Metadata-Version: 2.1
Name: cs-events
Version: 0.4.0
Summary: C#-style event handling mechanism for Python
Home-page: https://github.com/wise0704/python-cs-events
License: MIT
Keywords: python,event,c#
Author: Daniel Jeong
Author-email: wise0704@outlook.com
Requires-Python: >=3.10,<4.0
Classifier: Development Status :: 4 - Beta
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: MIT License
Classifier: Natural Language :: English
Classifier: Operating System :: OS Independent
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.10
Classifier: Programming Language :: Python :: 3.11
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.10
Classifier: Programming Language :: Python :: 3.11
Classifier: Topic :: Software Development :: Libraries
Classifier: Typing :: Typed
Requires-Dist: typing-extensions (>=4.0.1,<5.0.0) ; python_version < "3.11"
Project-URL: Issues, https://github.com/wise0704/python-cs-events/issues
Project-URL: Repository, https://github.com/wise0704/python-cs-events
Description-Content-Type: text/markdown

# C#-Style Event Handling Mechanism for Python

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C# provides a very simple syntax using the [observer pattern](https://en.wikipedia.org/wiki/Observer_pattern) for its event handling system.
The aim of this project is to implement the pattern in python as similarly as possible.

In C#, an "event" is a field or a property of the delegate type `EventHandler`.
Since delegates in C# can be combined and removed with `+=` and `-=` operators,
event handlers can easily subscribe to or unsubscribe from the event using those operators.

Python does not support an addition of two `Callable` types.
So the `Event[**TArgs]` class is provided to mimic delegates:

```python
from events import Event

changed = Event[str, object]()
```

> C# naming convention prefers present/past participles (`changing`/`changed`) instead of `on`+infinitive (`on_change`) for events.

Handlers can subscribe to and unsubscribe from the event with the same syntax:

```python
def changed_handler(key: str, value: object) -> None:
    ...

changed += changed_handler
changed -= changed_handler
```

An event can be raised by simply invoking it with the arguments:

```python
changed("state", obj)
```

Since `Event` acts just like a delegate from C#, it is not required to be bound to a class or an instance object.
This is the major difference to other packages that try to implement the C#-style event system, which usually revolve around a container object for events.

An example class with event fields may look like this:

```python
class EventExample:
    def __init__(self) -> None:
        self.updated: Event[str] = Event()
        self.__value: str = ""

    def update(self, value: str) -> None:
        if self.__value != value:
            self.__value = value
            self.updated(value)

obj = EventExample()
obj.updated += lambda value: print(f"obj.{value=}")
obj.update("new value")
```

A class decorator `@events` is provided as a shortcut for event fields:

```python
from events import Event, events

@events
class EventFieldsExample:
    item_added: Event[object]
    item_removed: Event[object]
    item_updated: Event[object, str]
```

C# also provides event properties with `add` and `remove` accessors:

```C#
public event EventHandler<T> Changed
{
    add { ... }
    remove { ... }
}
```

This feature is useful for classes that do not actually own the events, but need to forward the subscriptions to the underlying object that do own the events.

The `@event[**TArgs]` decorator and the `accessors[**TArgs]` type are provided to support this feature:

```python
from events import accessors, event, EventHandler

class EventPropertyExample:
    @event[str, object]
    def changed() -> accessors[str, object]:
        def add(self: Self, value: EventHandler[str, object]) -> None:
            ...
        def remove(self: Self, value: EventHandler[str, object]) -> None:
            ...
        return (add, remove)
```

Furthermore, the `EventHandlerCollection` interface is provided to support the functionalities of `System.ComponentModel.EventHandlerList` class from C#, along with the two implementations `EventHandlerList` and `EventHandlerDict` using a linked list and a dictionary respectively. The behaviour of `EventHandlerList` is exactly the same as of its counterpart from C#.

A typical usage of `EventHandlerList` in C# can be translated directly into the python code:

```python
class EventPropertyExample:
    __event_changed: Final = object()

    @event  # [str, object] is inferred
    def changed():  # -> accessors[str, object] is inferred
        def add(self: Self, value: EventHandler[str, object]) -> None:
            self.__events.add_handler(self.__event_changed, value)

        def remove(self: Self, value: EventHandler[str, object]) -> None:
            self.__events.remove_handler(self.__event_changed, value)

        return (add, remove)
    
    def __init__(self) -> None:
        self.__events = EventHandlerList()
    
    def perform_change(self, key: str, value: object) -> None:
        handler = self.__events[self.__event_changed]
        if handler:
            handler(key, value)
```

The class decorator `@events` also provides a shortcut for event properties.
The above code can be shortened to:

```python
@events(collection="__events")
class EventPropertyExample:
    changed: event[str, object]

    def __init__(self) -> None:
        self.__events = EventHandlerList()

    def perform_change(self, key: str, value: object) -> None:
        self.__events.invoke("changed", key, value)
```

## Installation

Install using [`pip`](https://pypi.org/project/pip/):

```console
pip install cs-events
```

