Timer

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

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Timer

You’re going to create a Timer GenServer which when spawned, will send itself a message every second to increment an internal count.

You should be able to retrieve the number of seconds that have passed since the Timer was spawned using a Timer.seconds/1 function.

{:ok, pid} = Timer.start_link([])
Timer.seconds(pid)
0
Process.sleep(1000)
Timer.seconds(pid)
2

Keep in mind, we cannot guarantee perfect timing when sending messages on an interval, and that is not the purpose of this exercise.

Example Solution

defmodule Timer do
  use GenServer

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

  def seconds(timer_pid) do
    GenServer.call(timer_pid, :seconds)
  end

  @impl true
  def init(_opts) do
    schedule_timer()
    {:ok, 0}
  end

  @impl true
  def handle_info(:increment, state) do
    schedule_timer()
    {:noreply, state + 1}
  end

  @impl true
  def handle_call(:seconds, _from, state) do
    {:reply, state, state}
  end

  defp schedule_timer do
    Process.send_after(self(), :increment, 1000)
  end
end

Implement the Timer module as documented below. You will also have to implement the necessary GenServer callback functions such as GenServer.init/1, GenServer.handle_info/2, and GenServer.handle_call/3.

 defmodule Timer do
   @moduledoc """
   Documentation for `Timer`
   """
   use GenServer

   @doc """
   Start the `Timer` process.

   ## Examples

       iex> {:ok, pid} = Timer.start_link([])
   """
   def start_link(_opts) do
   end

   @doc """
   Get the number of seconds that have elapsed since the `Timer` was started.

   ## Examples

     iex> {:ok, pid} = Timer.start_link([])
     iex> Timer.seconds(pid)
     0
     iex> Process.sleep(1200)
     iex> Timer.seconds(pid)
     1
   """
   def seconds(timer_pid) do
   end
 end

Bonus :timer

Re-implement your Timer module using the erlang :timer library. If already wrote your solution using the :timer library, then re-implement it using Process.send_after/4.

Example Solution

defmodule AlternateTimer do
  use GenServer

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

  def seconds(timer_pid) do
    GenServer.call(timer_pid, :seconds)
  end

  @impl true
  def init(_opts) do
    :timer.send_interval(1000, self(), :increment)
    {:ok, 0}
  end

  @impl true
  def handle_call(:seconds, _from, state) do
    {:reply, state, state}
  end

  @impl true
  def handle_info(:increment, state) do
    {:noreply, state + 1}
  end
end

defmodule AlternateTimer do
  @moduledoc """
  Documentation for `Timer`
  """
  use GenServer

  @doc """
  Start the `Timer` process.
  """
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, [])
  end

  @doc """
  Get the number of seconds that have elapsed since the `Timer` was started.
  """
  def seconds(timer_pid) do
    GenServer.call(timer_pid, :seconds)
  end
end

Mark As Completed

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

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "timer_reading"
    "exercises" -> "timer_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 timer-exercise
$ git add .
$ git commit -m "finish timer exercise"
$ git push origin timer-exercise

Create a pull request from your timer-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.

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