markDirty method
void
markDirty()
Implementation
void markDirty() {
_markDirty = true;
// this is done instead of just notifyListeners() so as to differentiate
// betweena adhoc calls to widgetsWithScreenPositions
// if you need to call notifyListeners() from within this class,
// it should always be with markDirty()
notifyListeners();
}