child property
The current child widget to display.
If there was an old child, it will be transitioned out using the secondary animation of the transitionBuilder, while the new child transitions in using the primary animation of the transitionBuilder.
If there was no old child, then this child will transition in using the primary animation of the transitionBuilder.
The child is considered to be "new" if it has a different type or Key (see Widget.canUpdate).
Implementation
final Widget? child;