defmodule Phoenix.LiveComponent do
@moduledoc ~S'''
LiveComponents are a mechanism to compartmentalize state, markup, and
events in LiveView.
LiveComponents are defined by using `Phoenix.LiveComponent` and are used
by calling `Phoenix.Component.live_component/1` in a parent LiveView.
They run inside the LiveView process but have their own state and
life-cycle. For this reason, they are also often called "stateful components".
This is a contrast to `Phoenix.Component`, also known as "function components",
which are stateless.
The smallest LiveComponent only needs to define a `c:render/1` function:
defmodule HeroComponent do
# If you generated an app with mix phx.new --live,
# the line below would be: use MyAppWeb, :live_component
use Phoenix.LiveComponent
def render(assigns) do
~H"""
<%= @content %>
"""
end
end
A LiveComponent is rendered as:
<.live_component module={HeroComponent} id="hero" content={@content} />
You must always pass the `module` and `id` attributes. The `id` will be
available as an assign and it must be used to uniquely identify the
component. All other attributes will be available as assigns inside the
LiveComponent.
## Life-cycle
Stateful components are identified by the component module and their ID.
Therefore, two stateful components with the same module and ID are treated
as the same component. We often tie the component ID to some application based ID:
<.live_component module={UserComponent} id={@user.id} user={@user} />
When [`live_component/1`](`Phoenix.Component.live_component/1`) is called,
`c:mount/1` is called once, when the component is first added to the page. `c:mount/1`
receives the `socket` as argument. Then `c:update/2` is invoked with all of the
assigns given to [`live_component/1`](`Phoenix.Component.live_component/1`).
If `c:update/2` is not defined all assigns are simply merged into the socket.
After the component is updated, `c:render/1` is called with all assigns.
On first render, we get:
mount(socket) -> update(assigns, socket) -> render(assigns)
On further rendering:
update(assigns, socket) -> render(assigns)
The given `id` is not automatically used as the DOM ID. If you want to set
a DOM ID, it is your responsibility to do so when rendering:
defmodule UserComponent do
use Phoenix.LiveComponent
def render(assigns) do
~H"""
<%= @user.name %>
"""
end
end
## Targeting Component Events
Stateful components can also implement the `c:handle_event/3` callback
that works exactly the same as in LiveView. For a client event to
reach a component, the tag must be annotated with a `phx-target`.
If you want to send the event to yourself, you can simply use the
`@myself` assign, which is an *internal unique reference* to the
component instance:
Say hello!
Note that `@myself` is not set for stateless components, as they cannot
receive events.
If you want to target another component, you can also pass an ID
or a class selector to any element inside the targeted component.
For example, if there is a `UserComponent` with the DOM ID of `"user-13"`,
using a query selector, we can send an event to it with:
Say hello!
In both cases, `c:handle_event/3` will be called with the
"say_hello" event. When `c:handle_event/3` is called for a component,
only the diff of the component is sent to the client, making them
extremely efficient.
Any valid query selector for `phx-target` is supported, provided that the
matched nodes are children of a LiveView or LiveComponent, for example
to send the `close` event to multiple components:
Dismiss
## Preloading and update
Stateful components also support an optional `c:preload/1` callback.
The `c:preload/1` callback is useful when multiple components of the
same type are rendered on the page and you want to preload or augment
their data in batches.
For each rendering, the optional `c:preload/1` and `c:update/2` callbacks
are called before `c:render/1`.
So on first render, the following callbacks will be invoked:
preload(list_of_assigns) -> mount(socket) -> update(assigns, socket) -> render(assigns)
On subsequent renders, these callbacks will be invoked:
preload(list_of_assigns) -> update(assigns, socket) -> render(assigns)
To provide a more complete understanding of why both callbacks are necessary,
let's see an example. Imagine you are implementing a component and the component
needs to load some state from the database. For example:
<.live_component module={UserComponent} id={user_id} />
A possible implementation would be to load the user on the `c:update/2`
callback:
def update(assigns, socket) do
user = Repo.get!(User, assigns.id)
{:ok, assign(socket, :user, user)}
end
However, the issue with said approach is that, if you are rendering
multiple user components in the same page, you have a N+1 query problem.
The `c:preload/1` callback helps address this problem as it is invoked
with a list of assigns for all components of the same type. For example,
instead of implementing `c:update/2` as above, one could implement:
def preload(list_of_assigns) do
list_of_ids = Enum.map(list_of_assigns, & &1.id)
users =
from(u in User, where: u.id in ^list_of_ids, select: {u.id, u})
|> Repo.all()
|> Map.new()
Enum.map(list_of_assigns, fn assigns ->
Map.put(assigns, :user, users[assigns.id])
end)
end
Now only a single query to the database will be made. In fact, the
preloading algorithm is a breadth-first tree traversal, which means
that even for nested components, the amount of queries are kept to
a minimum.
Finally, note that `c:preload/1` must return an updated `list_of_assigns`,
keeping the assigns in the same order as they were given.
## Slots
LiveComponent can also receive slots, in the same way as a `Phoenix.Component`:
<.live_component module={MyComponent} id={@data.id} >
Inner content here
If the LiveComponent defines an `c:update/2`, be sure that the socket it returns
includes the `:inner_block` assign it received.
See [the docs](Phoenix.Component.html#module-slots.md) for `Phoenix.Component` for more information.
## Live patches and live redirects
A template rendered inside a component can use `Phoenix.LiveView.Helpers.live_patch/2`
and `Phoenix.LiveView.Helpers.live_redirect/2` calls. The
[`live_patch/2`](`Phoenix.LiveView.Helpers.live_patch/2`) is always handled
by the parent`LiveView`, as components do not provide `handle_params`.
## Managing state
Now that we have learned how to define and use components, as well as
how to use `c:preload/1` as a data loading optimization, it is important
to talk about how to manage state in components.
Generally speaking, you want to avoid both the parent LiveView and the
LiveComponent working on two different copies of the state. Instead, you
should assume only one of them to be the source of truth. Let's discuss
the two different approaches in detail.
Imagine a scenario where a LiveView represents a board with each card
in it as a separate stateful LiveComponent. Each card has a form to
allow update of the card title directly in the component, as follows:
defmodule CardComponent do
use Phoenix.LiveComponent
def render(assigns) do
~H"\""
"\""
end
...
end
We will see how to organize the data flow to keep either the board LiveView or
the card LiveComponents as the source of truth.
### LiveView as the source of truth
If the board LiveView is the source of truth, it will be responsible
for fetching all of the cards in a board. Then it will call
[`live_component/1`](`Phoenix.Component.live_component/1`)
for each card, passing the card struct as argument to `CardComponent`:
<%= for card <- @cards do %>
<.live_component module={CardComponent} card={card} id={card.id} board_id={@id} />
<% end %>
Now, when the user submits the form, `CardComponent.handle_event/3`
will be triggered. However, if the update succeeds, you must not
change the card struct inside the component. If you do so, the card
struct in the component will get out of sync with the LiveView. Since
the LiveView is the source of truth, you should instead tell the
LiveView that the card was updated.
Luckily, because the component and the view run in the same process,
sending a message from the LiveComponent to the parent LiveView is as
simple as sending a message to `self()`:
defmodule CardComponent do
...
def handle_event("update_title", %{"title" => title}, socket) do
send self(), {:updated_card, %{socket.assigns.card | title: title}}
{:noreply, socket}
end
end
The LiveView then receives this event using `c:Phoenix.LiveView.handle_info/2`:
defmodule BoardView do
...
def handle_info({:updated_card, card}, socket) do
# update the list of cards in the socket
{:noreply, updated_socket}
end
end
Because the list of cards in the parent socket was updated, the parent
LiveView will be re-rendered, sending the updated card to the component.
So in the end, the component does get the updated card, but always
driven from the parent.
Alternatively, instead of having the component send a message directly to the
parent view, the component could broadcast the update using `Phoenix.PubSub`.
Such as:
defmodule CardComponent do
...
def handle_event("update_title", %{"title" => title}, socket) do
message = {:updated_card, %{socket.assigns.card | title: title}}
Phoenix.PubSub.broadcast(MyApp.PubSub, board_topic(socket), message)
{:noreply, socket}
end
defp board_topic(socket) do
"board:" <> socket.assigns.board_id
end
end
As long as the parent LiveView subscribes to the `board:` topic,
it will receive updates. The advantage of using PubSub is that we get
distributed updates out of the box. Now, if any user connected to the
board changes a card, all other users will see the change.
### LiveComponent as the source of truth
If each card LiveComponent is the source of truth, then the board LiveView
must no longer fetch the card structs from the database. Instead, the board
LiveView must only fetch the card ids, then render each component only by
passing an ID:
<%= for card_id <- @card_ids do %>
<.live_component module={CardComponent} id={card_id} board_id={@id} />
<% end %>
Now, each CardComponent will load its own card. Of course, doing so
per card could be expensive and lead to N queries, where N is the
number of cards, so we can use the `c:preload/1` callback to make it
efficient.
Once the card components are started, they can each manage their own
card, without concerning themselves with the parent LiveView.
However, note that components do not have a `c:Phoenix.LiveView.handle_info/2`
callback. Therefore, if you want to track distributed changes on a card,
you must have the parent LiveView receive those events and redirect them
to the appropriate card. For example, assuming card updates are sent
to the "board:ID" topic, and that the board LiveView is subscribed to
said topic, one could do:
def handle_info({:updated_card, card}, socket) do
send_update CardComponent, id: card.id, board_id: socket.assigns.id
{:noreply, socket}
end
With `Phoenix.LiveView.send_update/3`, the `CardComponent` given by `id`
will be invoked, triggering both preload and update callbacks, which will
load the most up to date data from the database.
## Cost of stateful components
The internal infrastructure LiveView uses to keep track of stateful
components is very lightweight. However, be aware that in order to
provide change tracking and to send diffs over the wire, all of the
components' assigns are kept in memory - exactly as it is done in
LiveViews themselves.
Therefore it is your responsibility to keep only the assigns necessary
in each component. For example, avoid passing all of LiveView's assigns
when rendering a component:
<.live_component module={MyComponent} {assigns} />
Instead pass only the keys that you need:
<.live_component module={MyComponent} user={@user} org={@org} />
Luckily, because LiveViews and LiveComponents are in the same process,
they share the data structure representations in memory. For example,
in the code above, the view and the component will share the same copies
of the `@user` and `@org` assigns.
You should also avoid using stateful components to provide abstract DOM
components. As a guideline, a good LiveComponent encapsulates
application concerns and not DOM functionality. For example, if you
have a page that shows products for sale, you can encapsulate the
rendering of each of those products in a component. This component
may have many buttons and events within it. On the opposite side,
do not write a component that is simply encapsulating generic DOM
components. For instance, do not do this:
defmodule MyButton do
use Phoenix.LiveComponent
def render(assigns) do
~H"\""
"\""
end
def handle_event("click", _, socket) do
_ = socket.assigns.on_click.()
{:noreply, socket}
end
end
Instead, it is much simpler to create a function component:
def my_button(%{text: _, click: _} = assigns) do
~H"\""
"\""
end
If you keep components mostly as an application concern with
only the necessary assigns, it is unlikely you will run into
issues related to stateful components.
## Limitations
### Live Components require a single HTML tag at the root
Live Components require a single HTML tag at the root. It is not possible
to have components that render only text or multiple tags.
### SVG support
Given components compartmentalize markup on the server, they are also
rendered in isolation on the client, which provides great performance
benefits on the client too.
However, when rendering components on the client, the client needs to
choose the mime type of the component contents, which defaults to HTML.
This is the best default but in some cases it may lead to unexpected
results.
For example, if you are rendering SVG, the SVG will be interpreted as
HTML. This may work just fine for most components but you may run into
corner cases. For example, the `` SVG tag may be rewritten to
the `` tag, since `` is an obsolete HTML tag.
Luckily, there is a simple solution to this problem. Since SVG allows
`