Events

JS.push/1

Read time: 5 minutes

In the previous lesson we learned how to use the phx-value-* binding to pass values into an event. To recap:

Mix.install([
  {:liveview_playground, "~> 0.1.1"}
])

defmodule PageLive do
  use LiveviewPlaygroundWeb, :live_view

  def mount(_params, _session, socket) do
    socket = assign(socket, temperature_celsius: 30)
    {:ok, socket}
  end

  def render(assigns) do
    ~H"""
    <div>
      Current temperature: <%= @temperature_celsius %>C
    </div>
    <div>
      <%= cond do %>
        <% @temperature_celsius > 40 -> %>
          <p>🔥 Impossible to live 🔥</p>
        <% @temperature_celsius > 30 -> %>
          <p>Its hot</p>
        <% @temperature_celsius > 20 -> %>
          <p>Kinda cool</p>
        <% @temperature_celsius > 10 -> %>
          <p>Chill</p>
        <% @temperature_celsius > 0 -> %>
          <p>Chill</p>
        <% true -> %>
          <p>❄️⛄️</p>
      <% end %>
    </div>

    <input type="button" value="+5" phx-click="add" phx-value-amount={+5} />
    <input type="button" value="+10" phx-click="add" phx-value-amount={+10} />
    <input type="button" value="-5" phx-click="add" phx-value-amount={-5} />
    <input type="button" value="-10" phx-click="add" phx-value-amount={-10} />
    """
  end

  def handle_event("add", %{"amount" => amount}, socket) do
    amount = String.to_integer(amount)
    socket = assign(socket, temperature_celsius: socket.assigns.temperature_celsius + amount)
    {:noreply, socket}
  end
end

LiveviewPlayground.start()

However, we talked about how values will be sent strictly as a strings. LiveView also has an alternative way to push events called JS.push/2 which is part of the so-called JS Commands. Write and run the js_push.exs file:


Mix.install([
  {:liveview_playground, "~> 0.1.1"}
])

defmodule PageLive do
  use LiveviewPlaygroundWeb, :live_view

  def mount(_params, _session, socket) do
    socket = assign(socket, temperature_celsius: 30)
    {:ok, socket}
  end

  def render(assigns) do
    ~H"""
    <div>
      Current temperature: <%= @temperature_celsius %>C
    </div>
    <div>
      <%= cond do %>
        <% @temperature_celsius > 40 -> %>
          <p>🔥 Impossible to live 🔥</p>
        <% @temperature_celsius > 30 -> %>
          <p>Its hot</p>
        <% @temperature_celsius > 20 -> %>
          <p>Kinda cool</p>
        <% @temperature_celsius > 10 -> %>
          <p>Chill</p>
        <% @temperature_celsius > 0 -> %>
          <p>Chill</p>
        <% true -> %>
          <p>❄️⛄️</p>
      <% end %>
    </div>

    <input type="button" value="+5" phx-click={JS.push("add", value: %{amount: +5})} />
    <input type="button" value="+10" phx-click={JS.push("add", value: %{amount: +10})} />
    <input type="button" value="-5" phx-click={JS.push("add", value: %{amount: -5})} />
    <input type="button" value="-10" phx-click={JS.push("add", value: %{amount: -10})} />
    """
  end

  def handle_event("add", %{"amount" => amount}, socket) do
    socket = assign(socket, temperature_celsius: socket.assigns.temperature_celsius + amount)
    {:noreply, socket}
  end
end

LiveviewPlayground.start()

We simplified our LiveView by using JS.push/2 directly in the phx-click binding and defining that we are pushing the "add" event and the value will be %{amount: INTEGER}. This phx-click will be serialized as JSON so that when the button is clicked the amount will be an integer as expected.

#Extra tip: using loops to prevent duplicated code

We could avoid some code duplication by using a simple :for:


Mix.install([
  {:liveview_playground, "~> 0.1.1"}
])

defmodule PageLive do
  use LiveviewPlaygroundWeb, :live_view

  def mount(_params, _session, socket) do
    socket = assign(socket, temperature_celsius: 30)
    {:ok, socket}
  end

  def render(assigns) do
    ~H"""
    <div>
      Current temperature: <%= @temperature_celsius %>C
    </div>
    <div>
      <%= cond do %>
        <% @temperature_celsius > 40 -> %>
          <p>🔥 Impossible to live 🔥</p>
        <% @temperature_celsius > 30 -> %>
          <p>Its hot</p>
        <% @temperature_celsius > 20 -> %>
          <p>Kinda cool</p>
        <% @temperature_celsius > 10 -> %>
          <p>Chill</p>
        <% @temperature_celsius > 0 -> %>
          <p>Chill</p>
        <% true -> %>
          <p>❄️⛄️</p>
      <% end %>
    </div>

    <input :for={value <- [5, 10, -5, -10]} type="button" value={"Add #{value}"} phx-click={JS.push("add", value: %{amount: value})} />
    """
  end

  def handle_event("add", %{"amount" => amount}, socket) do
    socket = assign(socket, temperature_celsius: socket.assigns.temperature_celsius + amount)
    {:noreply, socket}
  end
end

LiveviewPlayground.start()

This tip also works for the previous class with phx-value-amount. Take it as a home assignment to try out how to do this with the previous lesson code.

#Recap!

  • Using JS Commands we can switch from a combo of phx-click + phx-value-* bindings for just one phx-click binding containing a JS.push/2.
  • JS.push/2 simplifies the serialization of data that is not strings in values because integers are part of what JSON supports so the pushed data will be an integer as expected.

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