I think one reason this is less common in Elixir is that a lot of things you’d wait for are better accomplished by waiting to receive a message that either is the check or indicates that you can now check instead of polling. In other words if you perform a background task instead of polling to see if the task is complete, monitoring the background process and waiting for an exit message is probably better.

I’ve used ExUnit.Assertions — ExUnit v1.12.3 (assert_receive/3) for times where I’ve needed to do this

There’s nothing in core ExUnit. There are few third party libraries or you can do:

# try 10 times every 100ms
assert Stream.interval(100) 
       |> Stream.take(10)
       |> Enum.any?(fn _ -> …test end)

I know this post is old, but it came up when I looked for this exact problem, so putting my answer anyway.

I want to test a similar thing, and here is what I eventually came up with.

1. You extend the api of your log writer process so that any process can subscribe to an event in case a log has been written. That way your test can do an assert_receive on a log being written.
2. You can use something similar to assert_eventually to try an assertion a few times. This seems to be the easiest approach, but it might also point to a design issue in your code.

In my particular case, I had a work queue that divided tasks via work-stealing-like system. A queue keeps a list of tasks, and workers can request work from the queue. When work is available its sent to the workers. The API looks as follows:

  @doc """
  A process signals the queue that it is ready to ingest a message.
  """
  @spec request_work :: :ok
  def request_work do
    GenServer.cast(__MODULE__, {:request_work, self()})
  end

  @doc """
  Insert a work item in the queue.
  """
  @spec insert_work(message()) :: :ok
  def insert_work(message) do
    GenServer.call(__MODULE__, {:insert_work, message})
  end

I extended the API of my WorkQueue to allow a process to subscribe to events. When a work item is sent, a notification is sent, as well as when a worker reports it needs work.

  @doc """
  Send a message to the process when a worker asks for work, or when a task is
  sent to a worker.
  """
  @spec notify(pid()) :: :ok
  def notify(pid) do
    GenServer.call(__MODULE__, {:notify, pid})
  end

When the application starts, the WorkQueue has n workers waiting for work.
So, executing two work items should notify the test of two things:

• work_requested when the worker finishes the first task, and is ready for the next one.
• work_added when the second work item is put in the queue

My test looks as follows.

# let the queue tell us when a job is done, or when a worker asks for work
IngestQueue.notify(self())

Handler.handle_message([device.device_id, "data"], @telegram) |> tap(&IO.inspect(&1, label: ""))

# before the test all workers are waiting for work, so the first event is work being sent 
# when the worker is done, they will ask for new work 
# when the second task is done, that worker will ask for work again
assert_receive :sent_work
assert_receive :request_work

# I am now certain the queue executed at least two tasks.

Obviously this could be done a bit better, but with minimal effort I can now make assertions about the state of an asynchronous system.