cs-events 0.5.0
C#-style event handling mechanism for Python
License: MIT
Requires Python: <4.0,>=3.10
Keywords: python, event, c#

# 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

item_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 item_changed_handler(key: str, value: object) -> None:
    ...

item_changed += item_changed_handler
item_changed -= item_changed_handler
```

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

```python
item_changed("info", 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.__value = ""
        self.updated: Event[str] = Event()

    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[str, object]
```

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

```C#
public event EventHandler<ItemChangedEventArgs> ItemChanged
{
    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 item_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_item_changed: Final = object()

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

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

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

        return (add, remove)

    def _on_item_changed(self, key: str, value: object) -> None:
        handler = self.__events[self.__event_item_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:
    item_changed: event[str, object]

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

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

## Installation

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

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