Message passing with state[:pid] fails, freshly generated pid works

Question

My second test is failing.

 1) test it jumps when it can (RunnerTest)
     test/runner_test.exs:15
     No message matching %{y: 1} after 100ms.
     The process mailbox is empty.
     stacktrace:
       test/runner_test.exs:18: (test)

defmodule RunnerTest do
  use ExUnit.Case
  doctest Runner

  setup_all do
    {:ok, pid: spawn(fn -> Runner.input() end)}
  end

  test "it increases its x position", state do
    avatar = %{x: 0, y: 0}
    send state[:pid], {:run, self, avatar}
    assert_receive %{x: 1}
  end

  test "it jumps when it can", state do
    avatar = %{x: 0, y: 0}
    send state[:pid], {:jump, self, avatar}
    assert_receive %{y: 1}
  end

end

Runner Module:

defmodule Runner do
    def input do
      receive do
       {:run, sender, mover} -> send sender, run(mover)
       {:jump, sender, mover} -> send sender, jump(mover)
     end
    end

    defp run(mover) do
        Map.merge(mover, %{x: mover.x + 1})
    end

    defp jump(mover) do
        case {mover.y} do
          {0} ->
            Map.merge(mover, %{y: mover.y + 1})
          _ ->
            mover
        end
    end

end

This test passes:

 test "it jumps when it can", state do
    avatar = %{x: 0, y: 0}
    pid = spawn(fn -> Runner.input() end)
    send pid, {:jump, self, avatar}
    assert_receive %{y: 1}
  end

Why is 1 test (run) working with setup but not the other (jump)?

Answer 1

Why is 1 test (run) working with setup but not the other (jump)?

This is because your process only responds to one message and then dies and you're using setup_all instead of setup, which means the process is started only once, before the first test is ran, and the same pid is passed to all tests.

You can either change setup_all to setup if you want to spawn a fresh process for each test, or make your process respond to more than 1 message (possibly infinite using self recursion).