void LazyBlockExtentsChangedCallback(int firstIndex, int lastIndex, double firstStartOffset, double lastEndOffset, double minScrollOffset, double containerExtent)

Signature used by LazyBlockViewport to report its interior and exterior dimensions.

  • The firstIndex is the index of the child that is visible at the starting edge of the viewport.
  • The lastIndex is the index of the child that is visible at the ending edge of the viewport. This could be the same as the firstIndex if the child is bigger than the viewport or if it is the last child.
  • The firstStartOffset is the offset of the starting edge of the child with index firstIndex.
  • The lastEndOffset is the offset of the ending edge of the child with index lastIndex.
  • The minScrollOffset is the offset at which the starting edge of the first item in the viewport is aligned with the starting edge of the viewport. (As the scroll offset increases, items with larger indices are revealed in the viewport.) Typically the min scroll offset is 0.0, but because LazyBlockViewport uses dead reckoning, the min scroll offset might not always be 0.0. For example, if an item that's offscreen changes size, the visible items will retain their current scroll offsets even if the distance to the starting edge of the first item changes.
  • The containerExtent is the exterior dimension of the viewport (i.e., the amount of the thing inside the viewport that is visible from outside the viewport).

Source 

typedef void LazyBlockExtentsChangedCallback(int firstIndex, int lastIndex, double firstStartOffset, double lastEndOffset, double minScrollOffset, double containerExtent);