process method
Processes all changes to entities and executes all non-passive systems.
Implementation
void process([int group = 0]) {
assert(_frame.containsKey(group), 'No group $group exists');
// delete entites that have been deleted outside of a system
_deleteEntities();
_frame[group] = _frame[group]! + 1;
_time[group] = _time[group]! + delta;
for (final system in _systemsList
.where((system) => !system.passive && system.group == group)) {
_updateSystem(system);
system.process();
_deleteEntities();
}
}