dodos I created an example of a rig using Sliders with the Spine project you provided:
This skeleton has two sliders: Eye_Con_Y and Eye_Con_X.
And each of these sliders references the new animations I added, Eye_Con_Y and Eye_Con_X.
These animations define the range of eyes movement along the Y-axis and X-axis, respectively: Eye_Con_Y defines the range of eye movement along the Y-axis, while Eye_Con_X defines the range of eye movement along the X-axis.
By mapping these animations to the X and Y translate values of the Face_Con bone, the eyes automatically follow the direction of the face when the Face_Con bone moves.
There are several other changes made from your rig in other parts, so please refer to the actual Spine project for details. (Basically, the rig has been simplified.)
Perhaps this does not exactly match what you want to do, but I hope it gives you a general idea of how to use sliders.
