Add restore_state to modbus binary_sensor (#50922)

* Add restore_state to binary_sensor.

* Update return value in State.

homeassistant/components/modbus/binary_sensor.py

@@ -17,9 +17,11 @@ from homeassistant.const import (
     CONF_NAME,
     CONF_SCAN_INTERVAL,
     CONF_SLAVE,
+    STATE_ON,
 )
 from homeassistant.core import HomeAssistant
 from homeassistant.helpers import config_validation as cv
+from homeassistant.helpers.restore_state import RestoreEntity
 from homeassistant.helpers.typing import ConfigType, DiscoveryInfoType
 
 from .base_platform import BasePlatform
@@ -96,12 +98,17 @@ async def async_setup_platform(
     async_add_entities(sensors)
 
 
-class ModbusBinarySensor(BasePlatform, BinarySensorEntity):
+class ModbusBinarySensor(BasePlatform, RestoreEntity, BinarySensorEntity):
     """Modbus binary sensor."""
 
     async def async_added_to_hass(self):
         """Handle entity which will be added."""
         await self.async_base_added_to_hass()
+        state = await self.async_get_last_state()
+        if state:
+            self._value = state.state == STATE_ON
+        else:
+            self._value = None
 
     @property
     def is_on(self):

tests/components/modbus/test_binary_sensor.py

@@ -18,9 +18,12 @@ from homeassistant.const import (
     STATE_ON,
     STATE_UNAVAILABLE,
 )
+from homeassistant.core import State
 
 from .conftest import ReadResult, base_config_test, base_test, prepare_service_update
 
+from tests.common import mock_restore_cache
+
 
 @pytest.mark.parametrize("do_discovery", [False, True])
 @pytest.mark.parametrize(
@@ -130,3 +133,26 @@ async def test_service_binary_sensor_update(hass, mock_pymodbus):
         "homeassistant", "update_entity", {"entity_id": entity_id}, blocking=True
     )
     assert hass.states.get(entity_id).state == STATE_ON
+
+
+async def test_restore_state_binary_sensor(hass):
+    """Run test for binary sensor restore state."""
+
+    sensor_name = "test_binary_sensor"
+    test_value = STATE_ON
+    config_sensor = {CONF_NAME: sensor_name, CONF_ADDRESS: 17}
+    mock_restore_cache(
+        hass,
+        (State(f"{SENSOR_DOMAIN}.{sensor_name}", test_value),),
+    )
+    await base_config_test(
+        hass,
+        config_sensor,
+        sensor_name,
+        SENSOR_DOMAIN,
+        CONF_BINARY_SENSORS,
+        None,
+        method_discovery=True,
+    )
+    entity_id = f"{SENSOR_DOMAIN}.{sensor_name}"
+    assert hass.states.get(entity_id).state == test_value