Phoenixのフォームinputを共通ヘルパーとして使い回す

<%= f = form_for @changeset, "#", phx_submit: "submit-check-in-form" %>
  <%= input_tag f, :name %>
  <%= input_tag f, :phone %>
  <%= submit "Check In", phx_disable_with: "Saving ...", class: "btn btn-primary" %>
</form>

<!-- 提出前 -->
<div class="form-group">
  <label for="volunteer_name">Name *</label>
  <input type="text"
          class="form-control "
          id="volunteer_name"
          name="volunteer[name]"
          placeholder="Name">
</div>

<!-- 提出後BAD -->
<div class="form-group">
  <label for="volunteer_name">Name *</label>
  <input type="text"
          class="form-control is-invalid" <-- CSSが変化
          id="volunteer_name"
          name="volunteer[name]"
          placeholder="Name"
          value="">
  <span class="invalid-feedback d-inline-block" 
        phx-feedback-for="volunteer_name">
        can't be blank
  </span>
</div>

<!-- 提出後GOOD -->
<div class="form-group">
  <label for="volunteer_name">Name *</label>
  <input type="text"
          class="form-control is-valid" <-- CSSが変化
          id="volunteer_name"
          name="volunteer[name]"
          placeholder="Name"
          value="Masatoshi">
</div>

defmodule MnishiguchiWeb.InputHelpers do
  use Phoenix.HTML

  @custom_field_form_mapping %{
    "phone" => :telephone_input
  }

  @doc """
  Dynamically generates a Bootstrap 4 form input field.
  http://blog.plataformatec.com.br/2016/09/dynamic-forms-with-phoenix/

  ## Examples

      input_tag f, :name, placeholder: "Name", autocomplete: "off"
      input_tag f, :phone, using: :telephone_input, placeholder: "Phone", autocomplete: "off"

  """
  def input_tag(form, field, opts \\ []) do
    # Some input type can be inferred from the field name.
    input_fun_name = opts[:using] || Phoenix.HTML.Form.input_type(form, field, @custom_field_form_mapping)
    required = opts[:required] || form |> input_validations(field) |> Keyword.get(:required)
    label_text = opts[:label] || humanize(field)

    permitted_input_opts = Enum.filter(opts, &(elem(&1, 0) in [:id, :name, :autocomplete, :placeholder]))
    phx_attributes = Enum.filter(opts, &String.starts_with?(to_string(elem(&1, 0)), "phx_"))
    custom_class = [class: "form-control #{form_state_class(form, field)}"]

    input_opts =
      (permitted_input_opts ++ phx_attributes ++ custom_class)
      |> Enum.reject(&is_nil(elem(&1, 1)))

    content_tag :div, class: "form-group" do
      label = label_tag(form, field, label_text, required)
      input = apply(Phoenix.HTML.Form, input_fun_name, [form, field, input_opts])
      error = MnishiguchiWeb.ErrorHelpers.error_tag(form, field)

      [label, input, error]
    end
  end

  defp form_state_class(form, field) do
    cond do
      # Some forms may not use a Map as a source. E.g., :user
      !is_map(form.source) -> ""
      # Ignore Conn-based form.
      Map.get(form.source, :__struct__) == Plug.Conn -> ""
      # The form is not yet submitted.
      !Map.get(form.source, :action) -> ""
      # This field has an error.
      form.errors[field] -> "is-invalid"
      true -> "is-valid"
    end
  end
end

%{"email"    => :email_input,
  "password" => :password_input,
  "search"   => :search_input,
  "url"      => :url_input}

iex> alias Mnishiguchi.Volunteers.Volunteer
iex> import Ecto.Changeset

iex> changeset = %Volunteer{} |> cast(%{}, [:name]) |> validate_required([:name])
#Ecto.Changeset<
  action: nil,
  changes: %{},
  errors: [
    name: {"can't be blank", [validation: :required]},
  ],
  data: #Mnishiguchi.Volunteers.Volunteer<>,
  valid?: false
>

iex> form = Phoenix.HTML.Form.form_for changeset, "#"
%Phoenix.HTML.Form{
  action: "#",
  data: %Mnishiguchi.Volunteers.Volunteer{
    __meta__: #Ecto.Schema.Metadata<:built, "volunteers">,
    id: nil,
    inserted_at: nil,
    name: nil,
    updated_at: nil
  },
  errors: [],
  hidden: [],
  id: "volunteer",
  impl: Phoenix.HTML.FormData.Ecto.Changeset,
  index: nil,
  name: "volunteer",
  options: [method: "post"],
  params: %{},
  source: #Ecto.Changeset<
    action: nil,
    changes: %{},
    errors: [
      name: {"can't be blank", [validation: :required]},
    ],
    data: #Mnishiguchi.Volunteers.Volunteer<>,
    valid?: false
  >
}

# using default mapping
iex> Phoenix.HTML.Form.input_type(form, :name)
:text_input

iex> Phoenix.HTML.Form.input_type(form, :email)
:email_input

iex> Phoenix.HTML.Form.input_type(form, :search)
:search_input

iex> Phoenix.HTML.Form.input_type(form, :password)
:password_input

iex> Phoenix.HTML.Form.input_type(form, :url)
:url_input

# 第３引数にカスタムマッピングを指定するとデフォルトにマージされます。
iex> Phoenix.HTML.Form.input_type(form, :denwa, %{"denwa" => :telephone_input})
:telephone_input

iex> form |> input_validations(:name)
[required: true]

iex> form |> input_validations(:name) |> Keyword.get(:required)
true

iex> form |> input_validations(:hello) |> Keyword.get(:required)
false

iex> Phoenix.HTML.Tag.content_tag(:p, "hello")
{:safe, [60, "p", [], 62, "hello", 60, 47, "p", 62]}

iex> Phoenix.HTML.Tag.content_tag(:p, "hello") |> Phoenix.HTML.safe_to_string
"<p>hello</p>"

iex> Phoenix.HTML.Form.humanize("name")
"Name"

iex> Phoenix.HTML.Form.humanize("hello_world")
"Hello world"