Conditionally initalise Worker state

This commit updates the `Worker` `GenServer` so that the
`handle_continue/2` callback is only invoked when the service is set to
`DefaultService`.

This module already knows about `DefaultService` so this seems slightly
better than pattern matching on `MockService`, however if there are
alternative "production" services, other than `DefaultService` the
`Worker.init/1` would need to change.

Because we are unable to call `handle_continue/2` directly from the
`worker_test.ex` we need to add `initialise_foo/0` which finds the pid
and sends the `:initialise_foo` message. This same message is also used
in the `handle_continue/2` callback.

The tests can be run using:

```
mix test --only mocked_service
```

lib/example/worker.ex

@@ -3,15 +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
-    Application.get_env(:example, :service, DefaultService)
-  end
+  @service Application.get_env(:example, :service, DefaultService)
 
   def start_link(init_arg \\ []) do
     GenServer.start_link(__MODULE__, init_arg, name: __MODULE__)
@@ -21,28 +13,33 @@ defmodule Example.Worker do
     GenServer.call(__MODULE__, :get_foo)
   end
 
+  def initialise_foo() do
+    pid = Process.whereis(__MODULE__)
+    send(pid, :initialise_foo)
+  end
+
   def init(_init_arg) do
     initial_state = "no foo for you"
-    {:ok, initial_state, {:continue, :get_foo_from_service}}
+
+    case @service do
+      DefaultService ->
+        {:ok, initial_state, {:continue, :get_foo_from_service}}
+
+      _ ->
+        {:ok, initial_state}
+    end
+  end
+
+  def handle_continue(:get_foo_from_service, state) do
+    send(self(), :initialise_foo)
+
+    {:noreply, state}
   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
-
-    {:noreply, value_of_foo}
+  def handle_info(:initialise_foo, _state) do
+    IO.inspect("initialising foo now")
+    new_state = @service.foo()
+    {:noreply, new_state}
   end
 
   def handle_call(:get_foo, _from, state) do

test/worker_test.exs

@@ -4,29 +4,30 @@ defmodule Example.WorkerTest do
   alias Example.Worker
 
   describe "default service" do
+    @tag :default_service
     test "returns default service foo" do
       assert Worker.get_foo() =~ ~s(default says foo)
     end
   end
 
   describe "mocked service" do
+    setup :verify_on_exit!
+
     setup do
-      # Normally you would add this to `test_helper.ex`, or `support/mocks.ex
+      # Normally you would add this to `test_helper.ex`, or `support/mocks.ex`
       Mox.defmock(Example.MockService, for: Example.ServiceBehaviour)
 
-      Example.MockService
-      |> expect(:foo, fn -> "setup all says foo" end)
-
       :ok
     end
 
-    setup :verify_on_exit!
-
+    @tag :mocked_service
     test "returns mocked service foo" do
       Example.MockService
       |> expect(:foo, fn -> "mock says foo" end)
       |> allow(self(), Process.whereis(Worker))
 
+      Worker.initialise_foo()
+
       assert Worker.get_foo() =~ ~s(mock says foo)
     end
   end