apply_after_vs_send_after.ex

defmodule A do
  use GenServer
  
  def start do
    GenServer.start_link(__MODULE__, [])
  end
  
  def send_after(pid) do
    1..10 
    |> Enum.each(fn _ -> Process.send_after(pid, :message, 1_000) end)
  end    
  
  def apply_after do
    1..10 
    |> Enum.each(fn _ -> :timer.apply_after(1_000, __MODULE__, :message, [System.monotonic_time(:millisecond)]) end)
  end    
  
  def init(_args) do
    {:ok, System.monotonic_time(:millisecond)}
  end
  
  def handle_info(:message, state) do
    Process.sleep(2_000)
    IO.inspect({:message, (System.monotonic_time(:millisecond) - state)})
    {:noreply, System.monotonic_time(:millisecond)}
  end
  
  def message(time) do
    Process.sleep(2_000)
    IO.inspect({:message, (System.monotonic_time(:millisecond) - time)})    
  end    
end

{:ok, pid} = A.start()
A.send_after(pid)
A.apply_after()