selectAll method
Selects every index in 0..itemCount-1.
Implementation
void selectAll(int itemCount) {
if (itemCount <= 0) {
return;
}
var changed = false;
for (var i = 0; i < itemCount; i++) {
changed = _selected.add(i) || changed;
}
if (changed) {
notifyListeners();
}
}