Form bindings

A note about form helpers

LiveView works with the existing Phoenix.HTML form helpers. If you want to use helpers such as text_input/2, etc. be sure to use Phoenix.HTML at the top of your LiveView. If your application was generated with Phoenix v1.6, then mix phx.new automatically uses Phoenix.HTML when you use MyAppWeb, :live_view or use MyAppWeb, :live_component in your modules.

Using the generated :live_view and :live_component helpers will also import MyAppWeb.ErrorHelpers, a generated module where error_tag/2 resides (usually located at lib/my_app_web/views/error_helpers.ex).

Since ErrorHelpers is generated into your app, it is yours to modify – you may add additional helper functions here, such as those recommended when rendering feedback for upload_errors/1,2.

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. For example, to handle real-time form validation and saving, your template would use both phx_change and phx_submit bindings:

<.form let={f} for={@changeset} phx-change="validate" phx-submit="save">
  <%= label f, :username %>
  <%= text_input f, :username %>
  <%= error_tag f, :username %>

  <%= label f, :email %>
  <%= text_input f, :email %>
  <%= error_tag f, :email %>

  <%= submit "Save" %>
</.form>

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

def render(assigns) ...

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

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

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

The validate callback simply updates the changeset based on all form input values, then assigns the new changeset to the socket. If the changeset 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.

`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.ErrorHelpers may use this function:

def error_tag(form, field) do
  form.errors
  |> Keyword.get_values(field)
  |> Enum.map(fn error ->
    content_tag(:span, translate_error(error),
      class: "invalid-feedback",
      phx_feedback_for: input_name(form, field)
    )
  end)
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. The following css rules are generated in new projects to hide the errors:

.phx-no-feedback.invalid-feedback, .phx-no-feedback .invalid-feedback {
  display: none;
}

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 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:

<%= password_input f, :password, value: input_value(f, :password) %>
<%= password_input f, :password_confirmation, value: input_value(f, :password_confirmation) %>
<%= error_tag f, :password %>
<%= error_tag f, :password_confirmation %>

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 @uploads.avatar %>
</div>

See Phoenix.LiveView.Helpers.live_file_input/2 for more.

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

By default, all forms marked with phx-change 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 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

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:

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

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

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

To handle latent events, any HTML tag 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; }
  .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.

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