zoomLevelProperty should be discrete

[arouinfar]

For #237

The zoomLevelProperty is type NumberPropertyIO and accepts any value within the range 0-2. However, this range shouldn’t be treated continuously and it’s easy to break the sim with the current implementation. There are discrete zoom levels, each with its own rulers.

For an example of a discrete zoomLevelProperty see circuitConstructionKitDc.introScreen.model.zoomProperty.

[pixelzoom]

Here's circuitConstructionKitDc.introScreen.model.zoomProperty:

The implementation of zoom in Geometric Optics bears no resemblance to CCK. CCK is an enumeration and has only 2 zoom levels. Geometric Optics is more general - zoom level is an integer, uses as an index into an array of scale values.

zoomLevelProperty is already 'Integer', see screenshot below. The fact that you're able to enter non-integers is a Studio problem with the new "value entry" UI, and I believe the iO team is working on adding validation.

@arouinfar let me know if I missed something here, or if you want to discuss reimplementing zoom level in Geometric Optics.

[pixelzoom]

Also worth noting is that zoomLevelProperty is an integer NumberProperty because that's what is required by the common-code zoom controls. Doing what CCK is doing requires additonal code to map between integer and enumeration value, and the name zoomLevelProperty is then inconsistent with other sims. Since CCK is using a non-standard approach, perhaps it should consider renaming zoomLevelProperty to something non-standard that indicates that it's an enumertion.

[arouinfar]

Thanks for the clarification @pixelzoom. I think my confusion is that the zoom magnification levels are 0.25x, 0.5x, and 1x, so I missed that zoomLevelProperty was an integer. I recommend adding phetioDocumentation that connects the value of zoomLevelProperty to the actual zoom magnification.

[pixelzoom]

The actual scale value already exists as a DerivedProperty, so I've instrumented zoomScaleProperty. Note the validValues and documentation:

I improved the documentation for zoomIndexProperty:

What I do NOT want to do is put the actual scale values in the zoomIndexProperty documentation. That would be hard-coding something that is specified elsewhere in code, and could change in the future. So if (for some reason) the client wants to determine which zoomIndexProperty values correspond to which zoomScaleProperty values, they have to do a little work.

@arouinfar ready for review.

[arouinfar]

Thanks @pixelzoom. I think this is a big improvement and really clarifies things.