Common pitfalls

Six foot-guns that real egui_citizen apps hit. Each is presented as a concrete broken snippet, an explanation of why it fails, and the fix. None of them produce a panic or compile error — every one is a silent bug, which is what makes them worth their own chapter.

1. Constructing `CitizenState` fresh per frame

Broken:

match tab.kind {
    TabKind::Drc => DrcPanel::new(CitizenState::default())
                       .show(ui, &mut self),
}

What goes wrong: CitizenState::default() allocates fresh Arc<Mutex<...>> storage that the dispatcher knows nothing about. The dispatcher's activate(&drc_id) writes to its table; the panel reads from its freshly-allocated state; the two never agree. The DRC tab still highlights when clicked (egui_dock handles its own visual state), but anything reading drc_state.active.get() from elsewhere in the app sees false forever. Reactivity is silently severed.

Fix: obtain the CitizenState from Dispatcher::register(), store it somewhere durable (the app struct), and clone it into the per-frame panel:

struct App {
    dispatcher: Dispatcher,
    drc_state:  CitizenState,    // registered once, lives on the app
}

impl App {
    fn new(_: &eframe::CreationContext) -> Self {
        let mut dispatcher = Dispatcher::new();
        let drc_state = dispatcher.register(CitizenId::new("drc"));
        Self { dispatcher, drc_state }
    }
}

// In TabViewer:
match tab.kind {
    TabKind::Drc => DrcPanel::new(self.drc_state.clone())
                       .show(ui, &mut self),
}

Panels can be stateless; their CitizenState cannot. See Stored vs stateless panels and Reactive lifecycle: the trap.

2. Forgetting `drain_messages()`

Broken:

fn update(&mut self, ctx: &egui::Context, _: &mut eframe::Frame) {
    DockArea::new(&mut self.tabs).show(ctx, &mut self.tab_viewer);
    // (no dispatcher.drain_messages() anywhere)
}

What goes wrong: Dispatcher::activate() and any explicit Dispatcher::send() calls push into an internal Vec<CitizenMessage> that has no upper bound. If nothing drains it, the vec grows forever. The app keeps running, the UI keeps rendering, but RSS climbs every minute the user holds the app open. No panic, no error log — just a slow leak.

Fix: drain once per frame, after DockArea::show():

fn update(&mut self, ctx: &egui::Context, _: &mut eframe::Frame) {
    DockArea::new(&mut self.tabs).show(ctx, &mut self.tab_viewer);

    for msg in self.dispatcher.drain_messages() {
        // process or forward
    }
}

If you have nothing to do with the messages yet, drain into an ignored binding (let _ = self.dispatcher.drain_messages();) so the queue still empties. Don't leave the dispatcher's queue unattended — ever.

3. Calling `activate()` every frame unconditionally

Broken:

fn ui(&mut self, ui: &mut egui::Ui, tab: &mut Tab) {
    // The author wanted "this tab is rendering, mark it active":
    self.dispatcher.activate(&tab.citizen_id());
    tab.show(ui, self.app);
}

What goes wrong: ui() runs every frame for every visible tab. Each call to activate() emits an Activated message — even when the citizen was already active in the previous frame. The queue fills with redundant Activated events at the frame rate (60+/sec). Backend consumers see "fetch was just activated" 60 times a second and either dedupe defensively or kick off 60 fetches.

When two tabs are visible side-by-side, it's worse: each frame deactivates the other, so consumers see a flood of Activated/Deactivated pairs in alternation.

Fix: call activate() from TabViewer::on_tab_button gated on response.clicked(), never from ui():

fn on_tab_button(&mut self, tab: &mut Tab, response: &egui::Response) {
    if response.clicked() {
        self.dispatcher.activate(&tab.citizen_id());
    }
}

fn ui(&mut self, ui: &mut egui::Ui, tab: &mut Tab) {
    // ui() only renders. It does not write lifecycle state.
    tab.show(ui, self.app);
}

This is the single most common foot-gun and the one the problem chapter is built around. ui() is for rendering; on_tab_button is for state transitions. Keep them separate.

4. Mixing panel-local state into `CitizenState`

Broken:

// Trying to model "this panel's slider value" through CitizenState
// (which has fixed library-defined fields):
self.citizen_state.active.set(true);  // hijacking `active` as
                                      // "is showing project X"

…or, equivalently:

struct LoggerPanel {
    citizen_id: CitizenId,
    citizen_state: CitizenState,
    log_buffer: Dynamic<Vec<LogEntry>>,  // overkill; nothing else
                                         // outside the panel reads it
}

What goes wrong: CitizenState has a fixed shape — six Dynamic<T> fields with library-defined semantics (active, clicked, selected, moved, location, visible). Reusing those fields to mean something domain-specific (e.g. active = "showing project X") breaks the dispatcher's invariants the moment you call activate(), which clobbers your overload.

The variant is treating every panel-local field as reactive "because reactivity feels good." Reactivity is not free — every Dynamic<T> is an Arc plus a lock plus a notifier list. If only the panel itself reads its log buffer, a plain Vec<LogEntry> is the right type.

Fix: use a separate PanelState struct alongside CitizenState on the panel struct, and use plain (non-reactive) types unless a second reader actually exists:

struct LoggerPanelState {
    log_buffer: Vec<LogEntry>,
    filter_text: String,
    follow_tail: bool,
}

struct LoggerPanel {
    citizen_id: CitizenId,
    citizen_state: CitizenState,    // library lifecycle, fixed
    panel_state: LoggerPanelState,  // panel-author's fields
}

See Where does state live? for the full three-bucket model.

5. Expecting `visible` to track egui_dock's open/closed state

Broken:

// Assumes `visible` updates automatically when egui_dock opens or
// closes the tab — it doesn't.
if self.plot_state.visible.get() {
    self.start_streaming_data();
}

What goes wrong: egui_citizen and egui_dock are independent crates. egui_dock does not know egui_citizen exists, so it never calls state.visible.set() on its own. The visible field starts at false, stays at false, and your "is the panel showing?" check returns false even when the user is staring at the panel.

Fix: drive visible yourself, either by setting it directly when you detect a tab open/close in TabViewer, or — preferred — by routing through the VisibilityChanged message:

// Detect close in TabViewer:
fn on_close(&mut self, tab: &mut Tab) -> bool {
    self.dispatcher.send(CitizenMessage::VisibilityChanged {
        id: tab.citizen_id(),
        visible: false,
    });
    true
}

// In the drain loop, sync the reactive flag:
for msg in self.dispatcher.drain_messages() {
    if let CitizenMessage::VisibilityChanged { id, visible } = &msg {
        if let Some(state) = self.dispatcher.get(id) {
            state.visible.set(*visible);
        }
    }
}

egui_citizen provides the vocabulary (a Dynamic<bool> field, a message variant). The plumbing from egui_dock's tab-close into that vocabulary is your code's responsibility, by design — it's the boundary that keeps egui_citizen independent of the dock crate.

6. Two dispatchers in one app

Broken:

struct App {
    plot_dispatcher:     Dispatcher,
    settings_dispatcher: Dispatcher,
    /* ... */
}

What goes wrong: the one-hot activation invariant is per-dispatcher. plot_dispatcher.activate(&plot_id) deactivates every other citizen registered with plot_dispatcher — but it cannot deactivate a citizen registered with settings_dispatcher, because the two dispatchers maintain entirely separate HashMap<CitizenId, CitizenState> tables. Two panels — one registered to each dispatcher — can both be "active" simultaneously, which the rest of the codebase does not expect.

Fix: one Dispatcher per app. Always.

struct App {
    dispatcher: Dispatcher,    // exactly one
    /* ... */
}

If you find yourself wanting a second dispatcher because "these panels are unrelated to those panels," the right answer is still one dispatcher with all panels registered. Citizen ids are namespaced strings — use "editor.plot", "sidebar.settings", etc., to disambiguate. The dispatcher does not care about logical grouping; it only enforces the one-hot invariant across everything it knows about.

Summary

Five of the six foot-guns above are silent — no panic, no compile error, just behavior that drifts from what the author expected. The defenses are vocabulary-level: hold the CitizenState somewhere durable, drain the queue every frame, never write lifecycle state in ui(), keep panel-local data out of CitizenState, drive visible yourself, and never run two dispatchers in one app. Once these become habits, the rest of egui_citizen works out of the box.

egui-citizen