currentSize property
int currentSizeThe number of code units comprising the current rune.
Returns zero if there is no current rune (current is null).
Source
int get currentSize => _nextPosition - _position;
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dartlang.org/stable/1.24.3/dart-core/RuneIterator/currentSize.html