Cannot set enabledProperty of step buttons in TimeControlNode

[brandonLi8]

Over in phetsims/collision-lab#66, it was decided to disable the step backwards button when the sim is both paused and the elasticity isn't 100%.

Currently, collision-lab uses TimeControlNode for time controls, including the step buttons. However, TimeControlNode doesn't provided any public access to any of its StepButtons. I need access to be able to set the enabledProperty's value to what I need it to be, and I don't think (from quickly looking over the StepButton api) there is an option to do this.

Assigning @jessegreenberg because it looks like he was the main developer of TimeControlNode.

[jessegreenberg]

The current way to do this would be to pass an isPlayingProperty option to the TimeControlNode's step button. So your stepBackwardButtonOptions might look like

        stepBackwardButtonOptions: {
          listener: () => stepBackward(),
          isPlayingProperty: new DerivedProperty( [ playProperty, elasticityProperty ], ( playing, elasticity ) => {
            return playing && elasticity < 1;
          } ),
          radius: 20
        },

There is talk of changing this API in phetsims/sun#515 and #563, but that work hasn't been done yet.

[brandonLi8]

Thanks @jessegreenberg. This issue can be closed. I'll keep an eye out for when #563 is addressed.