Mailbox Server

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.8.0", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

Navigation

Return Home Report An Issue

Mailbox Server

Earlier you created a Process Mailbox using purely processes. Now you’re going to create a similar mailbox using GenServer.

To test your solution, you can use :sys.get_state/1 to get the current state of your mailbox. This should only be used for debugging purposes, not for retrieving values from state in a real-world application.

{:ok, mailbox_pid} = GenServer.start_link(Mailbox, [])

GenServer.cast(mailbox_pid, {:mail, %{title: "Title 1", content: "Content 1"}})
GenServer.cast(mailbox_pid, {:mail, %{title: "Title 2", content: "Content 2"}})

:sys.get_state(mailbox_pid)
[%{title: "Title 2", content: "Content 2"}, %{title: "Title 1", content: "Content 1"}]

Example Solution

defmodule Mailbox do
  use GenServer

  def start_link(state \\ []) do
    GenServer.start_link(__MODULE__, state)
  end

  def send(mailbox_pid, mail) do
    GenServer.cast(mailbox_pid, {:mail, mail})
  end

  def all_messages(mailbox_pid) do
    GenServer.call(mailbox_pid, :all_messages)
  end

  @impl true
  def init(state) do
    {:ok, state}
  end

  @impl true
  def handle_call(:all_messages, _from, state) do
    {:noreply, state, state}
  end

  @impl true
  def handle_cast({:mail, mail}, state) do
    {:noreply, [mail | state]}
  end

  def handle_info({:mail, mail}, state) do
    {:noreply, [mail | state]}
  end
end

Implement the Mailbox GenServer as documented below.

defmodule Mailbox do
  use GenServer

  @doc """
  Start a mailbox server.

  ## Examples

      iex> {:ok, pid} = Mailbox.start_link([])
      iex> :sys.get_state(pid)
      []

      Allow the mailbox to start with a default list of messages.

      iex> {:ok, pid} = Mailbox.start_link(["Welcome to your mailbox!"])
      iex> :sys.get_state(pid)
      ["Welcome to your mailbox!"]
  """
  def start_link(_opts) do
  end

  @doc """
  Asynchronously Send a message to a mailbox server.

  ## Examples

      iex> {:ok, pid} = Mailbox.start_link([])
      iex> Mailbox.send(pid, "Message 1")
      iex> Mailbox.send(pid, "Message 2")
      iex> :sys.get_state(pid)
      ["Message 2", "Message 1"]
  """
  def send(mailbox_pid, message) do
  end

  @doc """
  Synchonously retrieve all messages in a mailbox server.

  ## Examples

      iex> {:ok, pid} = Mailbox.start_link([])
      iex> Mailbox.all_messages(pid)
      []
  """
  def all_messages(mailbox_pid) do
  end

  @doc """
  Required callback to start a GenServer.

  ## Examples

      iex> {:ok, _pid} = GenServer.start_link(Mailbox, [])
  """
  @impl true
  def init(state) do
  end

  @doc """
  Callback to synchronously retrieve a `Mailbox`'s state.

  ## Examples

      iex> {:ok, pid} = GenServer.start_link(Mailbox, ["Welcome"])
      iex> GenServer.call(pid, :all_messages)
      ["Welcome"]
  """
  @impl true
  def handle_call(:all_messages, _from, state) do
  end

  @doc """
  Callback for receiving a `cast` message and storing it in the `Mailbox`'s state.

  ## Examples

      iex> {:ok, pid} = GenServer.start_link(Mailbox, [])
      iex> GenServer.cast(pid, {:mail, "Message 1"})
      iex> GenServer.cast(pid, {:mail, "Message 2"})
      iex> :sys.get_state(pid)
      ["Message 2", "Message 1"]
  """
  @impl true
  def handle_cast({:mail, mail}, state) do
  end

  @doc """
  Callback for receiving "regular" messages and storing them in the `Mailbox`'s state.

  ## Examples

      iex> {:ok, pid} = GenServer.start_link(Mailbox, [])
      iex> Process.send(pid, {:mail, "Message 1"}, [])
      iex> Process.send(pid, {:mail, "Message 2"}, [])
      iex> :sys.get_state(pid)
      ["Message 2", "Message 1"]
  """
  @impl true
  def handle_info({:mail, mail}, state) do
  end
end

handle_info Vs handle_cast/handle_call

The main difference between these functions is how you call them.

You call handle_cast with GenServer.cast and handle_call with GenServer.call. handle_info is called with Process.send.

It is useful for situations where you don’t have access to GenServer.call or when you need to send a message to many different GenServers without calling a bunch of different functions to do it.

Mark As Completed

file_name = Path.basename(Regex.replace(~r/#.+/, __ENV__.file, ""), ".livemd")

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "mailbox_server_reading"
    "exercises" -> "mailbox_server_exercise"
  end

progress_path = __DIR__ <> "/../progress.json"
existing_progress = File.read!(progress_path) |> Jason.decode!()

default = Map.get(existing_progress, save_name, false)

form =
  Kino.Control.form(
    [
      completed: input = Kino.Input.checkbox("Mark As Completed", default: default)
    ],
    report_changes: true
  )

Task.async(fn ->
  for %{data: %{completed: completed}} <- Kino.Control.stream(form) do
    File.write!(
      progress_path,
      Jason.encode!(Map.put(existing_progress, save_name, completed), pretty: true)
    )
  end
end)

form

Commit Your Progress

Run the following in your command line from the curriculum folder to track and save your progress in a Git commit. Ensure that you do not already have undesired or unrelated changes by running git status or by checking the source control tab in Visual Studio Code.

$ git checkout -b mailbox-server-exercise
$ git add .
$ git commit -m "finish mailbox server exercise"
$ git push origin mailbox-server-exercise

Create a pull request from your mailbox-server-exercise branch to your solutions branch. Please do not create a pull request to the DockYard Academy repository as this will spam our PR tracker.

DockYard Academy Students Only:

Notify your teacher by including @BrooklinJazz in your PR description to get feedback. You (or your teacher) may merge your PR into your solutions branch after review.

If you are interested in joining the next academy cohort, sign up here to receive more news when it is available.

Up Next