selection property

TextSelection selection

The currently selected text.

If the selection is collapsed, then this property gives the offset of the cursor within the text.

Implementation 

TextSelection get selection => value.selection;
void selection= (TextSelection newSelection)

Setting this will notify all the listeners of this TextEditingController that they need to update (it calls notifyListeners). For this reason, this value should only be set between frames, e.g. in response to user actions, not during the build, layout, or paint phases.

Implementation 

set selection(TextSelection newSelection) {
  if (newSelection.start > text.length || newSelection.end > text.length)
    throw FlutterError('invalid text selection: $newSelection');
  value = value.copyWith(selection: newSelection, composing: TextRange.empty);
}