The visual value of the control.
When the control is inactive, the value is false and this animation has the value 0.0. When the control is active, the value is either true or tristate is true and the value is null. When the control is active the animation has a value of 1.0. When the control is changing from inactive to active (or vice versa), value is the target value and this animation gradually updates from 0.0 to 1.0 (or vice versa).
CurvedAnimation get position => _position;