# LiveViewEvents

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LiveViewEvents provides a set of tools to send events between components.

## Installation

Add `live_view_events` to your list of dependencies in `mix.exs` like:

def deps do
    {:live_view_events, "~> 0.1.0"}

## Usage

Add `use LiveViewEvents` whenever you want to use any of the features of the libraries.

### Sending events to LiveView components

You can send events to LiveView components by using `notify_to/2` or `notify_to/3` (the only
difference being that the latter sends some extra params). These functions accept a target as
first argument and a message name as second. Targets can be any of:

  - `nil` makes the function call be a noop. Useful if you want to only send a message if an explicit target has been passed in.
  - `:self` to send to `self()`.
  - A PID.
  - A tuple of the form `{Module, "id"}` to send a message to a `LiveView.Component` in the same process.
  - A tuple of the form `{pid, Module, "id"}` to send a message to a `LiveView.Component` in a different process.

You an handle these messages on the `handle_info/2` callback on
your LiveView components. For being able to do so, you need to
use the `handle_info_or_assign/2` macro on the `update/3` callback of your component. 

## Example

In our view, we are going to have two components. The first one is a `Sender`
that can send an event to whatever is being passed. Let's dig into the

defmodule MyAppWeb.Components.Sender do
  use MyAppWeb, :live_component
  use LiveViewEvents

  def mount(socket) do
    socket = assign(socket, :notify_to, :self)

    {:ok, socket}

  def render(assigns) do
    ~H[<button type="button" phx-click="clicked" phx-target={@myself}>Send event</button>]

  def handle_event("clicked", _params, socket) do
    notify_to(socket.assigns.notify_to, :sender_event, :rand.uniform(100))
    {:noreply, socket}

This component will send a `:sender_event` message to whatever we pass to the `notify_to`
attribute. It sends a random number between 0 and up to 100 as parameter.

Let's dig into the receiver:

defmodule MyAppWeb.Components.Receiver do
  use MyAppWeb, :live_component
  use LiveViewEvents

  def mount(socket) do
    socket = assign(socket, :messages, [])

    {:ok, socket}

  def update(assigns, socket) do
    socket = handle_info_or_assign(socket, assigns)

    {:ok, socket}

  def render(assigns),
    do: ~H[<ul><li :for={m <- @messages}><%= m %></li></ul>]

  def handle_info({:sender_event, num}, socket) do
    {:noreply, update(socket, :messages, &[num | &1])}

This component will receive messages and handle them in `handle_info/2` because
it is using `handle_info_or_assign/2` in their `LiveComponent.update/2`.
It will add the received messages to `socket.assigns.messages` and display
them. As the reader can see, it is pattern matching against `:sender_event` messages.
When `notify_to/3` is used, the message sent is a tuple containing the event name
as first element, and the params as second the second element.

Finally, let's take a look at what the live view template would need to look like
for this to work:

<div class="contents">
    notify_to={{MyAppWeb.Components.Receiver, "receiver"}}
  <.live_component module={MyAppWeb.Components.Receiver} id="receiver" />

In this template, we set `notify_to` to the tuple `{MyAppWeb.Components.Receiver, "receiver"}`.
The first element of the tuple is the live component module and the second is the id.
Optionally, the tuple can contain an extra first element that needs to be a PID. Though
this might not be useful in the application code (there are way better ways to send events
between processes), it is quite useful when testing. When testing a `LiveView`
it creates a new process for it. Its PID can be accessed through ``.

## Why are messages without params still receiving empty params?

To normalize how to handle messages. If we could have `handle_info("message_name", _socket)`
or `handle_info({"message_name", _params}, socket)`, you would need to think about whether
the message contains params or not and then use a tuple. With this normalization, you only
care about the message name and whether you need the params or not. Also, after using similar
approaches in the past, sometimes you want to send params and sometimes you don't need to.
In cases like these, it is helpful not having a different behaviour notifying an event without
params and sending an event with _empty_ params.