Add Slack thread/reply support (#93384)

tests/components/slack/test_notify.py

@@ -6,6 +6,7 @@ from unittest.mock import AsyncMock, Mock
 from homeassistant.components import notify
 from homeassistant.components.slack import DOMAIN
 from homeassistant.components.slack.notify import (
+    ATTR_THREAD_TS,
     CONF_DEFAULT_CHANNEL,
     SlackNotificationService,
 )
@@ -93,3 +94,18 @@ async def test_message_icon_url_overrides_default() -> None:
     mock_fn.assert_called_once()
     _, kwargs = mock_fn.call_args
     assert kwargs["icon_url"] == expected_icon
+
+
+async def test_message_as_reply() -> None:
+    """Tests that a message pointer will be passed to Slack if specified."""
+    mock_client = Mock()
+    mock_client.chat_postMessage = AsyncMock()
+    service = SlackNotificationService(None, mock_client, CONF_DATA)
+
+    expected_ts = "1624146685.064129"
+    await service.async_send_message("test", data={ATTR_THREAD_TS: expected_ts})
+
+    mock_fn = mock_client.chat_postMessage
+    mock_fn.assert_called_once()
+    _, kwargs = mock_fn.call_args
+    assert kwargs["thread_ts"] == expected_ts