defmodule ExampleGenServer do
  use GenServer

  ## Client API
  # Starts detached process of GenServer
  def start_link do
    # Fisrt arg is a GenServer's module name, second one is its initial state and/or its type
    # Name of a process can be specified (name: :name) but it also prevents of running multiple process of the same type
    GenServer.start_link(__MODULE__, [])
  end

  # Sample client call function. First arg is a PID, other are of course optional
  def get_state(pid) do
    # second one a tuple in format: {:some_call, arg1, arg2,...}
    GenServer.call(pid, {:some_call})
  end

  # Sample client cast function. First arg is a PID, second one a tuple in format: {:some_cast, arg1, arg2,...}
  def set_state(pid) do
    GenServer.cast(pid, {:some_cast})
  end

  ## Server API / Callbacks
  # Purpose of this function is to return initial state in required format: {:ok, initial_state}
  # If it will not store any state we can skip it
  def init(initial_state) do
    {:ok, initial_state}
  end

  # When called, all below callbacks get access to current GenServet process state and they are resetting the state while returning response
  # Synchronous function which handles GenServer calls and returns reply tuple: {:reply, state, state}
  def handle_call({:some_call}, _from, state) do
    # stuff
    return_value = "return value"
    {:reply, return_value, state}
  end

  # Asynchronous function which handles GenServer casts and returns no-reply tuple: {:noreply, state}
  def handle_cast({:some_cast}, state) do
    # stuff
    {:noreply, state}
  end

  # Asynchronous function which handles messages from abny other places (than GenServer) and returns no-reply tuple: {:noreply, state}
  def handle_info({:some_info}, state) do
    # stuff
    {:noreply, state}
  end
end