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/Phoenix

Bindings

Phoenix supports DOM element bindings for client-server interaction. For example, to react to a click on a button, you would render the element:

<button phx-click="inc_temperature">+</button>

Then on the server, all LiveView bindings are handled with the handle_event callback, for example:

def handle_event("inc_temperature", _value, socket) do
  {:ok, new_temp} = Thermostat.inc_temperature(socket.assigns.id)
  {:noreply, assign(socket, :temperature, new_temp)}
end
Binding Attributes
Params phx-value-*
Click Events phx-capture-click, phx-click
Focus/Blur Events phx-window-focus, phx-window-blur, phx-focus, phx-blur
Key Events phx-window-keyup, phx-window-keydown, phx-keyup, phx-keydown
Form Events phx-auto-recover, phx-trigger-action, phx-disable-with, phx-feedback-for, phx-submit, phx-change
Rate Limiting phx-throttle, phx-debounce
DOM Patching phx-update
JS Interop phx-hook

Click Events

The phx-click binding is used to send click events to the server. When any client event, such as a phx-click click is pushed, the value sent to the server will be chosen with the following priority:

  • Any number of optional phx-value- prefixed attributes, such as:

    <div phx-click="inc" phx-value-myvar1="val1" phx-value-myvar2="val2">

    will send the following map of params to the server:

    def handle_event("inc", %{"myvar1" => "val1", "myvar2" => "val2"}, socket) do

    If the phx-value- prefix is used, the server payload will also contain a "value" if the element's value attribute exists.

  • When receiving a map on the server, the payload will also include user defined metadata of the client event, or an empty map if none is set. For example, the following LiveSocket client option would send the coordinates and altKey information for all clicks:

    let liveSocket = new LiveSocket("/live", Socket, {
      params: {_csrf_token: csrfToken},
      metadata: {
        click: (e, el) => {
          return {
            altKey: e.altKey,
            clientX: e.clientX,
            clientY: e.clientY
          }
        }
      }
    })

The phx-capture-click event is just like phx-click, but instead of the click event being dispatched to the closest phx-click element as it bubbles up through the DOM, the event is dispatched as it propagates from the top of the DOM tree down to the target element. This is useful when wanting to bind click events without receiving bubbled events from child UI elements. Since capturing happens before bubbling, this can also be important for preparing or preventing behaviour that will be applied during the bubbling phase.

Focus and Blur Events

Focus and blur events may be bound to DOM elements that emit such events, using the phx-blur, and phx-focus bindings, for example:

<input name="email" phx-focus="myfocus" phx-blur="myblur"/>

To detect when the page itself has received focus or blur, phx-window-focus and phx-window-blur may be specified. These window level events may also be necessary if the element in consideration (most often a div with no tabindex) cannot receive focus. Like other bindings, phx-value-* can be provided on the bound element, and those values will be sent as part of the payload. For example:

<div class="container"
    phx-window-focus="page-active"
    phx-window-blur="page-inactive"
    phx-value-page="123">
  ...
</div>

The following window-level bindings are supported:

  • phx-window-focus
  • phx-window-blur
  • phx-window-keydown
  • phx-window-keyup

Key Events

The onkeydown, and onkeyup events are supported via the phx-keydown, and phx-keyup bindings. Each binding supports a phx-key attribute, which triggers the event for the specific key press. If no phx-key is provided, the event is triggered for any key press. When pushed, the value sent to the server will contain the "key" that was pressed, plus any user-defined metadata. For example, pressing the Escape key looks like this:

%{"key" => "Escape"}

To capture additional user-defined metadata, the metadata option for keydown events may be provided to the LiveSocket constructor. For example:

let liveSocket = new LiveSocket("/live", Socket, {
  params: {_csrf_token: csrfToken},
  metadata: {
    keydown: (e, el) => {
      return {
        key: e.key,
        metaKey: e.metaKey,
        repeat: e.repeat
      }
    }
  }
})

To determine which key has been pressed you should use key value. The available options can be found on MDN or via the Key Event Viewer.

By default, the bound element will be the event listener, but a window-level binding may be provided via phx-window-keydown or phx-window-keyup, for example:

def render(assigns) do
  ~L"""
  <div id="thermostat" phx-window-keyup="update_temp">
    Current temperature: <%= @temperature %>
  </div>
  """
end

def handle_event("update_temp", %{"key" => "ArrowUp"}, socket) do
  {:ok, new_temp} = Thermostat.inc_temperature(socket.assigns.id)
  {:noreply, assign(socket, :temperature, new_temp)}
end

def handle_event("update_temp", %{"key" => "ArrowDown"}, socket) do
  {:ok, new_temp} = Thermostat.dec_temperature(socket.assigns.id)
  {:noreply, assign(socket, :temperature, new_temp)}
end

def handle_event("update_temp", _key, socket) do
  {:noreply, socket}
end

Rate limiting events with Debounce and Throttle

All events can be rate-limited on the client by using the phx-debounce and phx-throttle bindings, with the following behavior:

  • phx-debounce - Accepts either an integer timeout value (in milliseconds), or "blur". When an integer is provided, emitting the event is delayed by the specified milliseconds. When "blur" is provided, emitting the event is delayed until the field is blurred by the user. Debouncing is typically used for input elements.

  • phx-throttle - Accepts an integer timeout value to throttle the event in milliseconds. Unlike debounce, throttle will immediately emit the event, then rate limit it at once per provided timeout. Throttling is typically used to rate limit clicks, mouse and keyboard actions.

For example, to avoid validating an email until the field is blurred, while validating the username at most every 2 seconds after a user changes the field:

<form phx-change="validate" phx-submit="save">
  <input type="text" name="user[email]" phx-debounce="blur"/>
  <input type="text" name="user[username]" phx-debounce="2000"/>
</form>

And to rate limit a volume up click to once every second:

<button phx-click="volume_up" phx-throttle="1000">+</button>

Likewise, you may throttle held-down keydown:

<div phx-window-keydown="keydown" phx-throttle="500">
  ...
</div>

Unless held-down keys are required, a better approach is generally to use phx-keyup bindings which only trigger on key up, thereby being self-limiting. However, phx-keydown is useful for games and other use cases where a constant press on a key is desired. In such cases, throttle should always be used.

Debounce and Throttle special behavior

The following specialized behavior is performed for forms and keydown bindings:

  • When a phx-submit, or a phx-change for a different input is triggered, any current debounce or throttle timers are reset for existing inputs.

  • A phx-keydown binding is only throttled for key repeats. Unique keypresses back-to-back will dispatch the pressed key events.

LiveView Specific Events

The lv: event prefix supports LiveView specific features that are handled by LiveView without calling the user's handle_event/3 callbacks. Today, the following events are supported:

  • lv:clear-flash – clears the flash when sent to the server. If a phx-value-key is provided, the specific key will be removed from the flash.

For example:

<p class="alert" phx-click="lv:clear-flash" phx-value-key="info">
  <%= live_flash(@flash, :info) %>
</p>

© 2018 Chris McCord
Licensed under the MIT License.
https://hexdocs.pm/phoenix_live_view/bindings.html