calculate
Calculates the current offset of the bottom sheet. This method should be called when the child view is laid out.
Return
The offset of this bottom sheet within [-1,1] range. Offset increases as this bottom sheet is moving upward. From 0 to 1 the sheet is between collapsed and expanded states and from -1 to 0 it is between hidden and collapsed states. Returns -1 if the bottom sheet is not laid out (therefore it's hidden).