Coupling paths: UI-to-UI and UI-to-backend

egui_citizen gives you two distinct ways for a state change in one place to influence work somewhere else. They serve different jobs, they have different timing, and a single widget — what we'll call an atom (a widget inside a citizen panel) — can use either path or both at once.

Get this vocabulary clear before you design anything substantial.

The two paths

Path A — shared Dynamic<T> (panel to panel)

Two panels share a clone of the same Dynamic<T>. One writes, the other reads. That's the whole mechanism.

// settings_panel.rs
struct SettingsPanelState {
    pub slider_value: Dynamic<f32>,
}

// logger_panel.rs
struct LoggerPanelState {
    pub observed_slider: Dynamic<f32>, // clone of settings.slider_value
}

The two Dynamic<f32> handles point at the same Arc<Mutex<f32>>. When settings calls .set(...), the write lands in shared memory. When logger.ui() runs on the next frame and calls .get(), it sees the new value.

No subscription, no callback, no event bus. egui already redraws frequently enough that polling-per-frame is effectively free. Path A carries state, not events.

Path B — dispatcher messages (panel to backend)

The settings panel explicitly enqueues a message; the app's update loop drains it after DockArea::show() and forwards it onward — to a backend thread, a logger sink, a persistence layer.

// In settings_panel.rs, when the slider changes:
dispatcher.send(AppMessage::SliderChanged(local));

// In App::update(), once per frame:
for msg in dispatcher.drain_messages() {
    match msg {
        AppMessage::SliderChanged(v) => tx_backend.send(v).unwrap(),
        // ...
    }
}

Path B carries events, not state. Each .send() enqueues one record; drain_messages() consumes the queue. Nothing is "shared" — the message is a value, not a handle.

Aside on Dynamic::on_change. egui_mobius_reactive does provide a callback-style subscription on Dynamic<T> itself, which looks superficially like a third coupling option. It is — but it spawns one OS thread per subscriber, has no unsubscribe API, and doesn't coalesce wakeups. For most egui_citizen apps, Path B through the dispatcher is the better way to do "react off the UI thread when this value changes." The full mechanics live in Inside Dynamic<T>.

Atoms can wire to both

A single atom can fan out to both paths from the same user event. The fan-out happens at the change handler:

if ui.add(Slider::new(&mut local, 0.0..=100.0)).changed() {
    self.slider_value.set(local);                       // Path A
    dispatcher.send(AppMessage::SliderChanged(local));  // Path B
}

This is a common and correct pattern. The atom is the single write site; each path is a derived consequence of the one event. Readers on Path A see the new value next frame; consumers on Path B get the message on the next drain cycle.

When dual wiring is right

Dual-wire an atom when a change needs to:

• Update shared UI state, and
• Trigger side-effect work (log it, persist it, send it to a backend thread, recompute a derived value off-thread).

Single-path atoms are fine when the change only needs one of those. Don't dual-wire out of habit — extra messages with no consumers are noise.

Source-of-truth discipline

The trap: once an atom writes to both paths, downstream code can read from either one and the two representations can drift. Pick a discipline and hold it.

Discipline 1 — Dynamic<T> is canonical, the message is a ping.

dispatcher.send(AppMessage::SliderChanged);  // no value in the message!
// Consumers re-read `settings.slider_value.get()` when the message arrives.

Consumers of the message reach back into shared state for the current value. This guarantees consistency — there is exactly one value, the one in the Dynamic<f32>. The message says only "something happened, go look."

Discipline 2 — Message carries the value, Dynamic<T> is a UI mirror.

dispatcher.send(AppMessage::SliderChanged(local));

The message is the canonical record of the event. The Dynamic<f32> exists only so other panels can render the current value without intercepting messages. Consumers of the message trust the message and do not re-read the Dynamic.

Either discipline works. Mixing them silently — some consumers trusting the message, others reading the Dynamic — is where bugs live.

Timing

Within a single frame, the two paths do not tick in lockstep:

• Path A is instant. .set(v) returns after writing; any panel calling .get() on a clone sees v immediately.
• Path B is queued. .send(msg) appends to the dispatcher's queue; consumers don't see it until the update loop calls drain_messages().

Backend threads therefore observe the Path B message after the UI has already observed the Path A value. For typical use (the backend does work and replies asynchronously), that one-frame gap is invisible. For anything tighter — a dependency on in-frame ordering between UI and backend — you need to redesign, not lean harder on this.

The dispatcher is not a reactive bus

Worth saying explicitly, because it is the mistake everyone makes coming from other reactive systems:

The dispatcher does not observe Dynamic<T> writes.

It only knows about the CitizenState fields it registered, and its queue only fills from activate() and explicit send(). Your slider's Dynamic<f32> on SettingsPanelState is invisible to it until the panel explicitly bridges the two paths with a .send() call.

Shared state (Path A) and dispatcher messages (Path B) compose — they don't chain automatically.

Summary

• Two coupling paths. Path A for UI-to-UI state sharing (shared Dynamic<T>). Path B for UI-to-backend events (dispatcher.send() + drain_messages()).
• Atoms can wire to one path or both. Fan-out happens at the write site, not downstream.
• Dual-wired atoms require a source-of-truth discipline: Dynamic canonical with the message as a ping, or message canonical with the Dynamic as a UI mirror. Don't mix.
• Path A is in-frame; Path B lands at the next drain. Backend threads see changes one frame after the UI does.
• The dispatcher is a lifecycle registry plus an explicit outbound queue. It is not an automatic observer of reactive state.