PhET-iO customization of input listeners

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Related to #258 PhET-iO instrumentation...

Vectors (model elements) are instantiated and instrumented at startup in order to meet the needs of PhET-iO. This was a big expensive change that was done in #258.

VectorNodes (view elements) are not instantiated at startup and not instrumented. They are instantiated and disposed dynamically as the vectors exit/enter the toolbox. This means that it's not currently possible to customize input listeners for dragging, hotkeys, etc.

Options:

1. Instantiate all VectorNode (view) instances at startup, the same way that we do Vector (model) instances. Then the PhET-iO client can customize input listeners on a per-VectorNode (view) basis. This will be expensive, probably 20-25 hours.

2. Add Properties for customization of input listeners to each Vector instance in the model. Then the PhET-iO client can customize input listeners on a per-Vector (model) basis. This will be less expensive, probably 5-10 hours.

3. Add global Properties for customizing input listeners. These Properties would apply to all vector instances, so we loose the per-vector customization. For example, it would not be possible to allow dragging of vector 'a', but turn off dragging of vectors 'b' and 'c'. This is the least expensive options, probably 3-5 hours.

4. Do nothing for the 1.1 release. This will meet requirements for PhET Studio v1, but not PhET Studio v2 or PhET-iO Studio. This is the no-cost option.

Each of these options is quite different -- different implementation, and different PhET-iO API. With the exception of (4), switching to a different option in the future would be difficult/expensive, and would not have a clear PhET-iO migration path.

If (4) is not an options, then I'm leaning towards option (2).

@matthew-blackman thoughts?

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Related to this issue... Since VectorToolboxSlot is created at startup, we should be able to instrument its AddVectorKeyboardListener - which I did in b6991f3. But KeyboardListener appears to be ignoring tandem, even though it's part of its API.

I posted to Slack#phet-io:

Vector Addition has several KeyboardListeners for adding, removing, moving, scaling and rotating vectors. The API for KeyboardListener takes a tandem, and I’ve instrumented them for PhET-iO. But they are not appearing in the PhET-iO Studio tree or in the PhET-iO API. What is the status of PhET-iO support for KeyboardListener? And if it’s not supported, how do I support configurability of keyboard interactions in Vector Addition?

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I uninstrumented AddVectorKeyboardListener since it's not working. And I instrumented vectorToolBox.*Slot.inputEnabledProperty, which is probably a better approach for the toolbox.

But for vectors, inputEnabledProperty won't cut it. For example, a client may want to prevent vectors from being deleted (RemoveVectorKeyboardListener) while still allowing vectors to be selected (SelectVectorKeyboardListener).

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Over in phetsims/scenery#1720 (comment), @samreid discovered that adding phetioEnabledPropertyInstrumented: true will add the KeyboardListener and its enabledProperty to the PhET-iO API. So we can move forward with that approach in Vector Additions.

Also in phetsims/scenery#1720 (comment), I expressed concern that while this may be a path forward for Vector Addition, it may not be the best path forward, because there has been no PhET-iO design done for KeyboardListener.

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Based on the consensus at today's developer meeting (see phetsims/scenery#1720 (comment)), this issue will not be addressed for the 1.1 release, because it is not relevant for PhET Studio V1. It is deferred to a future release.

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As noted in #411, when this issue is addressed, there is an additional design consideration:

Should the vectors in the vector toolbox appear grayed out when the inputEnabledProperty is set to false, in order to indicate that they cannot be used?