event, handler)
+}
+
+/// Push-style event source.
+#[derive(Clone)]
+pub struct Event<T> {
+    inner: Rc<EventInner<T>>,
+}
+
+/// Disposable subscription handle.
+#[derive(Clone)]
+pub struct Subscription {
+    inner: Rc<SubscriptionInner>,
+}
+
+impl Reactor {
+    /// Creates an event source associated with this reactor.
+    pub fn event<T: 'static>(&self) -> Event<T> {
+        Event::new(self.clone())
+    }
+
+    /// Creates a reactive draining subscription for `event`.
+    pub fn on<T: Clone + 'static>(
+        &self,
+        event: &Event<T>,
+        handler: impl Fn(&T) + 'static,
+    ) -> Subscription {
+        let queue = Rc::new(RefCell::new(Vec::new()));
+        let direct = event.subscribe({
+            let queue = Rc::clone(&queue);
+            move |value| queue.borrow_mut().push(value.clone())
+        });
+        let effect = self.effect({
+            let event = event.clone();
+            let queue = Rc::clone(&queue);
+            move || {
+                event.observe();
+                let drained = {
+                    let mut queued = queue.borrow_mut();
+                    queued.drain(..).collect::<Vec<_>>()
+                };
+                for value in &drained {
+                    handler(value);
+                }
+            }
+        });
+
+        Subscription::new(move || {
+            direct.unsubscribe();
+            effect.dispose();
+        })
+    }
+}
+
+impl<T: 'static> Event<T> {
+    fn new(reactor: Reactor) -> Self {
+        let id = reactor.allocate_node();
+        Self {
+            inner: Rc::new(EventInner {
+                reactor,
+                id,
+                next_subscriber: Cell::new(1),
+                subscribers: RefCell::new(BTreeMap::new()),
+            }),
+        }
+    }
+
+    fn observe(&self) {
+        self.inner.reactor.observe(self.inner.id);
+    }
+
+    /// Emits a value to immediate subscribers, then notifies reactive dependents.
+    pub fn emit(&self, value: T) {
+        let subscribers = self
+            .inner
+            .subscribers
+            .borrow()
+            .values()
+            .cloned()
+            .collect::<Vec<_>>();
+        for subscriber in subscribers {
+            subscriber(&value);
+        }
+        self.inner.reactor.trigger(self.inner.id);
+    }
+
+    /// Adds an immediate subscriber to this event.
+    pub fn subscribe(&self, handler: impl Fn(&T) + 'static) -> Subscription {
+        let id = self.inner.next_subscriber.get();
+        self.inner.next_subscriber.set(id.wrapping_add(1));
+        self.inner
+            .subscribers
+            .borrow_mut()
+            .insert(id, Rc::new(handler) as Rc<SubscriberFn<T>>);
+
+        let inner = Rc::clone(&self.inner);
+        Subscription::new(move || {
+            inner.subscribers.borrow_mut().remove(&id);
+        })
+    }
+}
+
+impl<T> Drop for EventInner<T> {
+    fn drop(&mut self) {
+        self.reactor.dispose(self.id);
+    }
+}
+
+impl Subscription {
+    fn new(cancel: impl Fn() + 'static) -> Self {
+        Self {
+            inner: Rc::new(SubscriptionInner {
+                active: Cell::new(true),
+                cancel: Box::new(cancel),
+            }),
+        }
+    }
+
+    /// Cancels the subscription immediately.
+    pub fn unsubscribe(&self) {
+        self.inner.unsubscribe();
+    }
+
+    /// Returns `true` if the subscription is still active.
+    pub fn is_active(&self) -> bool {
+        self.inner.active.get()
+    }
+}
+
+struct EventInner<T> {
+    reactor: Reactor,
+    id: NodeId,
+    next_subscriber: Cell<usize>,
+    subscribers: RefCell<BTreeMap<usize, Rc<SubscriberFn<T>>>>,
+}
+
+struct SubscriptionInner {
+    active: Cell<bool>,
+    cancel: Box<dyn Fn() + 'static>,
+}
+
+impl SubscriptionInner {
+    fn unsubscribe(&self) {
+        if !self.active.replace(false) {
+            return;
+        }
+        (self.cancel)();
+    }
+}
+
+impl Drop for SubscriptionInner {
+    fn drop(&mut self) {
+        self.unsubscribe();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::cell::RefCell;
+    use std::rc::Rc;
+
+    use ruin_runtime::{queue_task, run};
+
+    use crate::{Reactor, event_in};
+
+    use super::{Subscription, on_in};
+
+    #[test]
+    fn emit_delivers_immediately_and_on_drains_reactively() {
+        let immediate = Rc::new(RefCell::new(Vec::new()));
+        let reactive = Rc::new(RefCell::new(Vec::new()));
+        let keep_alive = Rc::new(RefCell::new(Vec::<Subscription>::new()));
+
+        queue_task({
+            let immediate = Rc::clone(&immediate);
+            let reactive = Rc::clone(&reactive);
+            let keep_alive = Rc::clone(&keep_alive);
+            move || {
+                let reactor = Reactor::new();
+                let event = event_in::<usize>(&reactor);
+
+                let direct = event.subscribe({
+                    let immediate = Rc::clone(&immediate);
+                    move |value| immediate.borrow_mut().push(*value)
+                });
+                let draining = on_in(&reactor, &event, {
+                    let reactive = Rc::clone(&reactive);
+                    move |value| reactive.borrow_mut().push(*value)
+                });
+
+                event.emit(1);
+                event.emit(2);
+
+                assert_eq!(&*immediate.borrow(), &[1, 2]);
+                assert!(
+                    reactive.borrow().is_empty(),
+                    "reactive drain should be deferred"
+                );
+
+                keep_alive.borrow_mut().extend([direct, draining]);
+            }
+        });
+
+        run();
+        assert_eq!(&*reactive.borrow(), &[1, 2]);
+    }
+}
diff --git a/lib/reactivity/src/lib.rs b/lib/reactivity/src/lib.rs
index df5872e..2fb708d 100644
--- a/lib/reactivity/src/lib.rs
+++ b/lib/reactivity/src/lib.rs
@@ -5,11 +5,15 @@
 //! from the edges by updating state or emitting events.
 
 mod cell;
+mod effect;
+mod event;
 mod id;
 mod reactor;
 mod thunk;
 
 pub use cell::{Cell, cell, cell_in};
+pub use effect::{EffectHandle, effect, effect_in};
+pub use event::{Event, Subscription, event, event_in, on, on_in};
 pub use id::NodeId;
 pub use reactor::{ReactCycleError, Reactor, current};
-pub use thunk::{Thunk, thunk, thunk_in};
+pub use thunk::{Memo, Thunk, memo, memo_by, memo_by_in, memo_in, thunk, thunk_in};
diff --git a/lib/reactivity/src/thunk.rs b/lib/reactivity/src/thunk.rs
index 132ee33..ed75951 100644
--- a/lib/reactivity/src/thunk.rs
+++ b/lib/reactivity/src/thunk.rs
@@ -4,6 +4,9 @@ use std::rc::Rc;
 use crate::reactor::ObserverHook;
 use crate::{NodeId, Reactor, current};
 
+type ComputeFn<T> = dyn Fn() -> T + 'static;
+type EqualsFn<T> = dyn Fn(&T, &T) -> bool + 'static;
+
 /// Creates a [`Thunk`] in the current thread's default reactor.
 pub fn thunk<T: 'static>(compute: impl Fn() -> T + 'static) -> Thunk<T> {
     current().thunk(compute)
@@ -14,17 +17,67 @@ pub fn thunk_in<T: 'static>(reactor: &Reactor, compute: impl Fn() -> T + 'static
     reactor.thunk(compute)
 }
 
+/// Creates an equality-aware memo in the current thread's default reactor.
+pub fn memo<T: PartialEq + 'static>(compute: impl Fn() -> T + 'static) -> Memo<T> {
+    current().memo(compute)
+}
+
+/// Creates an equality-aware memo associated with `reactor`.
+pub fn memo_in<T: PartialEq + 'static>(
+    reactor: &Reactor,
+    compute: impl Fn() -> T + 'static,
+) -> Memo<T> {
+    reactor.memo(compute)
+}
+
+/// Creates a comparator-aware memo in the current thread's default reactor.
+pub fn memo_by<T: 'static>(
+    equals: impl Fn(&T, &T) -> bool + 'static,
+    compute: impl Fn() -> T + 'static,
+) -> Memo<T> {
+    current().memo_by(equals, compute)
+}
+
+/// Creates a comparator-aware memo associated with `reactor`.
+pub fn memo_by_in<T: 'static>(
+    reactor: &Reactor,
+    equals: impl Fn(&T, &T) -> bool + 'static,
+    compute: impl Fn() -> T + 'static,
+) -> Memo<T> {
+    reactor.memo_by(equals, compute)
+}
+
 /// Lazy computed node in the reactive graph.
 #[derive(Clone)]
 pub struct Thunk<T> {
     inner: Rc<ThunkInner<T>>,
 }
 
+/// Equality/comparator-aware computed node.
+#[derive(Clone)]
+pub struct Memo<T> {
+    inner: Rc<MemoInner<T>>,
+}
+
 impl Reactor {
     /// Creates a lazy computed thunk associated with this reactor.
     pub fn thunk<T: 'static>(&self, compute: impl Fn() -> T + 'static) -> Thunk<T> {
         Thunk::new(self.clone(), compute)
     }
+
+    /// Creates an equality-aware memo associated with this reactor.
+    pub fn memo<T: PartialEq + 'static>(&self, compute: impl Fn() -> T + 'static) -> Memo<T> {
+        self.memo_by(|left, right| left == right, compute)
+    }
+
+    /// Creates a comparator-aware memo associated with this reactor.
+    pub fn memo_by<T: 'static>(
+        &self,
+        equals: impl Fn(&T, &T) -> bool + 'static,
+        compute: impl Fn() -> T + 'static,
+    ) -> Memo<T> {
+        Memo::new(self.clone(), equals, compute)
+    }
 }
 
 impl<T: 'static> Thunk<T> {
@@ -61,10 +114,49 @@ impl<T: Clone + 'static> Thunk<T> {
     }
 }
 
+impl<T: 'static> Memo<T> {
+    fn new(
+        reactor: Reactor,
+        equals: impl Fn(&T, &T) -> bool + 'static,
+        compute: impl Fn() -> T + 'static,
+    ) -> Self {
+        let id = reactor.allocate_node();
+        let inner = Rc::new(MemoInner {
+            reactor: reactor.clone(),
+            id,
+            compute: Box::new(compute),
+            equals: Box::new(equals),
+            value: RefCell::new(None),
+            dirty: Cell::new(true),
+        });
+
+        let observer: Rc<dyn ObserverHook> = inner.clone();
+        reactor.register_observer(id, observer);
+        Self { inner }
+    }
+
+    /// Runs `f` with a shared reference to the current computed value.
+    pub fn with<R>(&self, f: impl FnOnce(&T) -> R) -> R {
+        self.inner.reactor.observe(self.inner.id);
+        self.inner.ensure_value();
+        let value = self.inner.value.borrow();
+        f(value
+            .as_ref()
+            .expect("memo should have a cached value after recomputing"))
+    }
+}
+
+impl<T: Clone + 'static> Memo<T> {
+    /// Clones and returns the current computed value.
+    pub fn get(&self) -> T {
+        self.with(T::clone)
+    }
+}
+
 struct ThunkInner<T> {
     reactor: Reactor,
     id: NodeId,
-    compute: Box<dyn Fn() -> T + 'static>,
+    compute: Box<ComputeFn<T>>,
     value: RefCell<Option<T>>,
     dirty: Cell<bool>,
 }
@@ -81,6 +173,36 @@ impl<T> ThunkInner<T> {
     }
 }
 
+struct MemoInner<T> {
+    reactor: Reactor,
+    id: NodeId,
+    compute: Box<ComputeFn<T>>,
+    equals: Box<EqualsFn<T>>,
+    value: RefCell<Option<T>>,
+    dirty: Cell<bool>,
+}
+
+impl<T> MemoInner<T> {
+    fn ensure_value(&self) {
+        if !self.dirty.get() {
+            return;
+        }
+        let _ = self.recompute();
+    }
+
+    fn recompute(&self) -> bool {
+        let next = self.reactor.run_in_context(self.id, || (self.compute)());
+        let mut value = self.value.borrow_mut();
+        let changed = match value.as_ref() {
+            Some(current) => !(self.equals)(current, &next),
+            None => true,
+        };
+        *value = Some(next);
+        self.dirty.set(false);
+        changed
+    }
+}
+
 impl<T: 'static> ObserverHook for ThunkInner<T> {
     fn notify(&self) {
         if self.dirty.replace(true) {
@@ -99,12 +221,37 @@ impl<T> Drop for ThunkInner<T> {
     }
 }
 
+impl<T: 'static> ObserverHook for MemoInner<T> {
+    fn notify(&self) {
+        if self.value.borrow().is_none() {
+            self.dirty.set(true);
+            return;
+        }
+
+        self.dirty.set(true);
+        if self.recompute() {
+            self.reactor.trigger(self.id);
+        }
+    }
+}
+
+impl<T> Drop for MemoInner<T> {
+    fn drop(&mut self) {
+        self.reactor.unregister_observer(self.id);
+        self.reactor.dispose(self.id);
+    }
+}
+
 #[cfg(test)]
 mod tests {
-    use std::cell::Cell as Counter;
+    use std::cell::{Cell as Counter, RefCell};
+    use std::panic::{AssertUnwindSafe, catch_unwind};
     use std::rc::Rc;
 
-    use crate::{Reactor, cell_in, thunk_in};
+    use ruin_runtime::{queue_task, run};
+
+    use super::Thunk;
+    use crate::{Cell, EffectHandle, Memo, Reactor, cell_in, memo_by_in, memo_in, thunk_in};
 
     #[test]
     fn thunk_caches_until_invalidated() {
@@ -170,4 +317,115 @@ mod tests {
         assert_eq!(double_count.get(), 2);
         assert_eq!(label_count.get(), 3);
     }
+
+    #[test]
+    fn cycle_detection_reports_two_thunks_reading_each_other() {
+        let reactor = Reactor::new();
+        let left_slot = Rc::new(RefCell::new(None::<Thunk<i32>>));
+        let right_slot = Rc::new(RefCell::new(None::<Thunk<i32>>));
+
+        let left = thunk_in(&reactor, {
+            let right_slot = Rc::clone(&right_slot);
+            move || {
+                right_slot
+                    .borrow()
+                    .as_ref()
+                    .expect("right thunk should exist")
+                    .get()
+                    + 1
+            }
+        });
+        let right = thunk_in(&reactor, {
+            let left_slot = Rc::clone(&left_slot);
+            move || {
+                left_slot
+                    .borrow()
+                    .as_ref()
+                    .expect("left thunk should exist")
+                    .get()
+                    + 1
+            }
+        });
+
+        *left_slot.borrow_mut() = Some(left.clone());
+        *right_slot.borrow_mut() = Some(right.clone());
+
+        let panic = catch_unwind(AssertUnwindSafe(|| {
+            let _ = left.get();
+        }))
+        .expect_err("mutually recursive thunks should panic");
+
+        let error = panic
+            .downcast::<crate::ReactCycleError>()
+            .expect("panic payload should be ReactCycleError");
+        assert_eq!(error.cycle().len(), 3);
+    }
+
+    #[test]
+    fn memo_suppresses_unchanged_results_and_memo_by_uses_custom_comparator() {
+        let reactive_seen = Rc::new(RefCell::new(Vec::new()));
+        let effect_slot = Rc::new(RefCell::new(None::<EffectHandle>));
+        let source_slot = Rc::new(RefCell::new(None::<Cell<usize>>));
+        let parity_slot = Rc::new(RefCell::new(None::<Memo<usize>>));
+        let bucket_slot = Rc::new(RefCell::new(None::<Memo<usize>>));
+
+        queue_task({
+            let reactive_seen = Rc::clone(&reactive_seen);
+            let effect_slot = Rc::clone(&effect_slot);
+            let source_slot = Rc::clone(&source_slot);
+            let parity_slot = Rc::clone(&parity_slot);
+            let bucket_slot = Rc::clone(&bucket_slot);
+            move || {
+                let reactor = Reactor::new();
+                let source = cell_in(&reactor, 1usize);
+                let parity = memo_in(&reactor, {
+                    let source = source.clone();
+                    move || source.get() % 2
+                });
+                let bucket = memo_by_in(
+                    &reactor,
+                    |left: &usize, right: &usize| left / 10 == right / 10,
+                    {
+                        let source = source.clone();
+                        move || source.get()
+                    },
+                );
+
+                let effect = reactor.effect({
+                    let reactive_seen = Rc::clone(&reactive_seen);
+                    let parity = parity.clone();
+                    let bucket = bucket.clone();
+                    move || {
+                        reactive_seen
+                            .borrow_mut()
+                            .push((parity.get(), bucket.get()))
+                    }
+                });
+
+                *source_slot.borrow_mut() = Some(source);
+                *parity_slot.borrow_mut() = Some(parity);
+                *bucket_slot.borrow_mut() = Some(bucket);
+                *effect_slot.borrow_mut() = Some(effect);
+            }
+        });
+
+        run();
+        assert_eq!(&*reactive_seen.borrow(), &[(1, 1)]);
+
+        queue_task({
+            let source_slot = Rc::clone(&source_slot);
+            move || {
+                let source = source_slot
+                    .borrow()
+                    .as_ref()
+                    .expect("source cell should still be alive")
+                    .clone();
+                source.set(3);
+                source.set(9);
+                source.set(10);
+            }
+        });
+        run();
+        assert_eq!(&*reactive_seen.borrow(), &[(1, 1), (0, 10)]);
+    }
 }