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a-note-about-form-helpers

A note about form helpers

If your application was generated with Phoenix v1.7, then mix phx.new automatically imports form helpers components defined in your applications CoreComponents module. You may also use Phoenix.HTML functions for building inputs.

form-events

Form Events

To handle form changes and submissions, use the phx-change and phx-submit events. In general, it is preferred to handle input changes at the form level, where all form fields are passed to the LiveView's callback given any single input change, but individual inputs may also track their own changes. For example, to handle real-time form validation and saving, your form would use both phx-change and phx-submit bindings:

<.form phx-change="validate" phx-submit="save">
  <.input type="text" field={@form[:username]} />
  <.input type="email" field={@form[:email]} />
  <button>Save</button>
</.form>

Next, your LiveView picks up the events in handle_event callbacks:

def render(assigns) ...

def mount(_params, _session, socket) do
  {:ok, assign(socket, form: to_form(Accounts.change_user(%User{}))}
end

def handle_event("validate", %{"user" => params}, socket) do
  form =
    %User{}
    |> Accounts.change_user(params)
    |> Map.put(:action, :insert)
    |> to_form()

  {:noreply, assign(socket, form: form)}
end

def handle_event("save", %{"user" => user_params}, socket) do
  case Accounts.create_user(user_params) do
    {:ok, user} ->
      {:noreply,
       socket
       |> put_flash(:info, "user created")
       |> redirect(to: Routes.user_path(MyAppWeb.Endpoint, MyAppWeb.User.ShowView, user))}

    {:error, %Ecto.Changeset{} = changeset} ->
      {:noreply, assign(socket, form: to_form(changeset))}
  end
end

The validate callback simply updates the changeset based on all form input values, then convert the changeset to a form and assign it to the socket. If the form changes, such as generating new errors, render/1 is invoked and the form is re-rendered.

Likewise for phx-submit bindings, the same callback is invoked and persistence is attempted. On success, a :noreply tuple is returned and the socket is annotated for redirect with Phoenix.LiveView.redirect/2 to the new user page, otherwise the socket assigns are updated with the errored changeset to be re-rendered for the client.

You may wish for an individual input to use its own change event or to target a different component. This can be accomplished by annotating the input itself with phx-change, for example:

<.form phx-change="validate" phx-submit="save">
  ...
  <.input field={f[:email]}  phx-change="email_changed" phx-target={@myself} />
</.form>

Then your LiveView or LiveComponent would handle the event:

def handle_event("email_changed", %{"user" => %{"email" => email}}, socket) do
  ...
end

Note: only the individual input is sent as params for an input marked with phx-change.

phx-feedback-for

phx-feedback-for

For proper form error tag updates, the error tag must specify which input it belongs to. This is accomplished with the phx-feedback-for attribute, which specifies the name (or id, for backwards compatibility) of the input it belongs to. Failing to add the phx-feedback-for attribute will result in displaying error messages for form fields that the user has not changed yet (e.g. required fields further down on the page).

For example, your MyAppWeb.CoreComponents may use this function:

def input(assigns) do
  ~H"""
  <div phx-feedback-for={@name}>
    <input
      type={@type}
      name={@name}
      id={@id || @name}
      value={Phoenix.HTML.Form.normalize_value(@type, @value)}
      class={[
        "phx-no-feedback:border-zinc-300 phx-no-feedback:focus:border-zinc-400",
        "border-zinc-300 focus:border-zinc-400 focus:ring-zinc-800/5",
      ]}
      {@rest}
    />
    <.error :for={msg <- @errors}><%%= msg %></.error>
  </div>
  """
end

def error(assigns) do
  ~H"""
  <p class="phx-no-feedback:hidden">
    <Heroicons.exclamation_circle mini class="mt-0.5 h-5 w-5 flex-none fill-rose-500" />
    <%= render_slot(@inner_block) %>
  </p>
  """
end

Now, any DOM container with the phx-feedback-for attribute will receive a phx-no-feedback class in cases where the form fields has yet to receive user input/focus. Using new CSS rules or tailwindcss variants allows you errors to be shown, hidden, and styled as feedback changes.

number-inputs

Number inputs

Number inputs are a special case in LiveView forms. On programmatic updates, some browsers will clear invalid inputs. So LiveView will not send change events from the client when an input is invalid, instead allowing the browser's native validation UI to drive user interaction. Once the input becomes valid, change and submit events will be sent normally.

<input type="number">

This is known to have a plethora of problems including accessibility, large numbers are converted to exponential notation, and scrolling can accidentally increase or decrease the number.

One alternative is the inputmode attribute, which may serve your application's needs and users much better. According to Can I Use?, the following is supported by 86% of the global market (as of Sep 2021):

<input type="text" inputmode="numeric" pattern="[0-9]*">

password-inputs

Password inputs

Password inputs are also special cased in Phoenix.HTML. For security reasons, password field values are not reused when rendering a password input tag. This requires explicitly setting the :value in your markup, for example:

<.input field={f[:password]} value={input_value(f[:password].value)} />
<.input field={f[:password_confirmation]} value={input_value(f[:password_confirmation].value)} />

nested-inputs

Nested inputs

Nested inputs are handled using .inputs_for function component. By default it will add the necessary hidden input fields for tracking ids of Ecto associations.

<.inputs_for :let={fp} field={f[:friends]}>
  <.input field={fp[:name]} type="text">
</.inputs_for>

file-inputs

File inputs

LiveView forms support reactive file inputs, including drag and drop support via the phx-drop-target attribute:

<div class="container" phx-drop-target={@uploads.avatar.ref}>
  ...
  <.live_file_input upload={@uploads.avatar} />
</div>

See Phoenix.Component.live_file_input/1 for more.

submitting-the-form-action-over-http

Submitting the form action over HTTP

The phx-trigger-action attribute can be added to a form to trigger a standard form submit on DOM patch to the URL specified in the form's standard action attribute. This is useful to perform pre-final validation of a LiveView form submit before posting to a controller route for operations that require Plug session mutation. For example, in your LiveView template you can annotate the phx-trigger-action with a boolean assign:

<.form :let={f} for={@changeset}
  action={Routes.reset_password_path(@socket, :create)}
  phx-submit="save"
  phx-trigger-action={@trigger_submit}>

Then in your LiveView, you can toggle the assign to trigger the form with the current fields on next render:

def handle_event("save", params, socket) do
  case validate_change_password(socket.assigns.user, params) do
    {:ok, changeset} ->
      {:noreply, assign(socket, changeset: changeset, trigger_submit: true)}

    {:error, changeset} ->
      {:noreply, assign(socket, changeset: changeset)}
  end
end

Once phx-trigger-action is true, LiveView disconnects and then submits the form.

recovery-following-crashes-or-disconnects

Recovery following crashes or disconnects

By default, all forms marked with phx-change and having id attribute will recover input values automatically after the user has reconnected or the LiveView has remounted after a crash. This is achieved by the client triggering the same phx-change to the server as soon as the mount has been completed.

Note: if you want to see form recovery working in development, please make sure to disable live reloading in development by commenting out the LiveReload plug in your endpoint.ex file or by setting code_reloader: false in your config/dev.exs. Otherwise live reloading may cause the current page to be reloaded whenever you restart the server, which will discard all form state.

For most use cases, this is all you need and form recovery will happen without consideration. In some cases, where forms are built step-by-step in a stateful fashion, it may require extra recovery handling on the server outside of your existing phx-change callback code. To enable specialized recovery, provide a phx-auto-recover binding on the form to specify a different event to trigger for recovery, which will receive the form params as usual. For example, imagine a LiveView wizard form where the form is stateful and built based on what step the user is on and by prior selections:

<form id="wizard" phx-change="validate_wizard_step" phx-auto-recover="recover_wizard">

On the server, the "validate_wizard_step" event is only concerned with the current client form data, but the server maintains the entire state of the wizard. To recover in this scenario, you can specify a recovery event, such as "recover_wizard" above, which would wire up to the following server callbacks in your LiveView:

def handle_event("validate_wizard_step", params, socket) do
  # regular validations for current step
  {:noreply, socket}
end

def handle_event("recover_wizard", params, socket) do
  # rebuild state based on client input data up to the current step
  {:noreply, socket}
end

To forgo automatic form recovery, set phx-auto-recover="ignore".

javascript-client-specifics

JavaScript client specifics

The JavaScript client is always the source of truth for current input values. For any given input with focus, LiveView will never overwrite the input's current value, even if it deviates from the server's rendered updates. This works well for updates where major side effects are not expected, such as form validation errors, or additive UX around the user's input values as they fill out a form.

For these use cases, the phx-change input does not concern itself with disabling input editing while an event to the server is in flight. When a phx-change event is sent to the server, the input tag and parent form tag receive the phx-change-loading CSS class, then the payload is pushed to the server with a "_target" param in the root payload containing the keyspace of the input name which triggered the change event.

For example, if the following input triggered a change event:

<input name="user[username]"/>

The server's handle_event/3 would receive a payload:

%{"_target" => ["user", "username"], "user" => %{"username" => "Name"}}

The phx-submit event is used for form submissions where major side effects typically happen, such as rendering new containers, calling an external service, or redirecting to a new page.

On submission of a form bound with a phx-submit event:

1. The form's inputs are set to readonly
2. Any submit button on the form is disabled
3. The form receives the "phx-submit-loading" class

On completion of server processing of the phx-submit event:

1. The submitted form is reactivated and loses the "phx-submit-loading" class
2. The last input with focus is restored (unless another input has received focus)
3. Updates are patched to the DOM as usual

To handle latent events, the <button> tag of a form can be annotated with phx-disable-with, which swaps the element's innerText with the provided value during event submission. For example, the following code would change the "Save" button to "Saving...", and restore it to "Save" on acknowledgment:

<button type="submit" phx-disable-with="Saving...">Save</button>

You may also take advantage of LiveView's CSS loading state classes to swap out your form content while the form is submitting. For example, with the following rules in your app.css:

.while-submitting { display: none; }
.inputs { display: block; }

.phx-submit-loading .while-submitting { display: block; }
.phx-submit-loading .inputs { display: none; }

You can show and hide content with the following markup:

<form phx-change="update">
  <div class="while-submitting">Please wait while we save our content...</div>
  <div class="inputs">
    <input type="text" name="text" value={@text}>
  </div>
</form>

Additionally, we strongly recommend including a unique HTML "id" attribute on the form. When DOM siblings change, elements without an ID will be replaced rather than moved, which can cause issues such as form fields losing focus.

triggering-phx-form-events-with-javascript

Triggering phx- form events with JavaScript

Often it is desirable to trigger an event on a DOM element without explicit user interaction on the element. For example, a custom form element such as a date picker or custom select input which utilizes a hidden input element to store the selected state.

In these cases, the event functions on the DOM API can be used, for example to trigger a phx-change event:

document.getElementById("my-select").dispatchEvent(
  new Event("input", {bubbles: true})
)

When using a client hook, this.el can be used to determine the element as outlined in the "Client hooks" documentation.

It is also possible to trigger a phx-submit using a "submit" event:

document.getElementById("my-form").dispatchEvent(
  new Event("submit", {bubbles: true, cancelable: true})
)