Hand crafted test services

config/test.exs

@@ -1,3 +1 @@
 use Mix.Config
-
-config :example, service: Example.MockService

lib/example/worker.ex

@@ -3,13 +3,7 @@ defmodule Example.Worker do
 
   alias Example.DefaultService
 
-  # When using ElixirLS, defining the service at compile time will result in an
-  # error because ElixirLS always compiles using MIX_ENV=test which mean @service
-  # will always be set to MockService, which does not have `foo/0`
-  # @service Application.get_env(:example, :service, DefaultService)
-  # @service DefaultService
-
-  def service() do
+  def service do
     Application.get_env(:example, :service, DefaultService)
   end
 
@@ -23,24 +17,12 @@ defmodule Example.Worker do
 
   def init(_init_arg) do
     initial_state = "no foo for you"
+
     {:ok, initial_state, {:continue, :get_foo_from_service}}
   end
 
   def handle_continue(:get_foo_from_service, _state) do
-    # And here lies the problem. We want to call our service to get
-    # whatever inital state it provides, but in doing so, we break
-    # in the test environment because the MockService doesn't have
-    # a function called `foo/0` until it can be defined in the expects
-    # block within the test - by that time, this code has already
-    # been executed because this GenServer is part of the staticly
-    # defined supervision tree in `application.ex`.
-
-    value_of_foo =
-      if function_exported?(service(), :foo, 0) do
-        service().foo()
-      else
-        "#{inspect(service())} does not support foo"
-      end
+    value_of_foo = service().foo()
 
     {:noreply, value_of_foo}
   end

test/support/test1_example.ex

@@ -0,0 +1,7 @@
+defmodule Test1Service do
+  @behaviour Example.ServiceBehaviour
+
+  def foo() do
+    "test 1 says foo"
+  end
+end

test/support/test2_example.ex

@@ -0,0 +1,7 @@
+defmodule Test2Service do
+  @behaviour Example.ServiceBehaviour
+
+  def foo() do
+    "test 2 says moo"
+  end
+end

test/worker_test.exs

@@ -1,33 +1,30 @@
 defmodule Example.WorkerTest do
   use ExUnit.Case
-  import Mox
   alias Example.Worker
 
-  describe "default service" do
-    test "returns default service foo" do
-      assert Worker.get_foo() =~ ~s(default says foo)
-    end
+  # In order to have the Worker use the test services we need to restart
+  # the process; killing it will suffice as it is a supervised process
+  def restart_worker() do
+    pid = Process.whereis(Worker)
+    Process.exit(pid, :kill)
+    Process.sleep(10)
   end
 
   describe "mocked service" do
-    setup do
-      # Normally you would add this to `test_helper.ex`, or `support/mocks.ex
-      Mox.defmock(Example.MockService, for: Example.ServiceBehaviour)
+    test "returns mocked service foo" do
+      Application.put_env(:example, :service, Test1Service)
 
-      Example.MockService
-      |> expect(:foo, fn -> "setup all says foo" end)
+      restart_worker()
 
-      :ok
+      assert Worker.get_foo() =~ ~s(test 1 says foo)
     end
 
-    setup :verify_on_exit!
+    test "returns mocked service moo" do
+      Application.put_env(:example, :service, Test2Service)
 
-    test "returns mocked service foo" do
-      Example.MockService
-      |> expect(:foo, fn -> "mock says foo" end)
-      |> allow(self(), Process.whereis(Worker))
+      restart_worker()
 
-      assert Worker.get_foo() =~ ~s(mock says foo)
+      assert Worker.get_foo() =~ ~s(test 2 says moo)
     end
   end
 end