v2: remove monitor

[rosesyrett]

simple PR to remove Monitor and SimMonitor from ophyd/v2/core.py.

It has been agreed that it makes sense if the backend only keeps track of one single callback. Therefore, we no longer need a Monitor to keep track of potentially several callbacks.

[rosesyrett]

I'm getting the following, seemingly unrelated, mypy errors:

(venv) rose@rose-Precision-3560:~/Documents/projects/ophyd$ mypy ophyd/v2/
ophyd/v2/core.py:805: error: Incompatible return value type (got "Union[Status, List[Any]]", expected "List[Any]")  [return-value]
ophyd/v2/core.py:811: error: Incompatible return value type (got "Union[Status, List[Any]]", expected "List[Any]")  [return-value]
ophyd/v2/core.py:888: error: Argument 1 to "__iadd__" of "list" has incompatible type "Union[Status, List[Any]]"; expected "Iterable[StandardReadable]"  [arg-type]
ophyd/v2/core.py:896: error: Argument 1 to "__iadd__" of "list" has incompatible type "Union[Status, List[Any]]"; expected "Iterable[StandardReadable]"  [arg-type]
Found 4 errors in 1 file (checked 9 source files)

[rosesyrett]

^ can confirm that I get these errors even on the master branch... I will # type: ignore them for now.

[coretl]

^ can confirm that I get these errors even on the master branch... I will # type: ignore them for now.

They will be fixed by #1123

[rosesyrett]

The docs are failing to build because of the changes made here... Specifically, the epics_demo.py in docs/user_v2/examples is failing.

Run it with ipython ipython -i docs/user_v2/examples/epics_demo.py and just type in,

RE(mov(samp.x, 100))
RE(mov(samp.x, 100))

The second invocation will fail with the assertion error;

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
File /usr/lib/python3.10/asyncio/tasks.py:408, in wait_for(fut, timeout)
    407 if timeout is None:
--> 408     return await fut
    410 if timeout <= 0:

File ~/Documents/projects/ophyd/ophyd/v2/epicsdemo/__init__.py:70, in Mover._move(self, new_position, watchers)
     69 await self.setpoint.set(new_position, wait=False)
---> 70 async for current_position in observe_value(self.readback):
     71     for watcher in watchers:

File ~/Documents/projects/ophyd/ophyd/v2/core.py:729, in observe_value(signal)
    728 q: asyncio.Queue[T] = asyncio.Queue()
--> 729 signal.subscribe_value(q.put_nowait)
    730 try:

File ~/Documents/projects/ophyd/ophyd/v2/core.py:670, in SignalR.subscribe_value(self, function)
    669 """Subscribe to updates in value of a device"""
--> 670 self._get_cache().subscribe(function, want_value=True)

File ~/Documents/projects/ophyd/ophyd/v2/core.py:645, in SignalR._get_cache(self)
    644 if not self._cache:
--> 645     self._cache = _SignalCache(self._backend, self)
    646 return self._cache

File ~/Documents/projects/ophyd/ophyd/v2/core.py:573, in _SignalCache.__init__(self, backend, signal)
    572 self.backend = backend
--> 573 backend.set_callback(self._callback)

File ~/Documents/projects/ophyd/ophyd/v2/_aioca.py:224, in CaSignalBackend.set_callback(self, callback)
    223 if callback:
--> 224     assert (
    225         not self.subscription
    226     ), "Cannot set a callback when one is already set"
    227     self.subscription = camonitor(
    228         self.read_pv,
    229         lambda v: callback(self.converter.reading(v), self.converter.value(v)),
    230         datatype=self.converter.read_dbr,
    231         format=FORMAT_TIME,
    232     )