Testing asynchronous code in Elixir

Question

I want to test a function which is using Task.async

In order to make my test pass, I need to make it sleep for 100ms before assertions, otherwise the test p

Answer 1

When I cannot use José's approach involving assert_receive, I use a small helper to repeatedly do assertion / sleep, until the assertion pass or finally times out.

Here is the helper module

defmodule TimeHelper do

  def wait_until(fun), do: wait_until(500, fun)

  def wait_until(0, fun), do: fun.()

  def wait_until(timeout, fun) defo
    try do
      fun.()
    rescue
      ExUnit.AssertionError ->
        :timer.sleep(10)
        wait_until(max(0, timeout - 10), fun)
    end
  end

end

It can be used like this in former example:

TSearch.search([q: "my query"])
wait_until fn ->
  assert called TStore.store("some tweet from my fixtures")
  assert called TStore.store("another one")
end

Answer 2

Since the question is a bit vague, I will give the general answer here. The usual technique is to monitor the process and wait for the down message. Something like this:

task = Task.async(fn -> "foo" end)
ref  = Process.monitor(task.pid)
assert_receive {:DOWN, ^ref, :process, _, :normal}, 500

Some important things:

• The fifth element of the tuple is the exit reason. I am asserting the Task exit is :normal. Change that accordingly if you are expecting another exit.

• The second value in assert_receive is the timeout. 500 miliseconds sounds like a reasonable amount given you currently have a 100 ms sleep.