• Phoenix LiveView Counter Tutorial
• Why? 🤷‍♀️
• What? 💭
  • LiveView?
• Who? 👤
  • Please Star! ⭐
  • Prerequisites: What you Need Before You Start 📝
• How? 💻
  • Step 0: Run the Finished Counter App on your localhost 🏃‍
    • Clone the Repository
    • Download the Dependencies
    • Run the App
  • Step 1: Create the App 🆕
    • Checkpoint 1: Run the Tests!
    • Checkpoint 1b: Run the New Phoenix App!
  • Step 2: Create the counter.ex File
    • Explanation of the Code
  • Step 3: Create the live Route in router.ex
    • 3.1 Update the Failing Test Assertion
    • Checkpoint: Run Counter App!
    • Recap: Working Counter Without Writing JavaScript!
  • Step 4: Share State Between Clients!
    • Code Explanation
    • Checkpoint: Run It!
  • Congratulations! 🎉
  • Tests! 🧪
    • Add excoveralls to Check/Track Coverage
    • Create a coveralls.json File
    • DELETE Unused Files
    • Add More Tests!
  • Bonus Level: Use a LiveView Component (Optional)
    • Create a LiveView Component
  • Moving state out of the LiveView
    • Update the Tests for GenServer State
  • How many people are viewing the Counter?
  • More Tests!
• Done! 🏁
• What's Next?
• Feedback 💬 🙏
• Credits + Thanks! 🙌
  • Phoenix LiveView for Web Developers Who Don't know Elixir
  • Relevant + Recommended Reading

Why? 🤷‍♀️

There are several apps around the Internet that use Phoenix LiveView but none include step-by-step instructions a complete beginner can follow ... 😕
This is the complete beginner's tutorial we wish we had when learning LiveView and the one you have been searching for! 🎉

What? 💭

A complete beginners tutorial for building the most basic possible Phoenix LiveView App with no prior experience necessary.

LiveView?

Phoenix LiveView allows you to build rich interactive web apps with realtime reactive UI (no page refresh when data updates) without writing JavaScript! This enables building incredible interactive experiences with considerably less code.

LiveView pages load instantly because they are rendered on the Server and they require far less bandwidth than a similar React, Vue.js, Angular, etc. because only the bare minimum is loaded on the client for the page to work.

For a sneak peak of what is possible to build with LiveView, watch @chrismccord's LiveBeats intro:

https://user-images.githubusercontent.com/576796/162234098-31b580fe-e424-47e6-b01d-cd2cfcf823a9.mp4

Tip: Enable the sound. It's a collaborative music listening experience. 🎶 Try the LiveBeats Demo: livebeats.fly.dev 😍 🤯 🙏

Who? 👤

This tutorial is aimed at people who have never built anything in Phoenix or LiveView. You can speed-run it in 10 minutes if you're already familiar with Phoenix or Rails.

If you get stuck at any point while following the tutorial or you have any feedback/questions, please open an issue on GitHub!

If you don't have a lot of time or bandwidth to watch videos, this tutorial will be the fastest way to learn LiveView.

Please Star! ⭐

This is the tutorial we wish we'd had when we first started using LiveView ...
If you find it useful, please give it a star ⭐ it on Github so that other people will discover it.

Thanks! 🙏

Prerequisites: What you Need Before You Start 📝

Before you start working through the tutorial, you will need:

a. Elixir installed on your computer. See: learn-elixir#installation

When you run the command:

elixir -v

You should expect to see output similar to the following:

Elixir 1.15.4 (compiled with Erlang/OTP 26)

This informs us we are using Elixir version 1.15.4 which is the latest version at the time of writing. Some of the more advanced features of Phoenix 1.7 during compilation time require elixir 1.14 although the code will work in previous versions.

b. Phoenix installed on your computer. see: hexdocs.pm/phoenix/installation

If you run the following command in your terminal:

mix phx.new -v

You should see something similar to the following:

Phoenix installer v1.7.7

If you have an earlier version, definitely upgrade to get the latest features!
If you have a later version of Phoenix, and you get stuck at any point, please open an issue on GitHub! We are here to help!

c. Basic familiarity with Elixir syntax is recommended but not essential;
If you know any programming language, you can pick it up as you go and ask questions if you get stuck! See: https://github.com/dwyl/learn-elixir

How? 💻

This tutorial takes you through all the steps to build and test a counter in Phoenix LiveView.
We always "begin with the end in mind" so we recommend running the finished app on your machine before writing any code.

💡 You can also try the version deployed to Fly.io: livecount.fly.dev

Step 0: Run the Finished Counter App on your localhost 🏃‍

Before you attempt to build the counter, we suggest that you clone and run the complete app on your localhost.
That way you know it's working without much effort/time expended.

Clone the Repository

On your localhost, run the following command to clone the repo and change into the directory:

git clone https://github.com/dwyl/phoenix-liveview-counter-tutorial.git
cd phoenix-liveview-counter-tutorial

Download the Dependencies

Install the dependencies by running the command:

mix setup

It will take a few seconds to download the dependencies depending on the speed of your internet connection; be patient. 😉

Run the App

Start the Phoenix server by running the command:

mix phx.server

Now you can visit localhost:4000 in your web browser.

💡 Open a second browser window (e.g. incognito mode), you will see the the counter updating in both places like magic!

You should expect to see something similar to the following:

With the finished version of the App running on your machine and a clear picture of where we are headed, it's time to build it!

Step 1: Create the App 🆕

In your terminal, run the following mix command to generate the new Phoenix app:

mix phx.new counter --no-ecto --no-mailer --no-dashboard --no-gettext

The flags after the counter (name of the project), tell mix phx.new generator:

• --no-ecto - don't create a Database - we aren't storing any data
• --no-mailer- this project doesn't send email
• --no-dashboard - we don't need a status dashboard
• --no-gettext - no translation required

This keeps our counter as simple as possible. We can always add these features later if needed.

Note: Since Phoenix 1.6 the --live flag is no longer required when creating a LiveView app. LiveView is included by default in all new Phoenix Apps. Older tutorials may still include the flag, everything is much easier now. 😉

When you see the following prompt:

Fetch and install dependencies? [Yn]

Type Y followed by the [Enter] key. That will download all the necessary dependencies.

Checkpoint 1: Run the Tests!

In your terminal, go into the newly created app folder:

cd counter

And then run the following mix command:

mix test

This will compile the Phoenix app and will take some time. ⏳
You should see output similar to this:

Compiling 13 files (.ex)
Generated counter app
.....
Finished in 0.00 seconds (0.00s async, 0.00s sync)
5 tests, 0 failures

Randomized with seed 29485

Tests all pass. ✅

This is expected with a new app. It's a good way to confirm everything is working.

Checkpoint 1b: Run the New Phoenix App!

Run the server by executing this command:

mix phx.server

Visit localhost:4000 in your web browser.

You should see something similar to the following:

Step 2: Create the counter.ex File

Create a new file with the path: lib/counter_web/live/counter.ex

And add the following code to it:

defmodule CounterWeb.Counter do
  use CounterWeb, :live_view

  def mount(_params, _session, socket) do
    {:ok, assign(socket, :val, 0)}
  end

  def handle_event("inc", _, socket) do
    {:noreply, update(socket, :val, &(&1 + 1))}
  end

  def handle_event("dec", _, socket) do
    {:noreply, update(socket, :val, &(&1 - 1))}
  end

  def render(assigns) do
    ~H"""
    <div>
    <h1 class="text-4xl font-bold text-center"> The count is: <%= @val %> </h1>

    <p class="text-center">
     <.button phx-click="dec">-</.button>
     <.button phx-click="inc">+</.button>
     </p>
     </div>
    """
  end
end

Explanation of the Code

The first line instructs Phoenix to use the Phoenix.LiveView behaviour. This just means that we need to implement certain functions for our LiveView to work.

The first function is mount/3 which, as it's name suggests, mounts the module with the _params, _session and socket arguments:

def mount(_params, _session, socket) do
  {:ok, assign(socket, :val, 0) }
end

In our case we are ignoring the _params and _session arguments, hence the prepended underscore. If we were using sessions, we would need to check the session variable, but in this simple counter example, we just ignore it.

mount/3 returns a tuple: {:ok, assign(socket, :val, 0)} which uses the assign/3 function to assign the :val key a value of 0 on the socket. That just means the socket will now have a :val which is initialized to 0.

The second function is handle_event/3 which handles the incoming events received. In the case of the first declaration of handle_event("inc", _, socket) it pattern matches the string "inc" and increments the counter.

def handle_event("inc", _, socket) do
  {:noreply, update(socket, :val, &(&1 + 1))}
end

handle_event/3 ("inc") returns a tuple of: {:noreply, update(socket, :val, &(&1 + 1))} where the :noreply just means "do not send any further messages to the caller of this function".

update(socket, :val, &(&1 + 1)) as it's name suggests, will update the value of :val on the socket to the &(&1 + 1) is a shorthand way of writing fn val -> val + 1 end. the &() is the same as fn ... end (where the ... is the function definition). If this inline anonymous function syntax is unfamiliar to you, please read: https://elixir-lang.org/crash-course.html#partials-and-function-captures-in-elixir

The third function is almost identical to the one above, the key difference is that it decrements the :val.

def handle_event("dec", _, socket) do
  {:noreply, update(socket, :val, &(&1 - 1))}
end

handle_event("dec", _, socket) pattern matches the "dec" String and decrements the counter using the &(&1 - 1) syntax.

In Elixir we can have multiple similar functions with the same function name but different matches on the arguments or different "arity" (number of arguments).
For more detail on Functions in Elixir, see: elixirschool.com/functions/#named-functions

Finally the forth function render/1 receives the assigns argument which contains the :val state and renders the template using the @val template variable.

The render/1 function renders the template included in the function. The ~H""" syntax just means "treat this multiline string as a LiveView template" The ~H sigil is a macro included when the use Phoenix.LiveView is invoked at the top of the file.

LiveView will invoke the mount/3 function and will pass the result of mount/3 to render/1 behind the scenes.

Each time an update happens (e.g: handle_event/3) the render/1 function will be executed and updated data (in our case the :val count) is sent to the client.

🏁 At the end of Step 2 you should have a file similar to: lib/counter_web/live/counter.ex

Step 3: Create the live Route in router.ex

Now that we have created our LiveView handler functions in Step 2, it's time to tell Phoenix how to find it.

Open the lib/counter_web/router.ex file and locate the block of code that starts with scope "/", CounterWeb do:

scope "/", CounterWeb do
  pipe_through :browser

  get "/", PageController, :index
end

Replace the line get "/", PageController, :index with live("/", Counter). So you end up with:

scope "/", CounterWeb do
  pipe_through :browser

  live("/", Counter)
end

3.1 Update the Failing Test Assertion

Since we have replaced the get "/", PageController, :index route in router.ex in the previous step, the test in test/counter_web/controllers/page_controller_test.exs will now fail:

Compiling 6 files (.ex)
Generated counter app
....

  1) test GET / (CounterWeb.PageControllerTest)
     test/counter_web/controllers/page_controller_test.exs:4
     Assertion with =~ failed
     code:  assert html_response(conn, 200) =~ "Peace of mind from prototype to production"
     left:  "<!DOCTYPE html>\n<html lang=\"en\">\n  <head>\n    <meta charset=\"utf-8\">\n    <meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">\n    <meta name=\"csrf-token\" content=\"EFt5PABkPz1nPg5FMAoaDSA6BCFtBCMO_4JmNTx_2vO6i3qxXjETTpal\">\n    <title data-suffix=\" · Phoenix Framework\">\nLiveViewCounter\n     · Phoenix Framework</title>\n    <link phx-track-static rel=\"stylesheet\" href=\"/assets/app.css\">\n    <script defer phx-track-static type=\"text/javascript\" src=\"/assets/app.js\">\n    </script>\n  </head>\n  <body class=\"bg-white antialiased\">\n<div data-phx-main=\"true\" data-phx-session=\"SFMyNTY.g2gDaA\" id=\"phx-Fyi3ICCa7vPqDADE\"><header class=\"px-4 sm:px-6 lg:px-8\">\n  <div class=\"flex items-center justify-between border-b border-zinc-100 py-3\">\n    <div class=\"flex items-center gap-4\">\n      <a href=\"/\">\n        <svg viewBox=\"0 0 71 48\" class=\"h-6\" aria-hidden=\"true\">\n          <path d=\"etc." fill=\"#FD4F00\"></path>\n        </svg>\n      </a>\n      <p class=\"rounded-full bg-brand/5 px-2 text-[0.8125rem] font-medium leading-6 text-brand\">\n        v1.7\n      </p>\n    </div>\n    <div class=\"flex items-center gap-4\">\n      <a href=\"https://twitter.com/elixirphoenix\" class=\"text-[0.8125rem] font-semibold leading-6 text-zinc-900 hover:text-zinc-700\">\n        @elixirphoenix\n      </a>\n      <a href=\"https://github.com/phoenixframework/phoenix\" class=\"text-[0.8125rem] font-semibold leading-6 text-zinc-900 hover:text-zinc-700\">\n        GitHub\n      </a>\n      <a href=\"https://hexdocs.pm/phoenix/overview.html\" class=\"rounded-lg bg-zinc-100 px-2 py-1 text-[0.8125rem] font-semibold leading-6 text-zinc-900 hover:bg-zinc-200/80 active:text-zinc-900/70\">\n        Get Started <span aria-hidden=\"true\">&rarr;</span>\n      </a>\n    </div>\n  </div>\n</header>\n<main class=\"px-4 py-20 sm:px-6 lg:px-8\">\n </p>\n  \n</div>\n<div>\n<h1 class=\"text-4xl font-bold text-center\"> The count is: 0 </h1>\n\n<p class=\"text-center\">\n <button class=\"phx-submit-loading:opacity-75 rounded-lg bg-zinc-900 hover:bg-zinc-700 py-2 px-3 text-sm font-semibold leading-6 text-white active:text-white/80 \" phx-click=\"dec\">\n  -\n</button>\n <button class=\"phx-submit-loading:opacity-75 rounded-lg bg-zinc-900 hover:bg-zinc-700 py-2 px-3 text-sm font-semibold leading-6 text-white active:text-white/80 \" phx-click=\"inc\">\n  +\n</button>\n </p>\n </div>\n  </div>\n</main></div>\n  </body>\n</html>"
     right: "Peace of mind from prototype to production"
     stacktrace:
       test/counter_web/controllers/page_controller_test.exs:6: (test)


Finished in 0.1 seconds (0.06s async, 0.09s sync)
5 tests, 1 failure

This just tells us that the test is looking for the string "Peace of mind from prototype to production" in the page and did not find it.

To fix the broken test, open the test/counter_web/controllers/page_controller_test.exs file and locate the line:

assert html_response(conn, 200) =~ "Peace of mind from prototype to production"

Update the string from "Peace of mind from prototype to production" to something we know is present on the page, e.g: "The count is"

🏁 The page_controller_test.exs.exs file should now look like this: test/counter_web/controllers/page_controller_test.exs#L6

Confirm the tests pass again by running:

mix test

You should see output similar to:

.....
Finished in 0.08 seconds (0.03s async, 0.05s sync)
5 tests, 0 failures

Randomized with seed 268653

Checkpoint: Run Counter App!

Now that all the code for the counter.ex is written, run the Phoenix app with the following command:

mix phx.server

Vist localhost:4000 in your web browser.

You should expect to see a fully functioning LiveView counter:

Recap: Working Counter Without Writing JavaScript!

Once the initial installation and configuration of LiveView was complete, the creation of the actual counter was remarkably simple. We created a single new file lib/counter_web/live/counter.ex that contains all the code required to initialise, render and update the counter. Then we set the live "/", Counter route to invoke the Counter module in router.ex.

In total our counter App is 25 lines of (relevant) code. 🤯

One important thing to note is that the counter only maintains state for a single web browser. Try opening a second browser window (e.g: in "incognito mode") and notice how the counter only updates in one window at a time:

If we want to share the counter state between multiple clients, we need to add a bit more code.

Step 4: Share State Between Clients!

One of the biggest selling points of using Phoenix to build web apps is the built-in support for WebSockets in the form of Phoenix Channels. Channels allow us to effortlessly sync data between clients and servers with minimal overhead; they are one of Elixir (Erlang/OTP) superpowers!

We can share the counter state between multiple clients by updating the counter.ex file with the following code:

defmodule CounterWeb.Counter do
  use CounterWeb, :live_view

  @topic "live"

  def mount(_session, _params, socket) do
    CounterWeb.Endpoint.subscribe(@topic) # subscribe to the channel
    {:ok, assign(socket, :val, 0)}
  end

  def handle_event("inc", _value, socket) do
    new_state = update(socket, :val, &(&1 + 1))
    CounterWeb.Endpoint.broadcast_from(self(), @topic, "inc", new_state.assigns)
    {:noreply, new_state}
  end

  def handle_event("dec", _, socket) do
    new_state = update(socket, :val, &(&1 - 1))
    CounterWeb.Endpoint.broadcast_from(self(), @topic, "dec", new_state.assigns)
    {:noreply, new_state}
  end

  def handle_info(msg, socket) do
    {:noreply, assign(socket, val: msg.payload.val)}
  end

  def render(assigns) do
    ~H"""
    <div class="text-center">
      <h1 class="text-4xl font-bold text-center"> Counter: <%= @val %> </h1>
      <.button phx-click="dec" class="w-20 bg-red-500 hover:bg-red-600">-</.button>
      <.button phx-click="inc" class="w-20 bg-green-500 hover:bg-green-600">+</.button>
    </div>
    """
  end
end

Code Explanation

The first change is on Line 4 @topic "live" defines a module attribute (think of it as a global constant), that lets us to reference @topic anywhere in the file.

The second change is on Line 7 where the mount/3 function now creates a subscription to the @topic:

CounterWeb.Endpoint.subscribe(@topic) # subscribe to the channel topic

Each client connected to the App subscribes to the @topic so when the count is updated on any of the clients, all the other clients see the same value.

Next we update the first handle_event/3 function which handles the "inc" event:

def handle_event("inc", _msg, socket) do
  new_state = update(socket, :val, &(&1 + 1))
  CounterWeb.Endpoint.broadcast_from(self(), @topic, "inc", new_state.assigns)
  {:noreply, new_state}
end

Assign the result of the update invocation to new_state so that we can use it on the next two lines. Invoking CounterWeb.Endpoint.broadcast_from sends a message from the current process self() on the @topic, the key is "inc" and the value is the new_state.assigns Map.

In case you are curious (like we are), new_state is an instance of the Phoenix.LiveView.Socket socket:

#Phoenix.LiveView.Socket<
  assigns: %{
    flash: %{},
    live_view_action: nil,
    live_view_module: CounterWeb.Counter,
    val: 1
  },
  changed: %{val: true},
  endpoint: CounterWeb.Endpoint,
  id: "phx-Ffq41_T8jTC_3gED",
  parent_pid: nil,
  view: CounterWeb.Counter,
  ...
}

The new_state.assigns is a Map that includes the key val where the value is 1 (after we clicked on the increment button).

The fourth update is to the "dec" version of handle_event/3

def handle_event("dec", _msg, socket) do
  new_state = update(socket, :val, &(&1 - 1))
  CounterWeb.Endpoint.broadcast_from(self(), @topic, "dec", new_state.assigns)
  {:noreply, new_state}
end

The only difference from the "inc" version is the &(&1 - 1) and "dec" in the broadcast_from.

The final change is the implementation of the handle_info/2 function:

def handle_info(msg, socket) do
  {:noreply, assign(socket, val: msg.payload.val)}
end

handle_info/2 handles Elixir process messages where msg is the received message and socket is the Phoenix.Socket.
The line {:noreply, assign(socket, val: msg.payload.val)} just means "don't send this message to the socket again" (which would cause a recursive loop of updates).

Finally we modified the HTML inside the render/1 function to be a bit more visually appealing.

🏁 At the end of Step 6 the file looks like: lib/counter_web/live/counter.ex

Checkpoint: Run It!

Now that counter.ex has been updated to broadcast the count to all connected clients, let's run the app in a few web browsers to show it in action!

In your terminal, run:

mix phx.server

Open localhost:4000 in as many web browsers as you have and test the increment/decrement buttons!

You should see the count increasing/decreasing in all browsers simultaneously!

Congratulations! 🎉

You just built a real-time counter that seamlessly updates all connected clients using Phoenix LiveView in less than 40 lines of code!

Tests! 🧪

before we get carried away celebrating that we've finished the counter, Let's make sure that all the functionality, however basic, is fully tested.

Add excoveralls to Check/Track Coverage

Open your mix.exs file and locate the deps list. Add the following line to the list:

# Track test coverage: github.com/parroty/excoveralls
{:excoveralls, "~> 0.16.0", only: [:test, :dev]},

e.g: mix.exs#L58

Then, still in the mix.exs file, locate the project definition, and replace:

deps: deps()

With the following lines:

deps: deps(),
test_coverage: [tool: ExCoveralls],
preferred_cli_env: [
  c: :test,
  coveralls: :test,
  "coveralls.detail": :test,
  "coveralls.json": :test,
  "coveralls.post": :test,
  "coveralls.html": :test,
  t: :test
]

e.g: mix.exs#L13-L22

Finally in the aliases section of mix.exs, add the following lines:

c: ["coveralls.html"],
s: ["phx.server"],
t: ["test"]

The mix c alias is the one we care about, we're going to use it immediately. The other two mix s and mix t are convenient shortcuts too. Hopefully you can infer what they do. 👌

With the the mix.exs file updated, run the following commands in your terminal:

mix deps.get
mix c

That will download the excoveralls dependency and execute the tests with coverage tracking.

You should see output similar to the following:

Randomized with seed 468341
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
 75.0% lib/counter/application.ex                     34        4        1
100.0% lib/counter_web.ex                            111        2        0
 15.9% lib/counter_web/components/core_componen      661      151      127
100.0% lib/counter_web/components/layouts.ex           5        0        0
100.0% lib/counter_web/controllers/error_html.e       19        1        0
100.0% lib/counter_web/controllers/error_json.e       15        1        0
  0.0% lib/counter_web/controllers/page_control        9        1        1
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
 33.3% lib/counter_web/live/counter.ex                32       12        8
100.0% lib/counter_web/live/counter_component.e       17        2        0
100.0% lib/counter_web/router.ex                      18        2        0
 80.0% lib/counter_web/telemetry.ex                   69        5        1
[TOTAL]  23.8%
----------------
Generating report...
Saved to: cover/
FAILED: Expected minimum coverage of 100%, got 23.8%.

This tells us that only 23.8% of the code in the project is covered by tests. 😕 Let's fix that!

Create a coveralls.json File

In the root of the project, create a file called coveralls.json and add the following code to it:

{
  "coverage_options": {
    "minimum_coverage": 100
  },
  "skip_files": [
    "lib/counter/application.ex",
    "lib/counter_web.ex",
    "lib/counter_web/channels/user_socket.ex",
    "lib/counter_web/telemetry.ex",
    "lib/counter_web/views/error_helpers.ex",
    "lib/counter_web/router.ex",
    "lib/counter_web/live/page_live.ex",
    "lib/counter_web/components/core_components.ex",
    "lib/counter_web/controllers/error_json.ex",
    "lib/counter_web/controllers/error_html.ex",
    "test/"
  ]
}

This file and the skip_files list specifically, tells excoveralls to ignore boilerplate Phoenix files we cannot test.

If you re-run mix c now you should see something similar to the following:

Randomized with seed 572431
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
100.0% lib/counter_web/components/layouts.ex           5        0        0
  0.0% lib/counter_web/controllers/page_control        9        1        1
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
 33.3% lib/counter_web/live/counter.ex                32       12        8
[TOTAL]  40.0%
----------------
Generating report...
Saved to: cover/
FAILED: Expected minimum coverage of 100%, got 40%.

This is already much better. There are only 2 files we need to focus on. Let's start by tidying up the unused files.

DELETE Unused Files

Given that this counter App doesn't use any "controllers", we can simply DELETE the lib/counter_web/controllers/page_controller.ex file.

git rm lib/counter_web/controllers/page_controller.ex

Rename the test/counter_web/controllers/page_controller_test.exs to: test/counter_web/live/counter_test.exs

Update the code in the test/counter_web/live/counter_test.exs to:

defmodule CounterWeb.CounterTest do
  use CounterWeb.ConnCase
  import Phoenix.LiveViewTest

  test "connected mount", %{conn: conn} do
    {:ok, _view, html} = live(conn, "/")
    assert html =~ "Counter: 0"
  end
end

Re-run the tests:

mix c

You should see:

Finished in 0.1 seconds (0.04s async, 0.07s sync)
6 tests, 0 failures

Randomized with seed 603239
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
100.0% lib/counter_web/components/layouts.ex           5        0        0
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
 33.3% lib/counter_web/live/counter.ex                32       12        8
[TOTAL]  42.9%
----------------
Generating report...
Saved to: cover/
FAILED: Expected minimum coverage of 100%, got 42.9%.

Open the coverage HTML file:

open cover/excoveralls.html

You should see:

This shows us which functions/lines are not being covered by our existing tests.

Add More Tests!

Open the test/counter_web/live/counter_test.exs and add the following tests:

test "Increment", %{conn: conn} do
  {:ok, view, _html} = live(conn, "/")
  assert render_click(view, :inc) =~ "Counter: 1"
end

test "Decrement", %{conn: conn} do
  {:ok, view, _html} = live(conn, "/")
  assert render_click(view, :dec) =~ "Counter: -1"
end

test "handle_info/2 broadcast message", %{conn: conn} do
  {:ok, view, _html} = live(conn, "/")
  {:ok, view2, _html} = live(conn, "/")

  assert render_click(view, :inc) =~ "Counter: 1"
  assert render_click(view2, :inc) =~ "Counter: 2"
end

Once you've saved the file, re-run the tests: mix c You should see:

........
Finished in 0.1 seconds (0.03s async, 0.09s sync)
8 tests, 0 failures

Randomized with seed 552859
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
100.0% lib/counter_web/components/layouts.ex           5        0        0
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
100.0% lib/counter_web/live/counter.ex                32       12        0
[TOTAL] 100.0%
----------------

Done. ✅

Bonus Level: Use a LiveView Component (Optional)

At present the render/1 function in counter.ex has an inline template:

def render(assigns) do
  ~H"""
  <div class="text-center">
    <h1 class="text-4xl font-bold text-center"> Counter: <%= @val %> </h1>
    <.button phx-click="dec" class="text-6xl pb-2 w-20 bg-red-500 hover:bg-red-600">-</.button>
    <.button phx-click="inc" class="text-6xl pb-2 w-20 bg-green-500 hover:bg-green-600">+</.button>
  </div>
  """

This is fine when the template is small like in this counter, but in a bigger App like our MPV it's a good idea to split the template into a separate file to make it easier to read and maintain.

This is where Phoenix.LiveComponent comes to the rescue! LiveComponents are a mechanism to compartmentalize state, markup, and events in LiveView.

Create a LiveView Component

Create a new file with the path: lib/counter_web/live/counter_component.ex

And type (or paste) the following code in it:

defmodule CounterComponent do
  use Phoenix.LiveComponent

  # Avoid duplicating Tailwind classes and show hot to inline a function call:
  defp btn(color) do
    "text-6xl pb-2 w-20 rounded-lg bg-#{color}-500 hover:bg-#{color}-600"
  end

  def render(assigns) do
    ~H"""
    <div class="text-center">
      <h1 class="text-4xl font-bold text-center"> Counter: <%= @val %> </h1>
      <button phx-click="dec" class={btn("red")}>
        -
      </button>
      <button phx-click="inc" class={btn("green")}>
        +
      </button>
    </div>
    """
  end
end

Then back in the counter.ex file, update the render/1 function to:

  def render(assigns) do
    ~H"""
    <.live_component module={CounterComponent} id="counter" val={@val} />
    """
  end

🏁 Your counter_component.ex should look like this: lib/counter_web/live/counter_component.ex

The component has a similar render/1 function to what was in counter.ex. That's the point; we just want it in a separate file for maintainability.

Re-run the counter App using mix phx.server and confirm everything still works:

The tests all still pass and we have 100% coverage:

........
Finished in 0.1 seconds (0.03s async, 0.09s sync)
8 tests, 0 failures

Randomized with seed 470293
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
100.0% lib/counter_web/components/layouts.ex           5        0        0
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
100.0% lib/counter_web/live/counter.ex                32       12        0
100.0% lib/counter_web/live/counter_component.e       17        2        0
[TOTAL] 100.0%
----------------

Moving state out of the LiveView

With this implementation you may have noticed that when we open a new browser window the count is always zero. As soon as we click plus or minus it adjusts and all the views get back in line. This is because the state is replicated across LiveView instances and coordinated via pub-sub. If the state was big and complicated this would get wasteful in resources and hard to manage.

Generally it is good practice to identify shared state and to manage that in a single location.

The Elixir way of managing state is the GenServer, using PubSub to update the LiveView about changes. This allows the LiveViews to focus on specific state, separating concerns; making the application both more efficient (hopefully) and easier to reason about and debug.

Create a file with the path: lib/counter_web/live/counter_state.ex and add the following:

defmodule Counter.Count do
  use GenServer
  alias Phoenix.PubSub
  @name :count_server

  @start_value 0

  # External API (runs in client process)

  def topic do
    "count"
  end

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, @start_value, name: @name)
  end

  def incr() do
    GenServer.call @name, :incr
  end

  def decr() do
    GenServer.call @name, :decr
  end

  def current() do
    GenServer.call @name, :current
  end

  def init(start_count) do
    {:ok, start_count}
  end

  # Implementation (Runs in GenServer process)

  def handle_call(:current, _from, count) do
     {:reply, count, count}
  end

  def handle_call(:incr, _from, count) do
    make_change(count, +1)
  end

  def handle_call(:decr, _from, count) do
    make_change(count, -1)
  end

  defp make_change(count, change) do
    new_count = count + change
    PubSub.broadcast(Counter.PubSub, topic(), {:count, new_count})
    {:reply, new_count, new_count}
  end
end

The GenServer runs in its own process. Other parts of the application invoke the API in their own process, these calls are forwarded to the handle_call functions in the GenServer process where they are processed serially.

We have also moved the PubSub publication here as well.

We are also going to need to tell the Application that it now has some business logic; we do this in the start/2 function in the lib/counter/application.ex file.

  def start(_type, _args) do
    children = [
      # Start the App State
+     Counter.Count,
      # Start the Telemetry supervisor
      CounterWeb.Telemetry,
      # Start the PubSub system
      {Phoenix.PubSub, name: Counter.PubSub},
      # Start the Endpoint (http/https)
      CounterWeb.Endpoint
      # Start a worker by calling: Counter.Worker.start_link(arg)
      # {Counter.Worker, arg}
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Counter.Supervisor]
    Supervisor.start_link(children, opts)
  end
...

Finally, we need make some changes to the LiveView itself, it now has less to do!

defmodule CounterWeb.Counter do
  use CounterWeb, :live_view
  alias Counter.Count
  alias Phoenix.PubSub

  @topic Count.topic

  def mount(_params, _session, socket) do
    PubSub.subscribe(Counter.PubSub, @topic)

    {:ok, assign(socket, val: Count.current()) }
  end

  def handle_event("inc", _, socket) do
    {:noreply, assign(socket, :val, Count.incr())}
  end

  def handle_event("dec", _, socket) do
    {:noreply, assign(socket, :val, Count.decr())}
  end

  def handle_info({:count, count}, socket) do
    {:noreply, assign(socket, val: count)}
  end

  def render(assigns) do
    ~H"""
    <div>
      <h1>The count is: <%= @val %></h1>
      <.button phx-click="dec">-</.button>
      <.button phx-click="inc">+</.button>
    </div>
    """
  end
end

The initial state is retrieved from the shared Application GenServer process and the updates are being forwarded there via its API. Finally, the GenServer to all the active LiveView clients.

Update the Tests for GenServer State

Given that the counter.ex is now using the GenServer State, two of the tests now fail because the count is not correct.

mix t

Generated counter app
.....

  1) test connected mount (CounterWeb.CounterTest)
     test/counter_web/live/counter_test.exs:6
     Assertion with =~ failed
     code:  assert html =~ "Counter: 0"
     left:  "<html lang=\"en\" class=\"[scrollbar-gutter:stable]\"><head><meta charset=\"utf-8\"/><meta name=\"viewport\" content=\"width=device-width, initial-scale=1\"/><meta name=\"csrf-token\" content=\"A2QQMHc0OgsbDl0mUCZdGDoHWhUtMC4CDUIv9XHhtx2p6_iLerkvIbbk\"/><title data-suffix=\" · Phoenix Framework\">\nCounter\n     · Phoenix Framework</title><link phx-track-static=\"phx-track-static\" rel=\"stylesheet\" href=\"/assets/app.css\"/><script defer=\"defer\" phx-track-static=\"phx-track-static\" type=\"text/javascript\" src=\"/assets/app.js\">\n    </script></head><body class=\"bg-white antialiased\"><div data-phx-main=\"data-phx-main\" data-phx-session=\"SFMyNTY.g2gDaAJhBXQAAAAIdwJpZG0AAAAUcGh4LUYzWm01LWgycVNXZW5RREJ3B3Nlc3Npb250AAAAAHcKcGFyZW50X3BpZHcDbmlsdwZyb3V0ZXJ3GEVsaXhpci5Db3VudGVyV2ViLlJvdXRlcncEdmlld3cZRWxpeGlyLkNvdW50ZXJXZWIuQ291bnRlcncIcm9vdF9waWR3A25pbHcJcm9vdF92aWV3dxlFbGl4aXIuQ291bnRlcldlYi5Db3VudGVydwxsaXZlX3Nlc3Npb25oAncHZGVmYXVsdG4IAPP26BwRWHYXbgYA2w20ookBYgABUYA.Zae9BLTboLn6SPPe0qmktsfuru8HX2W4CALIBZNpcqE\" data-phx-static=\"SFMyNTY.g2gDaAJhBXQAAAADdwJpZG0AAAAUcGh4LUYzWm01LWgycVNXZW5RREJ3BWZsYXNodAAAAAB3CmFzc2lnbl9uZXdqbgYA2w20ookBYgABUYA.uooN8p97RRE1JN4tmkVNqC9islv-Np5B8wrewhwLnKc\" id=\"phx-F3Zm5-h2qSWenQDB\"><header class=\"px-4 sm:px-6 lg:px-8\"><div class=\"flex items-center justify-between border-b border-zinc-100 py-3 text-sm\"><div class=\"flex items-center gap-4\"><a href=\"/\"><img src=\"/images/logo.svg\" width=\"36\"/></a><p class=\"bg-brand/5 text-brand rounded-full px-2 font-medium leading-6\">\n        v1.7.7\n      </p></div><div class=\"flex items-center gap-4 font-semibold leading-6 text-zinc-900\"><a href=\"https://github.com/dwyl/phoenix-liveview-counter-tutorial\" class=\"hover:text-zinc-700\">\n        GitHub\n      </a><a href=\"https://github.com/dwyl/phoenix-liveview-counter-tutorial#how-\" class=\"rounded-lg bg-zinc-100 px-2 py-1 hover:bg-zinc-200/80\">\n        Get Started ..."
     right: "Counter: 0"
     stacktrace:
       test/counter_web/live/counter_test.exs:8: (test)



  2) test Increment (CounterWeb.CounterTest)
     test/counter_web/live/counter_test.exs:11
     Assertion with =~ failed
     code:  assert render_click(view, :inc) =~ "Counter: 1"
     left:  "<header class=\"px-4 sm:px-6 lg:px-8\"><div class=\"flex items-center justify-between border-b border-zinc-100 py-3 text-sm\"><div class=\"flex items-center gap-4\"><a href=\"/\"><img src=\"/images/logo.svg\" width=\"36\"/></a><p class=\"bg-brand/5 text-brand rounded-full px-2 font-medium leading-6\">\n        v1.7.7\n      </p></div><div class=\"flex items-center gap-4 font-semibold leading-6 text-zinc-900\"><a href=\"https://github.com/dwyl/phoenix-liveview-counter-tutorial\" class=\"hover:text-zinc-700\">\n        GitHub\n      </a><a href=\"https://github.com/dwyl/phoenix-liveview-counter-tutorial#how-\" class=\"rounded-lg bg-zinc-100 px-2 py-1 hover:bg-zinc-200/80\">\n      etc..."
     right: "Counter: 1"
     stacktrace:
       test/counter_web/live/counter_test.exs:13: (test)

.
Finished in 0.1 seconds (0.02s async, 0.09s sync)
8 tests, 2 failures

The test is expecting the initial state to be 0 (zero) each time. But if we are storing the count in the GenServer, it will not be 0.

We can easily update the tests to check the state before incrementing/decrementing it. Open the test/counter_web/live/counter_test.exs file and replace the contents with the following:

defmodule CounterWeb.CounterTest do
  use CounterWeb.ConnCase
  import Phoenix.LiveViewTest

  test "Increment", %{conn: conn} do
    {:ok, view, html} = live(conn, "/")
    current = Counter.Count.current()
    assert html =~ "Counter: #{current}"
    assert render_click(view, :inc) =~ "Counter: #{current + 1}"
  end

  test "Decrement", %{conn: conn} do
    {:ok, view, html} = live(conn, "/")
    current = Counter.Count.current()
    assert html =~ "Counter: #{current}"
    assert render_click(view, :dec) =~ "Counter: #{current - 1}"
  end

  test "handle_info/2 Count Update", %{conn: conn} do
    {:ok, view, disconnected_html} = live(conn, "/")
    current = Counter.Count.current()
    assert disconnected_html =~ "Counter: #{current}"
    assert render(view) =~ "Counter: #{current}"
    send(view.pid, {:count, 2})
    assert render(view) =~ "Counter: 2"
  end
end

Re-run the tests mix c and watch them pass with 100% coverage:

.......
Finished in 0.1 seconds (0.04s async, 0.09s sync)
7 tests, 0 failures

Randomized with seed 614997
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
100.0% lib/counter_web/components/layouts.ex           5        0        0
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
100.0% lib/counter_web/live/counter.ex                31        7        0
100.0% lib/counter_web/live/counter_component.e       17        2        0
100.0% lib/counter_web/live/counter_state.ex          53       12        0
[TOTAL] 100.0%
----------------

How many people are viewing the Counter?

Phoenix has a very cool feature called Presence to track how many people (connected clients) are using our system. It does a lot more than count clients, but this is a counting app so ...

First of all we need to tell the Application we are going to use Presence. For this we need to create a lib/counter/presence.ex file and add the following lines of code:

defmodule Counter.Presence do
  use Phoenix.Presence,
    otp_app: :counter,
    pubsub_server: Counter.PubSub
end

and tell the application about it in the lib/counter/application.ex file (add it just below the PubSub config):

  def start(_type, _args) do
    children = [
      # Start the App State
      Counter.Count,
      # Start the Telemetry supervisor
      CounterWeb.Telemetry,
      # Start the PubSub system
      {Phoenix.PubSub, name: Counter.PubSub},
+     Counter.Presence,
      # Start the Endpoint (http/https)
      CounterWeb.Endpoint
      # Start a worker by calling: Counter.Worker.start_link(arg)
      # {Counter.Worker, arg}
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Counter.Supervisor]
    Supervisor.start_link(children, opts)
  end

The application doesn't need to know any more about the user count (it might, but not here) so the rest of the code goes into lib/counter_web/live/counter.ex.

1. We subscribe to and participate in the Presence system (we do that in mount)
2. We handle Presence updates and use the current count, adding joiners and subtracting leavers to calculate the current numbers 'present'. We do that in a pattern matched handle_info.
3. We publish the additional data to the client in render

defmodule CounterWeb.Counter do
  use CounterWeb, :live_view
  alias Counter.Count
  alias Phoenix.PubSub
+ alias Counter.Presence

  @topic Count.topic
+ @presence_topic "presence"

  def mount(_params, _session, socket) do
    PubSub.subscribe(Counter.PubSub, @topic)

+   Presence.track(self(), @presence_topic, socket.id, %{})
+   CounterWeb.Endpoint.subscribe(@presence_topic)
+
+   initial_present =
+     Presence.list(@presence_topic)
+     |> map_size

+   {:ok, assign(socket, val: Count.current(), present: initial_present) }
-   {:ok, assign(socket, val: Count.current()) }
  end

  def handle_event("inc", _, socket) do
    {:noreply, assign(socket, :val, Count.incr())}
  end

  def handle_event("dec", _, socket) do
    {:noreply, assign(socket, :val, Count.decr())}
  end

  def handle_info({:count, count}, socket) do
    {:noreply, assign(socket, val: count)}
  end

+ def handle_info(
+       %{event: "presence_diff", payload: %{joins: joins, leaves: leaves}},
+       %{assigns: %{present: present}} = socket
+    ) do
+   new_present = present + map_size(joins) - map_size(leaves)
+
+   {:noreply, assign(socket, :present, new_present)}
+ end

  def render(assigns) do
    ~H"""
    <.live_component module={CounterComponent} id="counter" val={@val} />
+   <.live_component module={PresenceComponent} id="presence" present={@present} />
    """
  end
end

You will have noticed that last addition in the render/1 function invokes a PresenceComponent. It doesn't exist yet, let's create it now!

Create a file with the path: lib/counter_web/live/presence_component.ex and add the following code to it:

defmodule PresenceComponent do
  use Phoenix.LiveComponent

  def render(assigns) do
    ~H"""
    <h1 class="text-center pt-2 text-xl">Connected Clients: <%= @present %></h1>
    """
  end
end

Now, as you open and close your incognito windows to localhost:4000, you will get a count of how many are running.

More Tests!

Once you have implemented the solution, you need to make sure that the new code is tested. Open the test/counter_web/live/counter_test.exs file and add the following tests:

test "handle_info/2 Presence Update - Joiner", %{conn: conn} do
  {:ok, view, html} = live(conn, "/")
  assert html =~ "Connected Clients: 1"
  send(view.pid, %{
    event: "presence_diff",
    payload: %{joins: %{"phx-Fhb_dqdqsOCzKQAl" => %{metas: [%{phx_ref: "Fhb_dqdrwlCmfABl"}]}},
                leaves: %{}}})
  assert render(view) =~ "Connected Clients: 2"
end

test "handle_info/2 Presence Update - Leaver", %{conn: conn} do
  {:ok, view, html} = live(conn, "/")
  assert html =~ "Connected Clients: 1"
  send(view.pid, %{
    event: "presence_diff",
    payload: %{joins: %{},
                leaves: %{"phx-Fhb_dqdqsOCzKQAl" => %{metas: [%{phx_ref: "Fhb_dqdrwlCmfABl"}]}}}})
  assert render(view) =~ "Connected Clients: 0"
end

With those tests in place, re-running the tests with coverage mix c, you should see the following:

.........
Finished in 0.1 seconds (0.04s async, 0.1s sync)
9 tests, 0 failures

Randomized with seed 958121
----------------
COV    FILE                                        LINES RELEVANT   MISSED
100.0% lib/counter.ex                                  9        0        0
100.0% lib/counter/presence.ex                         5        0        0
100.0% lib/counter_web/components/layouts.ex           5        0        0
100.0% lib/counter_web/controllers/page_html.ex        5        0        0
100.0% lib/counter_web/endpoint.ex                    46        0        0
100.0% lib/counter_web/live/counter.ex                51       13        0
100.0% lib/counter_web/live/counter_component.e       17        2        0
100.0% lib/counter_web/live/counter_state.ex          53       12        0
100.0% lib/counter_web/live/presence_component.        9        2        0
[TOTAL] 100.0%
----------------

Done! 🏁

That's it for this tutorial.
We hope you enjoyed learning with us!
If you found this useful, please ⭐️ and share the GitHub repo so we know you like it!

What's Next?

If you've enjoyed this basic counter tutorial and want something a bit more advanced, checkout our LiveView Chat Tutorial: github.com/dwyl/phoenix-liveview-chat-example 💬
Then if you want a more advanced "real world" App that uses LiveView extensively including Authentication and some client-side JS, checkout our MVP App /dwyl/mvp 📱⏳✅ ❤️

Feedback 💬 🙏

Several people in the Elixir / Phoenix community have found this tutorial helpful when starting to use LiveView, e.g: Kurt Mackey @mrkurt twitter.com/mrkurt/status/1362940036795219973

He deployed the counter app to a 17 region cluster using fly.io: https://liveview-counter.fly.dev

Code: https://github.com/fly-apps/phoenix-liveview-cluster/blob/master/lib/counter_web/live/counter.ex

Your feedback is always very much welcome! 🙏

Credits + Thanks! 🙌

Credit for inspiring this tutorial goes to Dennis Beatty @dnsbty for his superb post: https://dennisbeatty.com/2019/03/19/how-to-create-a-counter-with-phoenix-live-view.html and corresponding video: youtu.be/2bipVjOcvdI

We recommend everyone learning Elixir subscribe to his YouTube channel and watch all his videos as they are a superb resource!

The 3 key differences between this tutorial and Dennis' original post are:

1. Complete code commit (snapshot) at the end of each section (not just inline snippets of code).
   We feel that having the complete code speeds up learning significantly, especially if (when) we get stuck.
2. Latest Phoenix, Elixir and LiveView versions. Many updates have been made to LiveView setup since Dennis published his video, these are reflected in our tutorial which uses the latest release.
3. Broadcast updates to all connected clients. So when the counter is incremented/decremented in one client, all others see the update. This is the true power and "WOW Moment" of LiveView!

Phoenix LiveView for Web Developers Who Don't know Elixir

If you are new to LiveView (and have the bandwidth), we recommend watching James @knowthen Moore's intro to LiveView where he explains the concepts: youtu.be/U_Pe8Ru06fM

Watching the video is not required; you will be able to follow the tutorial without it.

Chris McCord (creator of Phoenix and LiveView) has github.com/chrismccord/phoenix_live_view_example
It's a great collection of examples for people who already understand LiveView. However we feel that it is not very beginner-friendly (at the time of writing). Only the default "start your Phoenix server" instructions are included, and the dependencies have diverged so the app does not compile/run for some people. We understand/love that Chris is focussed building Phoenix and LiveView so we decided to fill in the gaps and write this beginner-focussed tutorial.

If you haven't watched Chris' Keynote from ElixirConf EU 2019, we highly recommend watching it: youtu.be/8xJzHq8ru0M

Also read the original announcement for LiveView to understand the hype!
: https://dockyard.com/blog/2018/12/12/phoenix-liveview-interactive-real-time-apps-no-need-to-write-javascript

Sophie DeBenedetto's ElixirConf 2019 talk "Beyond LiveView: Building Real-Time features with Phoenix LiveView, PubSub, Presence and Channels (Hooks) is worth watching: youtu.be/AbNAuOQ8wBE

Related blog post: https://elixirschool.com/blog/live-view-live-component/

Relevant + Recommended Reading

• Real-Time Form Validation with Phoenix LiveView: https://blog.appsignal.com/2021/09/28/real-time-form-validations-with-phoenix-liveview.html
• Optimizing User Experience with LiveView: https://dockyard.com/blog/2020/12/21/optimizing-user-experience-with-liveview
• TDD with LiveView: https://youtu.be/KfW3l3qJPH8