counter_live.ex

defmodule MyTwitterWeb.CounterLive do
  use MyTwitterWeb, :live_view

  def render(assigns) do
    ~H"""
    <div>
      <.button phx-click="dec">-</.button>
      <%= @counter %>
      <.button phx-click="inc">+</.button>
    </div>
    """
  end

  def mount(_params, _session, socket) do
    socket =
      socket
      |> assign(:counter, 10)

    {:ok, socket}
  end

  def handle_event("dec", _, socket) do
    {:noreply, update(socket, :counter, fn count -> count - 1 end)}
  end

  def handle_event("inc", _, socket) do
    {:noreply, update(socket, :counter, fn count -> count + 1 end)}
  end
end