Add a Receiver.triggered method (#328)

This is an alternative to the `selected_from` method.

RELEASE_NOTES.md

@@ -10,7 +10,15 @@
 
 ## New Features
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- There is a new `Receiver.triggered` method that can be used instead of `selected_from`:
+
+  ```python
+  async for selected in select(recv1, recv2):
+      if recv1.triggered(selected):
+          print('Received from recv1:', selected.message)
+      if recv2.triggered(selected):
+          print('Received from recv2:', selected.message)
+  ```
 
 ## Bug Fixes
 

src/frequenz/channels/_receiver.py

@@ -155,11 +155,14 @@
 
 from abc import ABC, abstractmethod
 from collections.abc import Callable
-from typing import Generic, Self
+from typing import TYPE_CHECKING, Any, Generic, Self, TypeGuard
 
 from ._exceptions import Error
 from ._generic import MappedMessageT_co, ReceiverMessageT_co
 
+if TYPE_CHECKING:
+    from ._select import Selected
+
 
 class Receiver(ABC, Generic[ReceiverMessageT_co]):
     """An endpoint to receive messages."""
@@ -284,6 +287,30 @@ def filter(
         """
         return _Filter(receiver=self, filter_function=filter_function)
 
+    def triggered(
+        self, selected: Selected[Any]
+    ) -> TypeGuard[Selected[ReceiverMessageT_co]]:
+        """Check whether this receiver was selected by [`select()`][frequenz.channels.select].
+
+        This method is used in conjunction with the
+        [`Selected`][frequenz.channels.Selected] class to determine which receiver was
+        selected in `select()` iteration.
+
+        It also works as a [type guard][typing.TypeGuard] to narrow the type of the
+        `Selected` instance to the type of the receiver.
+
+        Please see [`select()`][frequenz.channels.select] for an example.
+
+        Args:
+            selected: The result of a `select()` iteration.
+
+        Returns:
+            Whether this receiver was selected.
+        """
+        if handled := selected._recv is self:  # pylint: disable=protected-access
+            selected._handled = True  # pylint: disable=protected-access
+        return handled
+
 
 class ReceiverError(Error, Generic[ReceiverMessageT_co]):
     """An error that originated in a [Receiver][frequenz.channels.Receiver].
@@ -373,9 +400,7 @@ def consume(self) -> MappedMessageT_co:  # noqa: DOC502
             ReceiverStoppedError: If the receiver stopped producing messages.
             ReceiverError: If there is a problem with the receiver.
         """
-        return self._mapping_function(
-            self._receiver.consume()
-        )  # pylint: disable=protected-access
+        return self._mapping_function(self._receiver.consume())
 
     def __str__(self) -> str:
         """Return a string representation of the mapper."""

src/frequenz/channels/_select.py

@@ -269,9 +269,7 @@ def selected_from(
     Returns:
         Whether the given receiver was selected.
     """
-    if handled := selected._recv is receiver:  # pylint: disable=protected-access
-        selected._handled = True  # pylint: disable=protected-access
-    return handled
+    return receiver.triggered(selected)
 
 
 class SelectError(Error):
@@ -378,14 +376,14 @@ async def select(  # noqa: DOC503
         import datetime
         from typing import assert_never
 
-        from frequenz.channels import ReceiverStoppedError, select, selected_from
+        from frequenz.channels import ReceiverStoppedError, select
         from frequenz.channels.timer import SkipMissedAndDrift, Timer, TriggerAllMissed
 
         timer1 = Timer(datetime.timedelta(seconds=1), TriggerAllMissed())
         timer2 = Timer(datetime.timedelta(seconds=0.5), SkipMissedAndDrift())
 
         async for selected in select(timer1, timer2):
-            if selected_from(selected, timer1):
+            if timer1.triggered(selected):
                 # Beware: `selected.message` might raise an exception, you can always
                 # check for exceptions with `selected.exception` first or use
                 # a try-except block. You can also quickly check if the receiver was
@@ -395,7 +393,7 @@ async def select(  # noqa: DOC503
                     continue
                 print(f"timer1: now={datetime.datetime.now()} drift={selected.message}")
                 timer2.stop()
-            elif selected_from(selected, timer2):
+            elif timer2.triggered(selected):
                 # Explicitly handling of exceptions
                 match selected.exception:
                     case ReceiverStoppedError():