refactor: simplify dependency injection approach

Rather than adding an app configuration key for every mockable
dependency of every module, we can standardize on a convention for
naming and defining the mocks, assisted by a small macro.

config/test.exs

@@ -159,26 +159,6 @@ config :screens, :screens_by_alert,
   screens_last_updated_ttl_seconds: 2,
   screens_ttl_seconds: 1
 
-config :screens, Screens.V2.ScreenData,
-  config_cache_module: Screens.Config.MockCache,
-  parameters_module: Screens.V2.ScreenData.MockParameters
-
-config :screens, Screens.V2.CandidateGenerator.DupNew, stop_module: Screens.Stops.MockStop
-
-config :screens, Screens.V2.CandidateGenerator.Dup.Departures,
-  headways_module: Screens.MockHeadways
-
-config :screens, Screens.V2.RDS,
-  departure_module: Screens.V2.MockDeparture,
-  route_pattern_module: Screens.RoutePatterns.MockRoutePattern,
-  stop_module: Screens.Stops.MockStop
-
-config :screens, Screens.V2.CandidateGenerator.Elevator.Closures,
-  stop_module: Screens.Stops.MockStop,
-  facility_module: Screens.Facilities.MockFacility,
-  alert_module: Screens.Alerts.MockAlert,
-  route_module: Screens.Routes.MockRoute
-
 config :screens, Screens.LastTrip,
   trip_updates_adapter: Screens.LastTrip.TripUpdates.Noop,
   vehicle_positions_adapter: Screens.LastTrip.VehiclePositions.Noop

lib/screens/inject.ex

@@ -0,0 +1,26 @@
+defmodule Screens.Inject do
+  @moduledoc "Conveniences for dependency injection and mocking."
+
+  @doc """
+  When the Mix env is not `test`, resolves to the given module name. When it is, resolves to the
+  module name with `.Mock` appended.
+
+  The mock module is not automatically defined (see e.g. `test/support/mocks.ex`).
+
+  Example usage:
+
+      defmodule Screens.Foo do
+        import Screens.Inject
+
+        @dependency injected(Screens.Dependency)
+
+        def do_something, do: @dependency.fetch_data()
+      end
+  """
+  defmacro injected(module) do
+    quote do
+      module = unquote(module)
+      if Mix.env() == :test, do: Module.concat(module, "Mock"), else: module
+    end
+  end
+end

lib/screens/v2/candidate_generator/dup/departures.ex

@@ -16,7 +16,9 @@ defmodule Screens.V2.CandidateGenerator.Dup.Departures do
   alias ScreensConfig.V2.Departures.Query.Params
   alias ScreensConfig.V2.Dup
 
-  @headways Application.compile_env(:screens, [__MODULE__, :headways_module], Screens.Headways)
+  import Screens.Inject
+
+  @headways injected(Screens.Headways)
 
   def departures_instances(
         %Screen{

lib/screens/v2/candidate_generator/dup_new/header.ex

@@ -6,11 +6,9 @@ defmodule Screens.V2.CandidateGenerator.DupNew.Header do
   alias ScreensConfig.V2.Dup
   alias ScreensConfig.V2.Header.{CurrentStopId, CurrentStopName}
 
-  @stop Application.compile_env(
-          :screens,
-          [Screens.V2.CandidateGenerator.DupNew, :stop_module],
-          Screens.Stops.Stop
-        )
+  import Screens.Inject
+
+  @stop injected(Screens.Stops.Stop)
 
   @spec instances(Screen.t(), DateTime.t()) :: [NormalHeader.t()]
   def instances(%Screen{app_params: %Dup{header: header_config}} = config, now) do

lib/screens/v2/candidate_generator/elevator/closures.ex

@@ -4,18 +4,19 @@ defmodule Screens.V2.CandidateGenerator.Elevator.Closures do
   require Logger
 
   alias Screens.Alerts.{Alert, InformedEntity}
-  alias Screens.Facilities.Facility
   alias Screens.Routes.Route
   alias Screens.Stops.Stop
   alias Screens.V2.WidgetInstance.ElevatorClosures
   alias Screens.V2.WidgetInstance.Serializer.RoutePill
   alias ScreensConfig.Screen
   alias ScreensConfig.V2.Elevator
 
-  @stop Application.compile_env(:screens, [__MODULE__, :stop_module], Stop)
-  @facility Application.compile_env(:screens, [__MODULE__, :facility_module], Facility)
-  @alert Application.compile_env(:screens, [__MODULE__, :alert_module], Alert)
-  @route Application.compile_env(:screens, [__MODULE__, :route_module], Route)
+  import Screens.Inject
+
+  @alert injected(Alert)
+  @facility injected(Screens.Facilities.Facility)
+  @route injected(Route)
+  @stop injected(Stop)
 
   @spec elevator_status_instances(Screen.t()) :: list(ElevatorClosures.t())
   def elevator_status_instances(%Screen{app_params: %Elevator{elevator_id: elevator_id}}) do

lib/screens/v2/rds.ex

@@ -9,6 +9,8 @@ defmodule Screens.V2.RDS do
   into widgets by screen-specific code.
   """
 
+  import Screens.Inject
+
   alias Screens.Lines.Line
   alias Screens.RoutePatterns.RoutePattern
   alias Screens.Routes.Route
@@ -40,15 +42,9 @@ defmodule Screens.V2.RDS do
     defstruct []
   end
 
-  @departure Application.compile_env(:screens, [__MODULE__, :departure_module], Departure)
-
-  @route_pattern Application.compile_env(
-                   :screens,
-                   [__MODULE__, :route_pattern_module],
-                   RoutePattern
-                 )
-
-  @stop Application.compile_env(:screens, [__MODULE__, :stop_module], Stop)
+  @departure injected(Departure)
+  @route_pattern injected(RoutePattern)
+  @stop injected(Stop)
 
   @max_departure_minutes 90
 

lib/screens/v2/screen_data.ex

@@ -8,18 +8,11 @@ defmodule Screens.V2.ScreenData do
   alias ScreensConfig.Screen
   alias __MODULE__.{ParallelRunSupervisor, Layout}
 
+  import Screens.Inject
   import Screens.V2.Template.Guards, only: [is_slot_id: 1, is_paged_slot_id: 1]
 
-  @config_cache Application.compile_env(
-                  :screens,
-                  [__MODULE__, :config_cache_module],
-                  Screens.Config.Cache
-                )
-  @parameters Application.compile_env(
-                :screens,
-                [__MODULE__, :parameters_module],
-                Screens.V2.ScreenData.Parameters
-              )
+  @config_cache injected(Screens.Config.Cache)
+  @parameters injected(Screens.V2.ScreenData.Parameters)
 
   @type t :: %{type: atom()}
   @type simulation_data :: %{full_page: t(), flex_zone: [t()]}

lib/screens/v2/screen_data/layout.ex

@@ -10,13 +10,10 @@ defmodule Screens.V2.ScreenData.Layout do
   alias Screens.V2.WidgetInstance
   alias ScreensConfig.Screen
 
+  import Screens.Inject
   import Template.Guards, only: [is_paged: 1, is_paged_slot_id: 1, is_non_paged_slot_id: 1]
 
-  @parameters Application.compile_env(
-                :screens,
-                [Screens.V2.ScreenData, :parameters_module],
-                Screens.V2.ScreenData.Parameters
-              )
+  @parameters injected(Screens.V2.ScreenData.Parameters)
 
   @type t :: {Template.layout(), %{Template.slot_id() => WidgetInstance.t()}}
   @type non_paged ::

test/screens/v2/candidate_generator/dup/departures_test.exs

@@ -7,7 +7,6 @@ defmodule Screens.V2.CandidateGenerator.Dup.DeparturesTest do
   alias ScreensConfig.V2.Departures.Header, as: SectionHeader
   alias ScreensConfig.V2.Departures.{Layout, Query, Section}
   alias ScreensConfig.V2.Dup, as: DupConfig
-  alias Screens.MockHeadways
   alias Screens.Predictions.Prediction
   alias Screens.Routes.Route
   alias Screens.Schedules.Schedule
@@ -19,9 +18,12 @@ defmodule Screens.V2.CandidateGenerator.Dup.DeparturesTest do
   alias Screens.V2.WidgetInstance.Departures, as: DeparturesWidget
   alias Screens.V2.WidgetInstance.OvernightDepartures
 
+  import Screens.Inject
   import Mox
   setup :verify_on_exit!
 
+  @headways injected(Screens.Headways)
+
   defp put_primary_departures(widget, primary_departures_sections) do
     %{
       widget
@@ -1151,7 +1153,7 @@ defmodule Screens.V2.CandidateGenerator.Dup.DeparturesTest do
         ])
 
       now = ~U[2020-04-06T10:00:00Z]
-      expect(MockHeadways, :get_with_route, 2, fn "place-B", "test", ^now -> {12, 16} end)
+      expect(@headways, :get_with_route, 2, fn "place-B", "test", ^now -> {12, 16} end)
 
       fetch_alerts_fn = fn
         [
@@ -1615,7 +1617,7 @@ defmodule Screens.V2.CandidateGenerator.Dup.DeparturesTest do
         ])
 
       now = ~U[2020-04-06T10:00:00Z]
-      expect(MockHeadways, :get_with_route, 2, fn "place-kencl", "test", ^now -> {7, 13} end)
+      expect(@headways, :get_with_route, 2, fn "place-kencl", "test", ^now -> {7, 13} end)
 
       fetch_alerts_fn = fn
         [
@@ -2510,7 +2512,7 @@ defmodule Screens.V2.CandidateGenerator.Dup.DeparturesTest do
         ])
 
       now = ~U[2020-04-06T10:00:00Z]
-      stub(MockHeadways, :get_with_route, fn "place-overnight", "Red", ^now -> {5, 8} end)
+      stub(@headways, :get_with_route, fn "place-overnight", "Red", ^now -> {5, 8} end)
 
       fetch_schedules_fn = fn
         _, nil ->

test/screens/v2/candidate_generator/dup_new_test.exs

@@ -4,14 +4,16 @@ defmodule Screens.V2.CandidateGenerator.DupNewTest do
   alias ScreensConfig.Screen
   alias ScreensConfig.V2.{Alerts, Departures, EvergreenContentItem, Header, Schedule}
   alias ScreensConfig.V2.Dup, as: DupConfig
-  alias Screens.Stops.MockStop
   alias Screens.Util.Assets
   alias Screens.V2.CandidateGenerator.DupNew
   alias Screens.V2.WidgetInstance.{DeparturesNoData, EvergreenContent, NormalHeader}
 
+  import Screens.Inject
   import Mox
   setup :verify_on_exit!
 
+  @stop injected(Screens.Stops.Stop)
+
   describe "candidate_instances/2" do
     @config %Screen{
       app_id: :dup_v2,
@@ -38,7 +40,7 @@ defmodule Screens.V2.CandidateGenerator.DupNewTest do
 
     test "returns header with stop name determined from stop ID" do
       config = put_in(@config.app_params.header, %Header.CurrentStopId{stop_id: "test_id"})
-      expect(MockStop, :fetch_stop_name, fn "test_id" -> "Test Name" end)
+      expect(@stop, :fetch_stop_name, fn "test_id" -> "Test Name" end)
 
       instances = DupNew.candidate_instances(config, @now)
 

test/screens/v2/candidate_generator/elevator/closures_test.exs

@@ -1,32 +1,35 @@
 defmodule Screens.V2.CandidateGenerator.Elevator.ClosuresTest do
   use ExUnit.Case, async: true
 
-  import Mox
-  setup :verify_on_exit!
-
-  alias Screens.Alerts.{Alert, MockAlert}
-  alias Screens.Facilities.MockFacility
-  alias Screens.Routes.{MockRoute, Route}
-  alias Screens.Stops.{MockStop, Stop}
+  alias Screens.Alerts.Alert
+  alias Screens.Routes.Route
+  alias Screens.Stops.Stop
   alias Screens.V2.CandidateGenerator.Elevator.Closures, as: ElevatorClosures
   alias ScreensConfig.Screen
   alias ScreensConfig.V2.Elevator
 
+  import Screens.Inject
+  import Mox
+  setup :verify_on_exit!
+
+  @alert injected(Alert)
+  @facility injected(Screens.Facilities.Facility)
+  @route injected(Route)
+  @stop injected(Stop)
+
   describe "elevator_status_instances/1" do
     test "Only returns alerts with effect of :elevator_closure" do
-      expect(MockFacility, :fetch_stop_for_facility, fn "111" ->
-        {:ok, %Stop{id: "place-test"}}
-      end)
+      expect(@facility, :fetch_stop_for_facility, fn "111" -> {:ok, %Stop{id: "place-test"}} end)
 
-      expect(MockStop, :fetch_parent_station_name_map, fn ->
+      expect(@stop, :fetch_parent_station_name_map, fn ->
         {:ok, %{"place-test" => "Place Test"}}
       end)
 
-      expect(MockRoute, :fetch, fn %{stop_id: "place-test"} ->
+      expect(@route, :fetch, fn %{stop_id: "place-test"} ->
         {:ok, [%Route{id: "Red", type: :subway}]}
       end)
 
-      expect(MockAlert, :fetch_elevator_alerts_with_facilities, fn ->
+      expect(@alert, :fetch_elevator_alerts_with_facilities, fn ->
         alerts = [
           struct(Alert,
             id: "1",
@@ -67,23 +70,21 @@ defmodule Screens.V2.CandidateGenerator.Elevator.ClosuresTest do
     end
 
     test "Groups outside closures by station" do
-      expect(MockFacility, :fetch_stop_for_facility, fn "111" ->
-        {:ok, %Stop{id: "place-test"}}
-      end)
+      expect(@facility, :fetch_stop_for_facility, fn "111" -> {:ok, %Stop{id: "place-test"}} end)
 
-      expect(MockStop, :fetch_parent_station_name_map, fn ->
+      expect(@stop, :fetch_parent_station_name_map, fn ->
         {:ok, %{"place-haecl" => "Haymarket"}}
       end)
 
-      expect(MockRoute, :fetch, 2, fn
+      expect(@route, :fetch, 2, fn
         %{stop_id: "place-haecl"} ->
           {:ok, [%Route{id: "Orange", type: :subway}]}
 
         %{stop_id: "place-test"} ->
           {:ok, [%Route{id: "Red", type: :subway}]}
       end)
 
-      expect(MockAlert, :fetch_elevator_alerts_with_facilities, fn ->
+      expect(@alert, :fetch_elevator_alerts_with_facilities, fn ->
         alerts = [
           struct(Alert,
             id: "1",
@@ -139,17 +140,15 @@ defmodule Screens.V2.CandidateGenerator.Elevator.ClosuresTest do
     end
 
     test "Filters alerts with no facilities or more than one facility" do
-      expect(MockFacility, :fetch_stop_for_facility, fn "111" ->
-        {:ok, %Stop{id: "place-test"}}
-      end)
+      expect(@facility, :fetch_stop_for_facility, fn "111" -> {:ok, %Stop{id: "place-test"}} end)
 
-      expect(MockStop, :fetch_parent_station_name_map, fn ->
+      expect(@stop, :fetch_parent_station_name_map, fn ->
         {:ok, %{"place-haecl" => "Haymarket"}}
       end)
 
-      expect(MockRoute, :fetch, fn _ -> {:ok, [%Route{id: "Red", type: :subway}]} end)
+      expect(@route, :fetch, fn _ -> {:ok, [%Route{id: "Red", type: :subway}]} end)
 
-      expect(MockAlert, :fetch_elevator_alerts_with_facilities, fn ->
+      expect(@alert, :fetch_elevator_alerts_with_facilities, fn ->
         alerts = [
           struct(Alert,
             id: "1",
@@ -184,19 +183,17 @@ defmodule Screens.V2.CandidateGenerator.Elevator.ClosuresTest do
     end
 
     test "Return empty routes on API error" do
-      expect(MockFacility, :fetch_stop_for_facility, fn "111" ->
-        {:ok, %Stop{id: "place-test"}}
-      end)
+      expect(@facility, :fetch_stop_for_facility, fn "111" -> {:ok, %Stop{id: "place-test"}} end)
 
-      expect(MockStop, :fetch_parent_station_name_map, fn ->
+      expect(@stop, :fetch_parent_station_name_map, fn ->
         {:ok, %{"place-test" => "Place Test"}}
       end)
 
-      expect(MockRoute, :fetch, fn %{stop_id: "place-test"} ->
+      expect(@route, :fetch, fn %{stop_id: "place-test"} ->
         :error
       end)
 
-      expect(MockAlert, :fetch_elevator_alerts_with_facilities, fn ->
+      expect(@alert, :fetch_elevator_alerts_with_facilities, fn ->
         alerts = [
           struct(Alert,
             id: "1",