Introduce ui.context as a simpler method to get Client, Slot-Stack etc

[rodja]

As I wrote #2811 (comment), it hit me that we could provide ui.context to get the current client object or Slot-Stack. I also think it would later also be a good place to cluster page_title, favicon, viewport, ... which currently can only be configured via page builder decorator.

[falkoschindler]

I wonder an API like ui.context.client was possible. This would require context to be a class instance and I'm not sure if this is possible and practical. It would also hide the fact that client isn't a constant field, but the result of a function. So we might just leave it as ui.context.get_client().

[Alyxion]

I wonder an API like ui.context.client was possible. This would require context to be a class instance and I'm not sure if this is possible and practical. It would also hide the fact that client isn't a constant field, but the result of a function. So we might just leave it as ui.context.get_client().

In the end always the balance between lazyness/simplicity and readability. In Flask world thats basically exactly what request does when you import it from Flask. It - where ever you are in the code execution right now as long as the HTTP API callback is in the call stack - will always contain the current context's / thread's request properties even though it looks like a global variable.

_cv_request: ContextVar[RequestContext] = ContextVar("flask.request_ctx")
request_ctx: RequestContext = LocalProxy(  # type: ignore[assignment]
    _cv_request, unbound_message=_no_req_msg
)
request: Request = LocalProxy(  # type: ignore[assignment]
    _cv_request, "request", unbound_message=_no_req_msg
)
session: SessionMixin = LocalProxy(  # type: ignore[assignment]
    _cv_request, "session", unbound_message=_no_req_msg
)

see https://github.com/pallets/flask/blob/main/src/flask/globals.py