Test code that a GenServer calls by injecting a mock client that sends a message to a named test process

genserver_test.ex

defmodule Example.Server do
  use GenServer
  
  defmodule Data do
    defstruct client: Example.Client
  end

  def init(%{client: client}), do: {:ok, %Data{client: client}, 500}
  def init(_), do: {:ok, %Data{}, 500}
  
  def handle_info(:timeout, %Data{client: client} = state) do
    client.request(nil)
    {:noreply, state}
  end
end

defmodule Example.Mock.Client do
  def request(message) do
    send(Example.TestReceiver, {:request, message})
    :ok
  end
end

test "test that the client is called after 500 ms" do
  Process.register(self(), Example.TestReceiver)
  assert {:ok, _pid} = Example.Server.start_link(%{client: Example.Mock.Client})
  assert_receive({:request, ^message}, 510)
end

test "get notified when the server shuts down" do
  Process.flag(:trap_exit, true)
  assert {:ok, pid} = Example.Server.start_link(%{client: Example.Mock.Client})
  assert_receive({:EXIT, ^pid, :normal})
end