childCount property

int childCount
override

Called to obtain a precise measure of the total number of children.

Must return the number that is one greater than the greatest index for which createChild will actually create a child.

This is used when createChild cannot add a child for a positive index, to determine the precise dimensions of the sliver. It must return an accurate and precise non-null value. It will not be called if createChild is always able to create a child (e.g. for an infinite list).

Implementation 

@override
int get childCount => widget.delegate.estimatedChildCount;