createChild method
Called during layout when a new child is needed. The child should be
inserted into the child list in the appropriate position, after the
after child (at the start of the list if after is null). Its index and
scroll offsets will automatically be set appropriately.
The index argument gives the index of the child to show. It is possible
for negative indices to be requested. For example: if the user scrolls
from child 0 to child 10, and then those children get much smaller, and
then the user scrolls back up again, this method will eventually be asked
to produce a child for index -1.
If no child corresponds to index, then do nothing.
Which child is indicated by index zero depends on the GrowthDirection
specified in the constraints of the RenderSliverMultiBoxAdaptor. For
example if the children are the alphabet, then if
SliverConstraints.growthDirection is GrowthDirection.forward then
index zero is A, and index 25 is Z. On the other hand if
SliverConstraints.growthDirection is GrowthDirection.reverse then
index zero is Z, and index 25 is A.
During a call to createChild it is valid to remove other children from the RenderSliverMultiBoxAdaptor object if they were not created during this frame and have not yet been updated during this frame. It is not valid to add any other children to this render object.
Implementation
@override
void createChild(int index, { required RenderBox? after }) {
assert(_currentlyUpdatingChildIndex == null);
owner!.buildScope(this, () {
final bool insertFirst = after == null;
assert(insertFirst || _childElements[index-1] != null);
_currentBeforeChild = insertFirst ? null : (_childElements[index-1]!.renderObject as RenderBox?);
Element? newChild;
try {
final SliverMultiBoxAdaptorWidget adaptorWidget = widget as SliverMultiBoxAdaptorWidget;
_currentlyUpdatingChildIndex = index;
newChild = updateChild(_childElements[index], _build(index, adaptorWidget), index);
} finally {
_currentlyUpdatingChildIndex = null;
}
if (newChild != null) {
_childElements[index] = newChild;
} else {
_childElements.remove(index);
}
});
}