ExUnit helper function to wait up to a timeout to determine whether a set of assertions completes. Perfect for testing async code

wait_for.ex

  @default_timeout 100
  @check_interval 10

  # Test Helpers

  defp wait_for(fun, timeout \\ @default_timeout) do
    start_time = System.monotonic_time(:millisecond)
    ref = make_ref()

    try do
      do_wait_for(fun, start_time, timeout, ref, nil)
    catch
      {:wait_for_timeout, ^ref, last_error} ->
        message = """
        Assertion did not succeed within #{timeout}ms.

        Last failure:
        #{Exception.format(:error, last_error, [])}
        """

        flunk(message)
    end
  end

  defp do_wait_for(fun, start_time, timeout, ref, _last_error) do
    try do
      fun.()
      :ok
    rescue
      error in [ExUnit.AssertionError] ->
        current_time = System.monotonic_time(:millisecond)

        if current_time - start_time < timeout do
          Process.sleep(@check_interval)
          do_wait_for(fun, start_time, timeout, ref, error)
        else
          throw({:wait_for_timeout, ref, error})
        end
    end
  end