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Presto

Presto is an Elixir library for creating Elm-like or React-like single page applications (SPAs) completely in Elixir.

It was presented at ElixirConfEU 2018. You can find the slides here.

Installation

Add this to mix.exs:

{:presto, "~> 0.1.2"}

Philosophy

Web development is too complciated. Front-ends, back-ends, multiple languages, markup, it's all too complicated. Things can be simpler.

We want:

  1. the feel and data model (mostly) of React.
  2. views to be a projection of the data.
  3. the simplicity of Elm's model/update/view functions.
  4. all of this in Elixir.

Model -> Update -> View
State -> Message -> Response

This is a GenServer.

How It Works

  1. A GenServer keeps the state for the user. It’s all on the server.
  2. For a single component root, there is one GenServer that comes to life when it gets a message.
  3. It receives DOM events from the browser over a channel, updating the GenServer state.
  4. UI updates are returned via the channel.

The GenServers are managed by a DynamicSupervisor.

Components are scoped to a visitor_id, which is unique to each browser.

Add Presto to mix.exs

mix.exs

  defp deps do
    [
      ...
      {:presto, "~> 0.1.2"},
      ...
    ]
  end

Create a component

lib/presto/single_counter.ex

defmodule PrestoDemoWeb.Presto.SingleCounter do
  use Presto.Component
  use Taggart.HTML
  require Logger

  @impl Presto.Component
  def initial_model(_model) do
    0
  end

  @impl Presto.Component
  def update(message, model) do
    case message do
      %{"event" => "click", "id" => "inc"} ->
        model + 1

      %{"event" => "click", "id" => "dec"} ->
        model - 1
    end
  end

  @impl Presto.Component
  def render(model) do
    div do
      "Counter is: #{inspect(model)}"

      button(id: "inc", class: "presto-click") do
        "More"
      end

      button(id: "dec", class: "presto-click") do
        "Less"
      end
    end
  end
end

Add the component to a view

index.html.eex

<%= Presto.render(Presto.component(PrestoDemoWeb.Presto.SingleCounter, assigns[:visitor_id])) %>

Wire up the javascript

assets/package.json

  ...
  "dependencies": {
    ...
    "presto": "file:../deps/presto"
  },
  ...

app.js

import {Presto} from "presto"
import unpoly from "unpoly/dist/unpoly.js"
let presto = new Presto(channel, up);

Wire Up A Presto Channel

user_socker.ex

defmodule PrestoDemoWeb.UserSocket do
  use Phoenix.Socket

  channel("presto:*", PrestoDemoWeb.CounterChannel)

  def connect(%{"token" => token} = _params, socket) do
    case PrestoDemoWeb.Session.decode_socket_token(token) do
      {:ok, visitor_id} ->
        {:ok, assign(socket, :visitor_id, visitor_id)}

      {:error, _reason} ->
        :error
    end
  end
  ...

component_channel.ex

defmodule PrestoDemoWeb.CounterChannel do
  ...
  def handle_in("presto", payload, socket) do
    %{visitor_id: visitor_id} = socket.assigns

    # send event to presto component
    {:ok, dispatch} = Presto.dispatch(PrestoDemoWeb.Presto.SingleCounter, visitor_id, payload)

    case dispatch do
      [] -> nil
      _ -> push(socket, "presto", dispatch)
    end

    {:reply, {:ok, payload}, socket}
  end
  ...
end

Setup user_token and visitor_id plugs

router.ex

  pipeline :browser do
    plug(:accepts, ["html"])
    plug(:fetch_session)
    plug(:fetch_flash)
    plug(:protect_from_forgery)
    plug(:put_secure_browser_headers)
    plug(PrestoDemoWeb.Plugs.VisitorIdPlug)
    plug(PrestoDemoWeb.Plugs.UserTokenPlug)
  end

user_token_plug.ex

defmodule PrestoDemoWeb.Plugs.UserTokenPlug do
  import Plug.Conn

  def init(default), do: default

  def call(conn, _default) do
    if visitor_id = conn.assigns[:visitor_id] do
      user_token = PrestoDemoWeb.Session.encode_socket_token(visitor_id)
      assign(conn, :user_token, user_token)
    else
      conn
    end
  end
end

visitor_id_plug.ex

defmodule PrestoDemoWeb.Plugs.VisitorIdPlug do
  import Plug.Conn

  @key :visitor_id

  def init(default), do: default

  def call(conn, _default) do
    visitor_id = get_session(conn, @key)

    if visitor_id do
      assign(conn, @key, visitor_id)
    else
      visitor_id = Base.encode64(:crypto.strong_rand_bytes(32))

      conn
      |> put_session(@key, visitor_id)
      |> assign(@key, visitor_id)
    end
  end
end

Testing

Testing is easy. It’s just a GenServer. Spin them up, update, test the response. Done.

Growing Your Application

Use the language. Growing your app is very simple with this approach. If your render() method gets too big, you just split it up in to helpers and modules and whatnot. If your update() method gets too big, you just split it up in to helpers and modules and whatnot.

Demos

Simple Counter

Here is the code for a simple counter demo

PrestoChange.io

This is a real application using Presto.

The code is here.

This is running on the West Coast of the USA:

www.prestochange.io

This is running in Central Europe:

eu.prestochange.io

About

Elixir-based SPA sites without the SPA

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