cardBuilder property
Function that builds each card in the stack.
The function is called with the index of the card to be built, the build context, the ratio
of vertical drag to threshold as a percentage, and the ratio of horizontal drag to threshold
as a percentage. The function should return a widget that represents the card at the given index.
It can return null, which will result in an empty card being displayed.
Implementation
final NullableCardBuilder cardBuilder;