[core] Drop the binary sensor follower

esphome/components/sensor/__init__.py

@@ -124,6 +124,7 @@ from esphome.core import (
 )
 from esphome.core.config import UNIT_OF_MEASUREMENT_MAX_LENGTH
 from esphome.core.entity_helpers import (
+    FollowEntity,
     SubEntities,
     entity_duplicate_validator,
     lazy_load_validator,
@@ -1109,6 +1110,10 @@ def sub_sensors(
     return SubEntities(new_sensor, config, parent)
 
 
+# Makes a plain sensor follow another sensor: follow_sensor(entity, source)
+follow_sensor = FollowEntity("add_on_state_callback", ((cg.float_, "x"),), "x")
+
+
 SENSOR_IN_RANGE_CONDITION_SCHEMA = cv.All(
     {
         cv.Required(CONF_ID): cv.use_id(Sensor),

esphome/components/text_sensor/__init__.py

@@ -22,6 +22,7 @@ from esphome.const import (
 )
 from esphome.core import CORE, ID, CoroPriority, coroutine_with_priority
 from esphome.core.entity_helpers import (
+    FollowEntity,
     SubEntities,
     entity_duplicate_validator,
     queue_entity_register,
@@ -238,6 +239,12 @@ def sub_text_sensors(
     return SubEntities(new_text_sensor, config, parent)
 
 
+# Makes a plain text sensor follow another text sensor: follow_text_sensor(entity, source)
+follow_text_sensor = FollowEntity(
+    "add_on_state_callback", ((cg.std_string_ref.operator("const"), "x"),), "x"
+)
+
+
 @coroutine_with_priority(CoroPriority.CORE)
 async def to_code(config):
     cg.add_global(text_sensor_ns.using)

esphome/core/entity_helpers.py

@@ -45,6 +45,7 @@ from esphome.core.config import (
     UNIT_OF_MEASUREMENT_MAX_LENGTH,
 )
 from esphome.cpp_generator import (
+    LambdaExpression,
     MockObj,
     MockObjClass,
     RawExpression,
@@ -519,6 +520,23 @@ class ApplyEntity:
 _APPLY_DOMAIN = "entity_apply"
 
 
+@dataclass(frozen=True, slots=True)
+class FollowEntity:
+    """A state callback, its parameters and the value an entity republishes from it."""
+
+    callback: str
+    params: tuple[tuple[SafeExpType, str], ...]
+    value: str
+
+    def __call__(self, entity: MockObj, source: MockObj, *args: SafeExpType) -> None:
+        """Make ``entity`` publish every state of ``source``; ``args`` precede the callback."""
+        publish = f"{parent_ref(entity).publish_state(RawExpression(self.value))};"
+        callback = LambdaExpression(
+            [publish], list(self.params), capture="", return_type=cg.void
+        )
+        add(getattr(source, self.callback)(*args, callback))
+
+
 @dataclass(frozen=True, slots=True)
 class SubEntities:
     """Create the child entity configured under key, set parent (if bound) and pass it to setter.

tests/unit_tests/core/test_entity_helpers.py

@@ -11,7 +11,7 @@ import pytest
 
 from esphome import final_validate as fv
 import esphome.codegen as cg
-from esphome.components import button, number, select, switch
+from esphome.components import button, number, select, sensor, switch, text_sensor
 from esphome.config import Config
 import esphome.config_validation as cv
 from esphome.config_validation import Invalid
@@ -1883,6 +1883,53 @@ async def test_sub_entities_create_from_whole_config(
     assert setup_test_environment == ["hub->set_rf_switch(rf_sw)"]
 
 
+@pytest.mark.parametrize(
+    ("follow", "expected"),
+    [
+        (
+            sensor.follow_sensor,
+            [
+                (
+                    "src->add_on_state_callback([](float x) -> void {\n"
+                    "    ::dst->publish_state(x);\n})"
+                )
+            ],
+        ),
+        (
+            text_sensor.follow_text_sensor,
+            [
+                (
+                    "src->add_on_state_callback([](const std::string & x) -> void {\n"
+                    "    ::dst->publish_state(x);\n})"
+                )
+            ],
+        ),
+    ],
+)
+def test_follow_entity(
+    setup_test_environment: list[str],
+    follow: Callable[[MockObj, MockObj], None],
+    expected: list[str],
+) -> None:
+    """The entity follows its source through a lambda that captures nothing."""
+    follow(MockObj("dst", "->"), MockObj("src", "->"))
+    assert setup_test_environment == expected
+
+
+def test_follow_entity_hub_callback(setup_test_environment: list[str]) -> None:
+    """A hub callback takes leading arguments and the published value is an expression."""
+    follow = entity_helpers.FollowEntity(
+        "register_listener", ((cg.uint8, "x"),), "x == 3"
+    )
+    follow(MockObj("dst", "->"), MockObj("hub", "->"), 7)
+    assert setup_test_environment == [
+        (
+            "hub->register_listener(7, [](uint8_t x) -> void {\n"
+            "    ::dst->publish_state(x == 3);\n})"
+        )
+    ]
+
+
 def test_entity_duplicate_validator_without_name() -> None:
     """A config with no name is returned untouched and not tracked."""
     validator = entity_duplicate_validator("sensor")