Unit testing a periodic timer via handle_info - unit-testing

Question

I have a GenServer which performs a periodic action like so:
defmodule Hello
use GenServer
def init(_) do
Process.send_after(self(), :timer, 1000)
{:ok, %{}}
end
def handle_info(:timer, state) do
# do stuff
Process.send_after(self(), :timer, 1000)
{:noreply, state}
end
end
I'm trying to figure out the best way to unit test it. My initial idea was to use mox to stub the Process.send_after call. This works fine, but then in my unit test, I tried something like this:
test "the timer callback does the right thing" do
MyMock |> expect(:timer, fn -> :ok end)
{:ok, pid} = start_supervised(Hello, [])
Process.send(pid, :timer)
# assert the right thing happens
end
However, this doesn't work because Process.send is async and doesn't return me anything. How else can I test the handle_info callback?

Answer 1

There are many options, like one might spawn an Agent from setup_all callback and use it in both mock and assertion:
expect(MyMock, :timer, fn -> Agent.update(MyAgent, & &1 + 1))
...
Process.sleep(1_100) # ensure all sent
assert Agent.get(MyAgent, & &1) == 1 # number of calls
The probably easiest approach would be to capture IO:
import ExUnit.CaptureLog
...
test "the timer callback does the right thing" do
MyMock |> expect(:timer, fn -> IO.inspect(:ok) end)
{:ok, pid} = start_supervised(Hello, [])
assert capture_log(fn ->
Process.send(pid, :timer)
Process.sleep(1_100)
end) =~ "ok"
end