Added add_event_listener and remove_event_listener

README.md

@@ -150,6 +150,82 @@ data.value = "final"
 # Callback2: test -> final
 ```
 
+#### Event listener
+I can register a listener for the `ObservableEvent.AFTER_PROPERTY_CHANGE` event.
+
+```python
+from dataclasses import dataclass
+from myutils.observable import make_observable, bind, add_event_listener, ObservableEvent
+
+@dataclass
+class Data:
+  number: int = 1
+  
+data = Data()
+make_observable(data)
+
+on_change_results = []
+on_after_change_results = []
+
+def on_change(old, new):
+  on_change_results.append((old, new))
+  return new + 1
+
+def on_after_change(attr, old, new, results):
+  on_after_change_results.append((attr, old, new, results))
+
+bind(data, 'number', on_change)
+add_event_listener(ObservableEvent.AFTER_PROPERTY_CHANGE, data, "number", on_after_change)
+
+data.number = 5
+data.number = 10
+
+assert on_change_results == [(1, 5), (5, 10)]
+assert on_after_change_results == [("number", 1, 5, [6]), ("number", 5, 10, [11])]
+```
+
+I can register for all attributes change events.
+
+```python
+
+from myutils.observable import make_observable, bind, add_event_listener, ObservableEvent
+
+
+class Data:
+  number: int = 1
+  value: str = "initial"
+
+
+data = Data()
+make_observable(data)
+
+
+def on_change_1(old, new):
+  return new
+
+
+def on_change_2(old, new):
+  return str(new) + "_1"
+
+
+on_after_change_results = []
+
+
+def on_after_change(attr, old, new, results):
+  on_after_change_results.append((attr, old, new, results))
+
+
+add_event_listener(ObservableEvent.AFTER_PROPERTY_CHANGE, data, "", on_after_change)
+bind(data, 'number', on_change_1)
+bind(data, 'number', on_change_2)
+bind(data, 'value', on_change_1)
+
+data.number = 5
+data.value = "new value"
+
+assert on_after_change_results == [("number", 1, 5, [5, "5_1"]), ("value", "initial", "new value", ["new value"])]
+```
+
 #### Using Helper Functions
 
 ```python
@@ -319,4 +395,5 @@ Special thanks to the Python and open-source community for their tools, inspirat
 
 * 0.1.0 : Initial release
 * 0.2.0 : Observable results can be collected using `collect_return_values`
-* 0.3.0 : Added `unbind`, `unbind_all`, `has_listeners` `get_listener_count` to Observable
+* 0.3.0 : Added `unbind`, `unbind_all`, `has_listeners` `get_listener_count` to Observable
+* 0.4.0 : Added `add_event_listener` and `remove_event_listener`