Migrating to Phoenix Liveview Streams
Tags: elixir, phoenix, liveview, streams
Published: 2023-04-19 06:19:49

Phoenix released 1.7.0 with Liveview 0.18.16 and introduced Streams for liveview. At first it was not clear to me how to replace my current prepend logic from the published blog  So I reached out to its creator of phoenix Chris McCord on twitter and he not only responded, even took out time to review my code and share what would be the way to get it done with stream. This is why elixir community is awesome.


![tweet thread](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/bmlpu6grbhxrd7s5y7i1.jpg)

The thing that was not clear from the blog post introducing streams was how would one send additional data over streams that is part of the page but not the actual stream.My [apps stream dashboard](https://dev.to/darnahsan/how-i-made-a-poor-man-s-personal-oblivious-dns-over-https-1o40) used prepend and it clearly could benefit from the streams and was an obvious choice for upgrade with streams release.

Here is how the old liveview `prepend` code

```elixir
defmodule KyubiWeb.Stream.LiveView do
  use KyubiWeb, :live_view

  @cache :kyubi_cachex
  def mount(_params, _session, socket) do
    if connected?(socket), do: KyubiWeb.Endpoint.subscribe("dns")
    {:ok, init_stream(socket), temporary_assigns: [cards: []]}
  end

  def handle_info(%{event: "stream", payload: query}, socket) do
    {:noreply,
     assign(
       socket,
       cards: Kyubi.Stream.Card.populate(query, socket.id),
       term: get_filter_term(socket.id),
       kyubi_node: Kyubi.Utility.Helper.nodename()
     )}
  end

  def handle_event(
        _event,
        %{"filter" => %{"term" => incoming_query_filter}},
        socket
      ) do
    if String.trim(incoming_query_filter) == "",
      do: Cachex.del(@cache, socket.id),
      else: Cachex.put(@cache, socket.id, {:ok, incoming_query_filter})

    {:noreply,
     assign(
       socket,
       cards: Kyubi.Stream.Card.new(),
       term: incoming_query_filter
     )}
  end

  def init_stream(socket) do
    assign(
      socket,
      cards: Kyubi.Stream.Card.new(),
      term: get_filter_term(socket.id),
      kyubi_node: Kyubi.Utility.Helper.nodename()
    )
  end

  def get_filter_term(socket_id) do
    case Cachex.get(@cache, socket_id) do
      {:ok, {:ok, filter}} ->
        filter

      {:ok, nil} ->
        ""
    end
  end
end

```
```html
 <div class="row mb-3 mx-3">
    <div id="dns-stream" phx-update="prepend" class="mb-3 mx-3 col">
      <%= for query <- @cards.queries do %>
        <p id={query.resource_event_id} />
        <div
          id={"dns-query-#{query.resource_status}%>-#{query.resource_event_id}"}
          class={"card text-white bg-dark
          border border-#{@cards.label.message_type} mb-3 mr-3 ml-3"}
        >
          <KyubiWeb.Components.Stream.card
            query={query}
            label={@cards.label}
            favicon={@cards.favicon}
            ad_tracker={@cards.ad_tracker}
          />
          <p class="card-text">
            <small class="text-muted">
            <%= query.client_user_agent %> <%= query.client_ip %>,
            <%= query.client_city %>
            </small>, &nbsp;<img
              src={~p"/kyubi/images/flags/#{@cards.country_flag_code}"}
              height="14"
            />
          </p>
        </div>
      <% end %>
    </div>
  </div>

```
Here is the updated liveview code based on streams

```elixir
defmodule KyubiWeb.Stream.LiveView do
  use KyubiWeb, :live_view

  @cache :kyubi_cachex
  def mount(_params, _session, socket) do
    if connected?(socket), do: KyubiWeb.Endpoint.subscribe("dns")
    {:ok, init_stream(socket)}
  end

  def handle_info(%{event: "stream", payload: query}, socket) do
    {:noreply,
     assign(
       socket,
       term: get_filter_term(socket.id),
       kyubi_node: Kyubi.Utility.Helper.nodename()
     )
     |> stream_insert(:cards, Kyubi.Stream.Card.populate(query, socket.id), at: 0)}
  end

  def handle_event(
        _event,
        %{"filter" => %{"term" => incoming_query_filter}},
        socket
      ) do
    if String.trim(incoming_query_filter) == "",
      do: Cachex.del(@cache, socket.id),
      else: Cachex.put(@cache, socket.id, {:ok, incoming_query_filter})

    {:noreply,
     assign(
       socket,
       term: incoming_query_filter
     )
     |> stream_insert(:cards, Kyubi.Stream.Card.new(), at: 0)}
  end

  def init_stream(socket) do
    assign(
      socket,
      term: get_filter_term(socket.id),
      kyubi_node: Kyubi.Utility.Helper.nodename()
    )
    |> stream(:cards, [Kyubi.Stream.Card.new()])
  end

  def get_filter_term(socket_id) do
    case Cachex.get(@cache, socket_id) do
      {:ok, {:ok, filter}} ->
        filter

      {:ok, nil} ->
        ""
    end
  end
end

```

```html
 <div class="row mb-3 mx-3">
    <div id="dns-stream" phx-update="prepend" class="mb-3 mx-3 col">
      <%= for query <- @cards.queries do %>
        <p id={query.resource_event_id} />
        <div
          id={"dns-query-#{query.resource_status}%>-#{query.resource_event_id}"}
          class={"card text-white bg-dark
          border border-#{@cards.label.message_type} mb-3 mr-3 ml-3"}
        >
          <KyubiWeb.Components.Stream.card
            query={query}
            label={@cards.label}
            favicon={@cards.favicon}
            ad_tracker={@cards.ad_tracker}
          />
          <p class="card-text">
            <small class="text-muted">
            <%= query.client_user_agent %> <%= query.client_ip %>,
            <%= query.client_city %>
            </small>, &nbsp;<img
              src={~p"/kyubi/images/flags/#{@cards.country_flag_code}"}
              height="14"
            />
          </p>
        </div>
      <% end %>
    </div>
  </div>

```

Streams definitely have a better developer experience compared to the append and prepend design. At times the release docs or blog posts don't cover all the use cases that are out there implemented and stalls upgrades. The elixir community makes it possible for the developers to adopt the latest changes seamlessly