'Process.send_after' doesn't work

Questions / Help
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1

Hi,

I’m trying to create a nerves app that will read from a sensor at 10s intervals. I’ve written code for reading from the device and it works. The problem is that when using the ‘Process.send_after’ function in order to read repeatedly, nothing happens.

Here’s my code:

defmodule HelloNerves do
  use GenServer

# Client
  def start_link(options\\[]) do
    GenServer.start_link __MODULE__, [], options
  end

  def find_device(pid) do
    GenServer.call(pid, {:find})
  end

  def start_reading(pid) do
    GenServer.call(pid, {:start})
  end

  def read_output(pid_1, pid_2) do
    GenServer.call(pid_1, {:read, pid_2})
  end  

# Server
  def init(_) do
    {:ok, []}
  end

  def handle_call({:find}, _from, state) do
    devices = Nerves.UART.enumerate
    case Map.has_key?(devices, "ttyUSB0") do
      true -> {:reply, :ok, state}
      false -> {:reply, :not_found, state}
    end
  end

  def handle_call({:start}, _from, state) do
    {:ok, pid} = Nerves.UART.start_link
    response = Nerves.UART.open(pid, "ttyUSB0", speed: 9600, active: false)
    read_data_interval(pid)
    case response == :ok do
      true -> {:reply, {:ok, pid}, state}
      false -> {:reply, :not_ok, state}
    end
  end

  def handle_call({:read, pid}, _from, state) do
    {:ok, output} = Nerves.UART.read(pid, 5000)
    IO.inspect(output, label: "The sensor data: ")
    read_data_interval(pid)
    case Kernel.is_bitstring(output) do
      true -> {:reply, :ok, state}
      false -> {:reply, :not_ok, state}
    end
  end

  defp read_data_interval(pid) do
    Process.send_after(self(), {:read, pid}, 5000)
  end
end

Any idea as to what my problem is?

Thank you

1 Like
2

Hello, you should handle messages sent from Process.send_after/2 with the function handle_info/2
Check https://hexdocs.pm/elixir/GenServer.html#c:handle_info/2

Also don’t forget to use Process.cancel_timer/1 when you are done with the timer.

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3

Thank you.

I managed to hack together an answer in the time between posting this and your answer.

Here it is , in case it’s relevant.

defmodule HelloNerves do
  use GenServer

# Client
  def start_link(options\\[]) do
    GenServer.start_link __MODULE__, [], options
  end

# Server
  def init(_) do
    IO.inspect("Test")
    {:ok, pid} = Nerves.UART.start_link
    Nerves.UART.open(pid, "ttyUSB0", speed: 9600, active: false)

    spawn(fn -> read_data_forever(pid) end)
    {:ok, []}
  end

  defp read_data_forever(pid) do
    {:ok, output} = Nerves.UART.read(pid, 5000)
    IO.inspect(output, label: "The sensor data: ")
    
    :timer.sleep(1000)
    read_data_forever(pid)
  end
end

Edit #1: Removed redundant code.

4

Glad you made it work!
If Nerves.UART.open/2 is blocking, an small improvement could be also to use the “new” handle_continue/2 callback. So that the OTP Application would not block when booting the init of the GenServer
An example (with nerves) is https://github.com/kpanic/nerves_morse/blob/master/fw/lib/nerves_morse/worker.ex#L33-L36 and https://github.com/kpanic/nerves_morse/blob/master/fw/lib/nerves_morse/worker.ex#L42-L45

See also the official doc https://hexdocs.pm/elixir/GenServer.html#c:handle_continue/2

btw, nerves is really nice! :+1:
Happy hacking!

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