Observer pattern

switch.py と issue に示されている Widget/YDoc の実装から始め、その後、psygnal と traitlets が監視対象の属性をどのように表現しているかを確認します。実用的な observer 設計で YMap の値エントリとネストされた YDoc 構造をカバーできるか判断します。実装範囲について合意され、これらの構造を監視するための明確なアプローチが定まれば完了とします。

In https://github.com/jupyter-widgets/ipywidgets/issues/3293#issuecomment-1433848240 @tlambert03 mentioned psygnal, that could be used to implement the equivalent of traitlets.
I think it is a good idea, but I'm not sure how it could fit here. For instance, take the Switch model, currently implemented as:

from .ypywidgets import Widget


class Switch(Widget):

    def __init__(self, value: bool = False, open_comm: bool = True) -> None:
        super().__init__(name="switch", open_comm=open_comm)
        self.yvalue = self.ydoc.get_map("value")
        self._set(value)

    def _set(self, value: bool) -> None:
        with self.ydoc.begin_transaction() as t:
            self.yvalue.set(t, "value",  value)

    @property
    def value(self) -> bool:
        return self.yvalue["value"]

    @value.setter
    def value(self, value: bool):
        if value == self.value:
            return

        self._set(value)

    def toggle(self):
        self.value = not self.value

Its YDoc consists of a YMap named value, with a single entry in it, also named value, which contains the boolean value of the switch. Here the observer pattern is manually implemented using a getter and a setter function for the value attribute.
If we were to use psygnal, we would need a way to point to the YDoc's structure (self.yvalue and value entry in it) from the psygnal's attribute.
I think that's the reason why traitlets use Python's descriptor protocol, which allows to get the name of an attribute at runtime.
Also, I'm not sure how we could connect nested YDoc structures (e.g. a YMap can contain other Y structures), but I don't think it was possible with traitlets anyway, so that would be an improvement.