Add focus maintence device to DemoCamera

[ianhi]

I guess what I want is demo microscope not so much democamera.

For testing/developing it would be nice to have both a stage and a focus maintence device (a la PFS-Offset) available as a demo device.

[marktsuchida]

Would be nice, but the bad news (at least if you want this for the purpose of testing applications) is that there would need to be several demo devices, simulating Nikon, Zeiss, Leica, etc. They behave differently enough that I don't think it is possible to create a single demo device such that code that works with the demo device would be guaranteed to work with all the real devices. I guess it is still worth having a handful of demo devices implementing the different patterns. Or maybe it is useful enough to have a single "simple" demo device that would work for some types of testing but would not guarantee anything about whether your code will work with real devices (this could be useful if the scope of testing is limited to, say, MDA-like usage with no human interaction -- SequenceTester has an AutoFocus device for this purpose).

This is probably one of the most challenging areas of our device control -- what we ought to have (but lack) is a carefully designed device API that is actually capable of abstracting the differences between the devices. I think it is possible to come up with one, but it is not easy. It will certainly require deep familiarity with the different devices.

Examples of things that differ between the devices include:

• Whether the focus offset can be changed while focus is "locked"
• Whether the focus offset can be changed while focus is not "locked"
• Whether the Z drive can be moved when the focus is "locked" (and what happens if not and you try)
• Whether the focus offset can be measured when the focus is not "locked"
• Whether a numerical "offset" can be obtained (as opposed to a non-ordered indicator of focus position)
• Whether a continuous (vs periodic) mode is available

And probably many more. It almost reminds me of the difficulty of handling people's names.