keen_web_multiselect

FORM INTEGRATION
================

Pass a Phoenix.HTML.FormField and the component fills in id, name, and the
initial value; the underlying element writes a hidden input named after the
field, so phx-change / phx-submit see the selection like a native <select multiple>.

    <.simple_form for={@form} phx-change="validate" phx-submit="save">
      <.web_multiselect
        field={@form[:tags]}
        options={@tag_options}
        hook={true}
      />
    </.simple_form>

WHAT `field` DOES
-----------------
Keenmate.WebMultiselect.FormHelpers.assign_from_field/1 derives from the FormField:
  - id    (from the field id)
  - name  (the input name, e.g. "post[tags][]" style)
  - value (the field's current value -> initial selection)

Explicit assigns WIN over the field-derived ones — pass id=/name=/value= to
override any single piece.

WHERE THE VALUE SHOWS UP
------------------------
The component renders a hidden input named after the field. On phx-change /
phx-submit the selection arrives in params under the form name + field, exactly
like a native multi-select:

    params[form_name]["tags"]   #=> ["elixir", "phoenix"]

So Ecto changesets, cast/3, and validation work unchanged — treat it as an
array field.

VALUE ENCODING
--------------
value_format controls how the hidden input serializes the value (e.g. array vs
CSV) when a plain array of param entries isn't what your backend expects. Default
behaves like a native multi-select. Only set it if your form handling needs a
different wire format.

DO I NEED hook={true} IN A FORM?
--------------------------------
- For pure form submit (the hidden input carries the value), NO — the hidden
  input is enough on submit.
- Add hook={true} if you also want live server events on each change (e.g.
  dependent fields, or to push_update another control). phx-change on the form
  will already see the value via the hidden input without the hook.

SINGLE-SELECT FIELD
-------------------
    <.web_multiselect field={@form[:category]} multiple={false} options={@categories} />

Params then carry a single value string instead of a list.

VALIDATION EXAMPLE
------------------
    def handle_event("validate", %{"post" => post_params}, socket) do
      changeset = Post.changeset(%Post{}, post_params) |> Map.put(:action, :validate)
      {:noreply, assign(socket, form: to_form(changeset))}
    end

See also: data-and-options.txt (option shapes), liveview-events.txt (change
events), cookbook.txt (form recipe).