In the previous video we learned what makes Phoenix LiveView unique. Now let’s build a basic LiveView from scratch to see how to react to user events. This video is from our free course, and all the steps are outlined below so you can easily recreate it yourself. 🏗

A Simple Light Controller

Building this simple light controller is a great way to get our feet wet with LiveView. It starts at 10% brightness. From there we can turn it off, turn it all the way on, or adjust the brightness where we want it.

Live Demo

Create the Phoenix App

In the video above we started in Git branch that already had a generated Phoenix 1.5 app. To get to the same starting point, first create a Phoenix 1.5 app using the --live option:

mix phx.new live_view_studio --live

Then change into the live_view_studio directory and set up the app:

cd live_view_studio

mix setup

Now you’re ready to fire up the Phoenix server:

mix phx.server

Then if you then browse to http://localhost:4000 you should see the standard Phoenix welcome page. 🔥

Add a Live Route

Next we need a live route which we define using the live macro in router.ex like so:

scope "/", LiveViewStudioWeb do
  pipe_through :browser

  ...

  live "/light", LightLive
end

The live macro takes a path and a LiveView module to use. So in this case a request to /light gets routed to the LightLive module.

Let’s define that module…

Create a LiveView Module

By convention LiveView modules reside in the live_view_studio_web/live/ directory. Create a light_live.ex file in that directory and then define the LightLive module like so:

defmodule LiveViewStudioWeb.LightLive do
  use LiveViewStudioWeb, :live_view
end

All the good stuff we need to be able to create a LiveView gets pulled in by use LiveViewStudioWeb, :live_view.

Define mount

A LiveView module needs to define three callback functions:

  • mount assigns the initial state of the LiveView process

  • handle_event changes the state of the process

  • and render renders a new view for the newly-updated state

Let’s start with the mount callback since it’s the first callback that’s invoked when the request comes in through the router:

defmodule LiveViewStudioWeb.LightLive do
  use LiveViewStudioWeb, :live_view

  def mount(_params, _session, socket) do
  end
end

The mount callback takes three arguments:

  • params is a map containing the current query params, as well as any router parameters

  • session is a map containing private session data

  • socket is a struct representing the websocket connection

For this example we only need the socket argument, so we’ve ignored the others. It’s in the socket struct that we store the state of a LiveView process. And it’s in mount that we assign the initial state.

Let’s say we want the light to initially be set at a warm glow of 10% brightness. To do that we use the assign function, passing it the socket and a key/value pair to assign to the socket:

def mount(_params, _session, socket) do
  socket = assign(socket, :brightness, 10)
  {:ok, socket}
end

Calling assign this way returns a new socket struct with :brightness set to 10. And mount must return an :ok tuple with the new socket.

Rather than using the temporary socket variable you’ll often see it inlined like this:

def mount(_params, _session, socket) do
  {:ok, assign(socket, :brightness, 10)}
end

Notice we passed the key and value as separate arguments to assign, but if you had multiple key/value pairs you would pass a keyword list.

Define render

Next up we’ll define the render callback:

def render(assigns) do
  ~L"""
  """
end

The render function takes assigns which is the map of key/value pairs we assigned to the socket in mount. The one-and-only job of render is to return rendered content, which we create using a LiveView template.

In this case we used the special ~L sigil which defines an inlined LiveView template. Alternatively, we could put this template code in a separate file, but we’ll co-locate the template since it’ll be relatively short.

Inside the template we can access the current brightness value using @brightness and interpolate it in an EEx tag:

def render(assigns) do
  ~L"""
  <h1>Front Porch Light</h1>
  <%= @brightness %>%
  """
end

Now if you hop into the browser and go to http://localhost:4000/light, you should see the initial brightness of 10%.

Good!

Show a Light Meter

Now let’s dress it up a bit by using the brightness value to display a light meter, kinda like a progress bar. We’ll use a span with a width based on the current brightness value:

def render(assigns) do
  ~L"""
  <h1>Front Porch Light</h1>
  <div class="meter">
    <span style="width: <%= @brightness %>%">
      <%= @brightness %>%
    </span>
  </div>
  """
end

A splash of CSS is needed to bring this to life, and to save you the trouble go ahead and plunk this at the bottom of live_view_studio/assets/css/app.scss:

.meter {
  display: flex;
  overflow: hidden;
  background-color: #e2e8f0;
  border-radius: 0.75rem;
  margin-bottom: 2rem;
  height: 4rem;

  span {
    display: flex;
    flex-direction: column;
    justify-content: center;
    background-color: #faf089;
    text-align: center;
    white-space: nowrap;
    font-weight: 700;
    font-size: 1.5rem;
    transition: width 2s ease;
  }
}

Not too shabby, eh?

Make It Interactive: On and Off Buttons

Now let’s make it interactive by adding buttons to turn the light on and off. They’re part of the view, so they go in the the render function:

<button>
  Off
</button>

<button>
  On
</button>

When a button is clicked we need to send an event to our LightLive process to change the brightness. To do that, we use the special phx-click HTML attribute (it’s unique to LiveView) and its value is the name of the event to send. Because we’re creative, we’ll have the “Off” button send an off event and the “On” button send an on event:

<button phx-click="off">
  Off
</button>

<button phx-click="on">
  On
</button>

LiveView refers to the phx-click attribute (and others like it) as a binding. It binds a click event to the button so when it’s clicked the event is sent via the websocket to the LightLive process.

Handle On and Off Events

Then to handle those inbound events, we need to define matching handle_event callbacks. We’ll start by handling the on event:

def handle_event("on", _, socket) do
  socket = assign(socket, :brightness, 100)
  {:noreply, socket}
end

The handle_event callback takes three arguments:

  • the name of the event (on in this case)
  • metadata related to the event which we’ll ignore
  • the socket which, remember, has the current state of our live view assigned to it

To turn the light on we use assign to set the brightness value to 100. And then handle_event must return a tuple with :noreply and the new socket.

Now, whenever a live view’s state changes (in our case brightness changed from 10 to 100), the render function is automatically called to render a new view with the newly-updated state.

Then we also need a handle_event callback for the off event:

def handle_event("off", _, socket) do
  socket = assign(socket, :brightness, 0)
  {:noreply, socket}
end

And that’s all there is to reacting to user events. 👍

Dim the Lights

Now that we’ve got the hang of this, let’s add a couple more buttons to dim the lights down and up, which will introduce us to something new.

No surprise, these buttons emit corresponding down and up events:

<button phx-click="down">
  Down
</button>

<button phx-click="up">
  Up
</button>

Then we need to handle those two inbound events. The down event will dial back the brightness by 10 and the up event will bump up the brightness by 10.

To do that, the respective handle_event callbacks need to access the current brightness value. And since the socket struct holds the current state of a LiveView process, we can get the current brightness value using socket.assigns.brightness. So the handlers look like this:

def handle_event("down", _, socket) do
  brightness = socket.assigns.brightness - 10
  socket = assign(socket, :brightness, brightness)
  {:noreply, socket}
end

def handle_event("up", _, socket) do
  brightness = socket.assigns.brightness + 10
  socket = assign(socket, :brightness, brightness)
  {:noreply, socket}
end

That works, but there’s a convenient shortcut. When updating a value, we can use the update function, like so:

def handle_event("down", _, socket) do
  socket = update(socket, :brightness, fn b -> b - 10 end)
  {:noreply, socket}
end

def handle_event("up", _, socket) do
  socket = update(socket, :brightness, fn b -> b + 10 end)
  {:noreply, socket}
end

Notice that update takes a function as its third argument. That function receives the current key’s value (the brightness value in this case) and returns an updated value which is then assigned to the key in the socket. Thus, the value is updated.

You can make this more concise using the Elixir shorthand capture syntax:

def handle_event("down", _, socket) do
  socket = update(socket, :brightness, &(&1 - 10))
  {:noreply, socket}
end

def handle_event("up", _, socket) do
  socket = update(socket, :brightness, &(&1 + 10))
  {:noreply, socket}
end

For bonus points, you could limit how high or low the light can go by using max and min:

def handle_event("down", _, socket) do
  socket = update(socket, :brightness, &max(&1 - 10, 0))
  {:noreply, socket}
end

def handle_event("up", _, socket) do
  socket = update(socket, :brightness, &min(&1 + 10, 100))
  {:noreply, socket}
end

Now if you head over to http://localhost:4000/light, you should have a fully-functioning light meter. 💡

Summary

Stepping back, you gotta admit that’s a lot of functionality for such a minimal amount of code:

  • mount initialize the state of the LiveView process by assigning values to the socket

  • render uses a LiveView template to render a view for the state in the socket

  • handle_event functions handle each inbound event by changing the state in the socket using either assign or update

And whenever the state changes render is automatically called to render a new view for the updated state. 🌀

It’s a fairly simple (and fun!) programming model, all written in Elixir. Not one line of custom JavaScript! But there’s more going on behind the scenes than meets the eye. And we’ll look at the lifecycle of a LiveView in more detail in the next video.

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