Override this is descendants to actually send value_changed event witk keyValue[ValueIndex]. You are guaranteed that ValueIndex is between 0 and FdKey.Count - 1, but you have to check yourself whether it's also between 0 and FdKeyValue.Count - 1 (although spec requires that key and keyValue fields have the same size, we have to protect against invalid files).
In what key range we currently are. In other words, which keyValue was last send (passed to SendValueChanged).
We have to keep track of this information, per each node, as specification clearly says that value_changed should not be generated continously on each set_fraction, but only when value actually changes.
Valid values here are between -1 and FdKey.Count - 1. Values between 0 and FdKey.Count - 1 indicate which keyValue was last send. Value -1 means that SendValueChanged was not yet called, this is the default value (after construction).