The real purpose these two callbacks serve is exactly what an ordinary
parameter is perfect for:
* Each has just one call site, at the top of the function.
* They're not done for side effects; the point is what they return.
* The function doesn't pass them any arguments of its own, or
otherwise express any internal knowledge that doesn't just as
properly belong to its caller.
So, push the calls to these callbacks up into the function's caller,
and pass in the data they return instead.
This greatly simplifies the interface of `handle_text_input` and of
`typing_status` in general.
This is intended as a pure refactor, making the data flow clearer in
preparation for further changes. In particular, this makes it
manifest that the calls to `get_recipient` and `is_valid_conversation`
don't depend on anything else that has happened during the call to
`handle_text_input`.
This is indeed a pure refactor because
* is_valid_conversation itself has no side effects, either in the
implementation in typing.js or in any reasonable implementation,
so calling it sooner doesn't affect anything else;
* if we do reach it, the only potentially-side-effecting code it's
moving before is a call to `stop_last_notification`, and that in
turn (with the existing, or any reasonable, implementation of
`notify_server_stop`) has no effect on the data consulted by
the implementation of `is_valid_conversation`.
This adds the general machinery required, and sets it up for the file
`typing_status.js` as a first use case.
Co-authored-by: Anders Kaseorg <anders@zulipchat.com>
