Improving event subscription interface

tangostationcontrol/tangostationcontrol/common/events/change_events.py

 #  Copyright (C) 2024 ASTRON (Netherlands Institute for Radio Astronomy)
 #  SPDX-License-Identifier: Apache-2.0
 
-from typing import Dict
-
 import numpy
 
+from tangostationcontrol.common.case_insensitive_string import CaseInsensitiveString
+from tangostationcontrol.common.case_insensitive_dict import CaseInsensitiveDict
 from tango.server import Device
 
 
@@ -17,7 +17,15 @@ class ChangeEvents:
     def __init__(self, device: Device):
         self.device = device
 
-        self.prev_values: Dict[str, object] = {}
+        # keep track of which attributes we manage
+        self.attributes: list[CaseInsensitiveString] = []
+
+        # previous values of attributes, to avoid
+        # emitting CHANGE_EVENTs when nothing changed.
+        self.prev_values: CaseInsensitiveDict[str, object] = {}
+
+    def is_configured(self, attr_name: str) -> bool:
+        return attr_name in self.attributes
 
     def configure_attribute(self, attr_name: str):
         """Prepares an attribute for emitting custom change events."""
@@ -33,6 +41,8 @@ class ChangeEvents:
         # so detecting it ourselves is easier.
         self.device.set_change_event(attr_name, True, False)
 
+        self.attributes.append(CaseInsensitiveString(attr_name))
+
     def send_change_event(self, attr_name: str, value: object | None):
         """Emits a CHANGE_EVENT if the attribute has changed value."""