use getFocusDirection when switching objects

[ianhi]

pymmcore-plus/napari-micromanager#119 added some logic to move the Z position on switching an objective

pymmcore-widgets/src/pymmcore_widgets/_objective_widget.py

Lines 104 to 112 in 2eed0a7

	def _pre_change_hook(self) -> None:
	if not self._mmc.getFocusDevice():
	# drop focus motor
	return
	self._mmc.waitForDevice(self._device_label)
	zdev = self._mmc.getFocusDevice()
	self._previous_z = self._mmc.getZPosition()
	self._mmc.setPosition(zdev, 0)
	self._mmc.waitForDevice(zdev)

I think that in order to be fully safe that logic needs to check the focus device direction with core.getFocusDirection.

From the docs:

Returns +1 if increasing position brings objective closer to sample, -1 if increasing position moves objective away from sample, or 0 if unknown. (Make sure to check for zero!)

https://micro-manager.org/apidoc/MMCore/latest/class_c_m_m_core.html#ace092b60e0d8138c7c68cf6baebfc8af

If it's 0 then I think we can pop up a dialog asking what to do. For -1 I think the move it query the stage limits and then move to the max position.

(I have no sense of if any devices have -1 for this, but I'd hate to be person who found out that my device does :) )

[tlambert03]

Yep sounds good. Note that pymmcore-plus/napari-micromanager#119 didn't add that, it's been there the whole time, just refactored there to maintain the existing behavior.

I agree that for a safety mechanism it makes too many assumptions... and as noted in the refactor, it also needs a preference (since moving anything should probably be opt in in some way)