Restore_state helper to restore entity states from the DB on startup (#4614)

* Restore states

* feedback

* Remove component move into recorder

* space

* helper

* Address my own comments

* Improve test coverage

* Add test for light restore state

tests/helpers/test_restore_state.py

@@ -0,0 +1,42 @@
+"""The tests for the Restore component."""
+import asyncio
+from unittest.mock import patch, MagicMock
+
+from homeassistant.const import EVENT_HOMEASSISTANT_START
+from homeassistant.core import CoreState, State
+import homeassistant.util.dt as dt_util
+
+from homeassistant.helpers.restore_state import (
+    async_get_last_state, DATA_RESTORE_CACHE)
+
+
+@asyncio.coroutine
+def test_caching_data(hass):
+    """Test that we cache data."""
+    hass.config.components.add('recorder')
+    hass.state = CoreState.starting
+
+    states = [
+        State('input_boolean.b0', 'on'),
+        State('input_boolean.b1', 'on'),
+        State('input_boolean.b2', 'on'),
+    ]
+
+    with patch('homeassistant.helpers.restore_state.last_recorder_run',
+               return_value=MagicMock(end=dt_util.utcnow())), \
+            patch('homeassistant.helpers.restore_state.get_states',
+                  return_value=states):
+        state = yield from async_get_last_state(hass, 'input_boolean.b1')
+
+    assert DATA_RESTORE_CACHE in hass.data
+    assert hass.data[DATA_RESTORE_CACHE] == {st.entity_id: st for st in states}
+
+    assert state is not None
+    assert state.entity_id == 'input_boolean.b1'
+    assert state.state == 'on'
+
+    hass.bus.async_fire(EVENT_HOMEASSISTANT_START)
+
+    yield from hass.async_block_till_done()
+
+    assert DATA_RESTORE_CACHE not in hass.data