adaptite

Fine-grained reactivity for runite programs.

Adaptite provides reactivity primitives for dependency tracking and incremental computation. Those primitives are:

• Signal<T>: a tracked-state value cell, primitively observable — the "signal" familiar from other fine-grained reactivity libraries.
• effect: a primitive observer that runs once, observes its dependencies, and runs again whenever its dependencies change.
• Thunk<T>: a tracked-state recomputable value defined by a closure, recomputed on read after any of its dependencies change. A Thunk is both an observer and an observable.
• Memo<T>: a Thunk with an equality (or custom comparator) gate — if a recomputation produces an equal value, downstream observers do not re-run.
• Event<T>: a push-style source of events of type T. Immediate subscribers (subscribe) run inline with emit; draining subscriptions (on) queue values and deliver them in order on the microtask flush. Subscriptions cancel on drop.
• Resource<T>: reactive async state — a value fetched by a future, re-fetched (with stale fetches aborted) whenever its tracked inputs change.
• watch: an explicitly-scoped observer — only its source closure is tracked, and its handler runs untracked with the new and previous values.
• scope/on_cleanup/owner: ownership for reactive subgraphs — dispose a whole tree of effects at once, register teardown that runs before an effect re-runs, and re-attach async work to its owner after an .await.
• Observable: the common trait over everything readable (Signal, Thunk, Memo, Resource), with DynObservable<T> for type-erased handles in component APIs.
• Source: a low-level observable node for building custom reactive data structures with sub-container granularity (per-key, per-field), including is_observed for garbage-collecting dependency units nobody reads, and source_with_hooks for tying an external resource — a socket, a file watcher, an upstream subscription — to whether anyone is actually observing the node.
• writable(get, set): a two-way bindable derived value — a memo bundled with a setter that translates assignments into upstream writes. WritableObservable (Observable + set) makes signals and writable computeds interchangeable in component APIs, so a form binding is one handle rather than a (memo, callback) pair.

Adaptite is built for the runite runtime: effects, draining subscriptions, and resources are flushed or spawned on runite's queues, so anything that schedules work must run on a runtime-managed thread. (Pure signal, thunk, and memo graphs can be read and written without a runtime; nothing that reacts can.)

Because that scheduling goes through runite's thread-local queues, adaptite and the application must resolve the same runite: two copies in one dependency tree means two queues, and adaptite's reactive work is flushed by a runtime nobody is driving. Adaptite therefore depends on a single runite minor at a time (runite = "0.2" for adaptite 0.2), and reaching a newer runite minor requires an adaptite release against it. An application should not pin runite itself; take whatever adaptite resolves.

Adaptite does not function across thread boundaries. It tracks dependencies between entities on the same thread only. Async work feeds the graph from the edges by updating signals or emitting events.

The reactivity model

Lazy, glitch-free propagation

Writes are cheap: setting a signal marks its direct dependents dirty and their transitive dependents "check", and nothing recomputes until it is read. On read, a computed node verifies whether its recorded inputs actually changed (refreshing them first) and recomputes only if so. This makes propagation glitch-free — a computation can never observe a half-updated ("torn") view of the graph — and guarantees each node recomputes at most once per change, even in diamond-shaped graphs.

Signal::set suppresses writes of equal values entirely. A Memo whose recomputation produces an equal value (under PartialEq or a custom comparator via memo_by) does not propagate further, so downstream effects skip their re-runs.

Effects and scheduling

Effects never run inline with the write that triggered them. They are queued on the reactor's job queue and flushed on the runtime's microtask queue, so consecutive writes within one task coalesce into a single effect run — batching is implicit. Host integrations that need synchronous propagation (for example, native resize loops) can call Reactor::flush_now.

That default lane is not the only one. effect_with(scheduler, f) hands each ready run to a consumer-supplied EffectScheduler — any Fn(EffectRun) — which decides when it runs. Marking, coalescing, and dependency verification stay in the reactor; only the when moves. Consumers build effect phases out of that: one queue per phase, drained in whatever order and at whatever moment suits the host, so a render lane can run inside a paint callback instead of on the microtask queue. Adaptite deliberately ships no opinion about what the phases are.

use std::cell::RefCell;
use std::rc::Rc;

use adaptite::{EffectRun, Reactor, signal_in};

let reactor = Reactor::new();
let size = signal_in(&reactor, 1);
let painted = Rc::new(RefCell::new(Vec::new()));

// The render lane. Nothing in it runs until the host drains it.
let lane: Rc<RefCell<Vec<EffectRun>>> = Rc::new(RefCell::new(Vec::new()));

let effect = reactor.effect_with(
    { let lane = Rc::clone(&lane); move |ready: EffectRun| lane.borrow_mut().push(ready) },
    { let size = size.clone(); let painted = Rc::clone(&painted);
      move || painted.borrow_mut().push(size.get()) },
);

// Later, inside the host's paint callback:
reactor.external_flush(|| {
    for ready in lane.borrow_mut().drain(..) {
        ready.run();
    }
});
assert_eq!(*painted.borrow(), [1]);
# effect.dispose();

Draining inside Reactor::external_flush gives the whole drain one flush epoch, which keeps the debug divergence guard meaningful across it and reports the drain to diagnostic consumers as a single flush. A run executed outside any flush opens one of its own. EffectRun::run must happen on the reactor's thread — verification and the effect body always do.

Discarding an EffectRun instead of running it is legal: the effect keeps its dirty mark and is scheduled again on its next invalidation, so a lane may drop work for a subtree that is no longer visible without stranding it.

Feedback loops

An effect may write state it depends on, as long as the loop converges — for example clamping a value, normalizing input, or syncing two representations. Convergence is reached when the rewritten value is equal to the existing one and the write is suppressed. A loop that never converges is a bug: in debug builds, an effect that runs more than 100 times in a single flush panics with the effect's creation site instead of hanging the event loop.

Synchronous read cycles (a thunk whose computation reads itself, directly or transitively) have no convergent interpretation and always panic, reporting the cycle path with the source location of each node. For "reduction"-style computations that want their own previous value, use memo_with_prev, which passes the last value into the compute closure without creating a cycle.

Ownership and cleanup

Effects and event subscriptions created during another effect's run (or inside scope(...)) are owned by it: they stay alive without their handles being held and are disposed when the owner re-runs or is disposed. on_cleanup registers teardown against the innermost owner; it runs before the owning effect's next run and on disposal. Top-level effects are owned by their EffectHandle — dropping the last handle disposes the effect, and leak() opts out. Subscription handles follow the same rules. To opt out of adoption entirely, unowned(|| ...) runs a closure with no current owner: everything created inside is kept alive by its handles alone, even when called from inside an effect or scope.

Ownership is established by where code runs, which async work escapes: after an .await, the original owner is no longer on the stack. Capture it first with owner() and re-enter with Owner::run_in so effects created after the suspension are still disposed with their scope.

Error boundaries

One buggy widget's panicking effect otherwise unwinds out of the whole flush. scope_catch(f, on_error) is a scope that confines the blast radius: a panic from any effect it owns, at any depth, is delivered to the handler as an ErrorInfo — payload, message, and the failing effect's creation site — instead of propagating. The nearest enclosing boundary wins, and boundaries nest.

The panicking effect is disposed before the handler runs: its dependency tracking was cut short mid-run, and a panic during dependency verification re-queues it, so leaving it live would re-run and re-panic immediately. The failure is terminal for that effect, and the handler decides what replaces it. Siblings and the rest of the scope keep running.

Boundaries are for bugs. A fetch that 404s or a parse that fails should stay in the graph as a Result value so downstream nodes can react to it. Under panic = "abort" there is nothing to catch and the boundary is never invoked.

Async data

resource(source, fetch) connects the graph to runite's async side: source runs tracked and produces the fetch input; fetch returns a future that is spawned on the runtime. When the input changes (equality-gated) or refetch() is called, a new fetch starts and the superseded one is aborted; a stale completion can never overwrite a newer value. The resource exposes the latest value (None until first completion) and a separately-tracked loading flag, so a UI can render stale data with a spinner during refetch.

The ambient reactor, and state that outlives a component

The free constructors (signal, memo, effect, …) create nodes on the thread's default reactor, obtained from current(). Creating long-lived reactive state this way — outside any component, from a key handler or a registry that owns it for the life of the process — is supported: those nodes join the ambient reactor, which is the same graph a host framework running on that thread flushes. A pane registry owning a signal per process, read by components that come and go, is a correct use of the library.

What is not guaranteed for free is that the ambient reactor stays the same one. The thread default is cached weakly, so it lives only as long as some node, Reactor handle, or guard keeps it alive. If everything referencing it is dropped, the next current() installs a fresh, unrelated reactor — and because writes to a node on an unflushed graph mark dependents stale without scheduling anything, the symptom is "this value changes and nothing reacts", with no panic to point at the cause. Adaptite logs that re-install at warn level on the adaptite::graph target.

An application or framework that owns a graph should make it a fact rather than a coincidence:

// Once, for the lifetime of the application. The guard holds a *strong*
// reference, so the reactor cannot expire and be replaced underneath you.
let reactor = adaptite::Reactor::new();
let guard = reactor.enter();

// Anything created from here on joins that reactor, with no handle in scope.
assert_eq!(adaptite::current().id(), reactor.id());
let pane_title = adaptite::signal(String::from("shell"));
assert_eq!(pane_title.get(), "shell");

drop(guard);

and try_current() returns Option<Reactor> without installing anything, for code where a missing reactor should be an error rather than a new graph. Two handles address the same graph exactly when Reactor::id() matches, which is how a consumer confirms its state landed where it expected.

Deriving without the clone dance

Every closure over a reactive handle otherwise starts with let x = x.clone();. Observable::map internalizes that for the dominant case — deriving one value from another:

use adaptite::{Observable, signal};

let base = signal(2);
let doubled = base.map(|value| value * 2);   // no manual clone; `base` stays usable

assert_eq!(doubled.get(), 4);
base.set(5);
assert_eq!(doubled.get(), 10);

The result is an ordinary Memo, equality-suppressed like any other, built on the receiver's own reactor rather than the thread default. It does not help an effect body that captures several handles; that case is still explicit, and a clone! macro is deliberately deferred until real usage shows it is needed.

Untracked reads

untrack(|| ...) suspends dependency recording, and signal.peek() / with_peek(...) read a single value without recording. Computed nodes are still brought up to date before an untracked read.

Examples

Observe a signal using an effect

use std::time::Duration;

use adaptite::{effect, signal};
use runite::{main, time::set_timeout};

#[main]
fn main() {
    // Creates an observable value. Calling `.get` from within an observer will create a dependency, and calling `.set`
    // will trigger updates to any dependent observers.
    let v = signal(5);

    // Creates an observer that prints the value of `v` whenever it changes.
    // Calling `.leak()` on the effect handle allows it to run for the lifetime of the program without automatically
    // disposing when dropped.
    effect({
        let v = v.clone();
        move || {
            println!("v is: {}", v.get());
        }
    })
    .leak();

    // Schedule a callback to run after 5 seconds and update `v`. This will trigger
    // the effect to run again and print the new value.
    set_timeout(Duration::from_secs(5), {
        let v = v.clone();
        move || {
            v.set(v.get() + 20);
        }
    });
}

Observe a thunk using an effect

use std::time::Duration;

use adaptite::{effect, signal, thunk};
use runite::{
    main,
    time::{set_interval, set_timeout},
};

#[main]
fn main() {
    // Two primitive observable values.
    let x = signal(5);
    let y = signal(10);

    // A derived observable value that depends on `x` and `y`. The closure will only run when `x` or `y` change, and the
    // result will be cached until then.
    let z = thunk({
        let x = x.clone();
        let y = y.clone();
        move || {
            println!("calculating z...");
            x.get() + y.get()
        }
    });

    // The effect observes `z`, so it will run whenever `z` changes. Because `z` depends on `x` and `y`, the effect will
    // run whenever `x` or `y` change.
    effect({
        let z = z.clone();
        move || {
            println!("z is: {}", z.get());
        }
    })
    .leak();

    // Update `x` and `y` every second. This will trigger the effect to run and print the new value of `z`.
    let interval = set_interval(Duration::from_secs(1), {
        let x = x.clone();
        let y = y.clone();
        move || {
            x.update(|value| *value += 1);
            y.update(|value| *value += 2);
        }
    });

    // After 10 seconds, clear the interval to stop updating `x` and `y`. Once the interval is cleared, the queue will
    // empty and the program will exit since there are no more pending tasks.
    set_timeout(Duration::from_secs(10), move || {
        println!("clearing interval...");
        interval.cancel();
    });
}

Use an event to handle intra-thread messaging

use std::{cell::Cell, rc::Rc, time::Duration};

use adaptite::event;
use runite::{
    main,
    time::{set_interval, sleep},
};

#[main]
fn main() {
    let my_event = event::<String>();

    // Subscriptions are cancelled when dropped; leak this one so it lives for the whole program.
    my_event
        .subscribe(|message| {
            println!("got event with message: {message}");
        })
        .leak();

    // Emit an event every 250ms with an incrementing count.
    let interval = set_interval(Duration::from_millis(250), {
        let counter = Rc::new(Cell::new(0));
        move || {
            let count = counter.get();
            my_event.emit(format!("the count is {}", count));
            counter.set(count + 1);
        }
    });

    // After 5 seconds, clear the interval to stop emitting events.
    runite::spawn(async move {
        sleep(Duration::from_secs(5)).await;
        interval.cancel();
    });
}

More complete demonstrations live in examples/: a dependency-switching spreadsheet, a streaming stock ticker with running statistics and threshold alerts, and an ownership-based screen router.

Tracing

Adaptite emits tracing diagnostics under the targets adaptite::graph, adaptite::signal, adaptite::thunk, adaptite::memo, adaptite::effect, adaptite::event, adaptite::scope, and adaptite::resource. See examples/tracing_subscriber_showcase.rs for a suggested subscriber setup.

Causal diagnostics

Performance tools can subscribe to a reactor's structured scheduling stream:

use adaptite::{DiagnosticEvent, Reactor};

let reactor = Reactor::new();
let subscription = reactor.subscribe_diagnostics(|event| {
    if let DiagnosticEvent::ReactiveWrite { cause, .. } = event {
        eprintln!(
            "node {} created at {} changed at {}",
            cause.node.get(),
            cause.node_origin,
            cause.write_origin
        );
    }
});

The stream preserves the root write through thunk and memo propagation and reports effect invalidation, queue coalescing, flush epochs, effect run/skip/dispose outcomes, and source locations. Delivery is synchronous on the reactor thread. Callbacks should append the event to an external trace sink and return without reading or mutating the graph.

Diagnostics are available in release builds. Without a subscription, the path is dormant and mutation/scheduling sites perform only a boolean check. Dropping DiagnosticSubscription removes the callback.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT license (LICENSE-MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.