1. override
double estimateTotalExtent(int firstIndex, int lastIndex, double minOffset, double firstStartOffset, double lastEndOffset)

Returns the estimated total height of the children, in pixels.

If there's an infinite number of children, this should return double.INFINITY.

The provided values can be used to estimate the total extent.

The firstIndex and lastIndex values give the integers that were passed to buildItem to build the respective widgets.

The minOffset is the offset of the widget with index 0. Unless the firstIndex is 0, the minOffset is only itself an estimate.

The firstStartOffset is the offset of the widget with firstIndex, in the same coordinate space as minOffset.

The lastEndOffset is the offset of the widget that would be after lastIndex, in the same coordinate space as minOffset. (In other words, it's the offset to the end of the lastIndex widget.)

A simple algorithm for this function, which works well when there are many children, the exact child count is known, and the children near the top of the list are more or less representative of the length of the other children, is the following:

// childCount is the number of children
return (lastEndOffset - minOffset) * childCount / (lastIndex + 1);

Source 

@override
double estimateTotalExtent(int firstIndex, int lastIndex, double minOffset, double firstStartOffset, double lastEndOffset) {
  final int childCount = children.length;
  if (childCount == 0)
    return 0.0;
  return (lastEndOffset - minOffset) * childCount / (lastIndex + 1);
}