Combo box (external piece)

Authoring

use day_piece_combobox::combo_box;

let flavors = Signal::new(vec!["vanilla".into(), "chocolate".into()]);
let flavor = Signal::new(String::new());
combo_box(flavors, flavor).placeholder("Type or pick a flavor").id("flavor")

combo_box(items, text) binds text two-way: typing writes the signal per keystroke, picking a dropdown item writes the item’s string, and setting the signal patches the control (echo-guarded, §4.4: the guard remembers the last value that arrived from the native control so bind_seeded does not patch it straight back). items is reactive: a change patches the native dropdown live, and the typed text survives the swap. .placeholder(impl IntoText) sets the empty-state prompt (read once at build). ComboBox implements Piece, so .id()/.a11y()/.frame() chain via Decorate. Like text_field it is a width-growing leaf (grow_w = true, natural single-line height).

Every backend reports both change paths (typing and picking) through the one Event::TextChanged(String), so dayscript’s input: step drives the entry on every backend. The front-end also answers a synthetic Event::SelectionChanged(i) by mapping i through the current items, so dayscript’s select: step is the scripted menu path (native backends never emit it: a native pick already arrives as text).

Per-backend native realization

iOS and HarmonyOS have no native combo-box control, so the piece carries no renderer there: day renders its placeholder leaf, and the showcase adds a footnote saying why. Use picker or text_field on those platforms. The change plumbing per backend:

• AppKit: one per-node delegate serves both halves, NSControlTextEditingDelegate::controlTextDidChange: for keystrokes and NSComboBoxDelegate::comboBoxSelectionDidChange: for picks. The selection notification fires before the control writes the pick into its own stringValue, so the handler reads the selected item’s string instead. setCompletes(true) gives inline autocompletion. Programmatic setStringValue fires neither (no suppression needed).
• GTK: GtkComboBoxText::with_entry(); the internal GtkEntry’s changed signal is the single change path (a pick writes the entry). It also fires on programmatic set_text, so a per-node suppress cell guards the sync. GTK 4.10 deprecated GtkComboBoxText without an editable replacement (GtkDropDown has no entry), so the renderer keeps it under a commented allow(deprecated).
• Qt / XAML: each carries its own C++ shim inside this crate (src/lib-qt-shim.cpp, src/lib-xaml-shim.cpp), compiled by the crate’s build.rs. The Qt shim is a QComboBox with setEditable(true) + NoInsert; editTextChanged is the single change path, and programmatic setters sit in blockSignals. The XAML shim boxes an editable ComboBox through the day_xaml_box / day_xaml_unbox functions. Documented divergence: XAML’s ComboBox has no per-keystroke text event, so free-form text commits on Enter or focus loss (TextSubmitted / LostFocus) while picks report immediately (SelectionChanged).
• Android: carries its own Java factory (src/DayCombo.java), folded into the app’s Gradle build via [package.metadata.day.android] with no edits to day-android. Android’s combo box is AutoCompleteTextView: suggestions prefix-filter while typing, and a tap or focus pops the dropdown open so the list is reachable without typing. One TextWatcher reports both paths as DayBridge.K_TEXT_CHANGED; the programmatic setters guard on equality.

Verification

The showcase Controls page (controls.rs flavor_block) binds a combo_box to a flavor signal with a localized three-item list, an Add button that pushes the typed text into the items, and a readout mirroring the signal. The walkthrough drives all three behaviors: select index 2 (menu path, asserted by localized key), input a literal that is in no list (free-form path), then Add + select index 3 (an index that exists only after the add), which shows the reactive item list round-trips. Runs on macOS-AppKit, GTK, Qt, iOS-sim (placeholder + synthetic steps), and the Android emulator. Rust is clippy-clean (-D warnings) and cargo fmt-clean for every backend feature.

Follow-ups

• Reactive placeholder (currently fixed at build).
• Event::Submitted on Return for “commit” semantics distinct from per-keystroke changes.
• Disabled/enabled state; a max-visible-items hint for the dropdown.

XAML is CI-only (built under cfg(windows) + the Windows SDK); it isn’t buildable on the macOS/Linux dev hosts.