Selection value-type contract

ForAccordion, ForToggleGroup, ForListbox, and ForSelect expose value as model<readonly string[]>; the generic ForCombobox uses model<readonly T[]>. That uniform array is the FormValueControl<readonly T[]> (or readonly T[]) backing that the [formField] directive auto-wires to. One control covers both single and multi selection because the shape never changes with mode.

The array is treated immutably: the primitive replaces it ([...], .filter(), a fresh Set then spread) rather than mutating it in place. You write to it through [(value)].

With multiple unset (the default) the array stays at length ≤ 1. Activating an option replaces the current entry rather than appending, and an empty array means nothing is selected. There is no separate "single value" model to keep in sync.

Each primitive exposes a read-only, single-mode view derived with computed():

It returns the sole element when the array holds exactly one entry, otherwise null (empty, or multiple in multi mode). This is a convenience accessor, not a second source of truth: writes still go through [(value)].

In single mode, bind [formField] to a field you model as readonly T[], exactly as in multi mode, and keep it at length ≤ 1. There is no adapter to insert: [formField] pushes disabled, readonly, required, invalid, errors and touched into the control and routes focus() to the primitive's real focus target (the listbox option, the select trigger, the combobox input) on its own.

readonly model = signal({ country: [] as readonly string[] });
readonly profile = form(this.model);
// <div forSelect [formField]="profile.country">

A FieldTree<T | null> cannot bind to these controls, and it is deliberately not bridged. Angular's template type-checker adds a two-way [value] binding between the field and every directive on the host owning a value model, so the value type has to match in both directions; a hand-written FieldTree view that mapped null ⇄ [] was only expressible as reflection over @angular/forms/signals internals and was retired in #1579. Map to a T | null shape at the edge that needs it (a request payload, a persisted record) rather than in the binding, and read the picked value for display off selected / selectedItem.

Selection-for-display primitives that model a single value (ForTabs and ForRadioGroup) are out of this array contract. It covers the array-backed, multi-capable controls only.