Make sure groups are initialized before template sensors (#37766)

* Make sure groups are initialized before template sensors

This way users may use the `expand` function in templates to expand
groups and have HA listen for changes to group members.

Fixes #35872

* Patch async_setup_platform instead of async_setup

* Cleanup

* Use an event to avoid sleep

* Update tests/components/template/test_sensor.py

Co-authored-by: J. Nick Koston <nick@koston.org>

Co-authored-by: J. Nick Koston <nick@koston.org>

homeassistant/components/template/manifest.json

@@ -3,5 +3,6 @@
   "name": "Template",
   "documentation": "https://www.home-assistant.io/integrations/template",
   "codeowners": ["@PhracturedBlue", "@tetienne"],
-  "quality_scale": "internal"
+  "quality_scale": "internal",
+  "after_dependencies": ["group"]
 }

tests/components/template/test_sensor.py

@@ -1,11 +1,16 @@
 """The test for the Template sensor platform."""
+from asyncio import Event
+from unittest.mock import patch
+
+from homeassistant.bootstrap import async_from_config_dict
 from homeassistant.const import (
+    EVENT_COMPONENT_LOADED,
     EVENT_HOMEASSISTANT_START,
     STATE_OFF,
     STATE_ON,
     STATE_UNAVAILABLE,
 )
-from homeassistant.setup import async_setup_component, setup_component
+from homeassistant.setup import ATTR_COMPONENT, async_setup_component, setup_component
 
 from tests.common import assert_setup_component, get_test_home_assistant
 
@@ -438,6 +443,45 @@ class TestTemplateSensor:
         )
 
 
+async def test_creating_sensor_loads_group(hass):
+    """Test setting up template sensor loads group component first."""
+    order = []
+    after_dep_event = Event()
+
+    async def async_setup_group(hass, config):
+        # Make sure group takes longer to load, so that it won't
+        # be loaded first by chance
+        await after_dep_event.wait()
+
+        order.append("group")
+        return True
+
+    async def async_setup_template(
+        hass, config, async_add_entities, discovery_info=None
+    ):
+        order.append("sensor.template")
+        return True
+
+    async def set_after_dep_event(event):
+        if event.data[ATTR_COMPONENT] == "sensor":
+            after_dep_event.set()
+
+    hass.bus.async_listen(EVENT_COMPONENT_LOADED, set_after_dep_event)
+
+    with patch(
+        "homeassistant.components.group.async_setup", new=async_setup_group,
+    ), patch(
+        "homeassistant.components.template.sensor.async_setup_platform",
+        new=async_setup_template,
+    ):
+        await async_from_config_dict(
+            {"sensor": {"platform": "template", "sensors": {}}, "group": {}}, hass
+        )
+        await hass.async_block_till_done()
+
+    assert order == ["group", "sensor.template"]
+
+
 async def test_available_template_with_entities(hass):
     """Test availability tempalates with values from other entities."""
     hass.states.async_set("sensor.availability_sensor", STATE_OFF)