If you need to group your entities together, e.g. tanks going into "units" group or explosions into "effects", then use this manager. You must retrieve it using world instance.

An Entity can only belong to several groups (0,n) at a time.

class GroupManager extends Manager {
 final Map<String, Bag<Entity>> _entitiesByGroup;
 final Map<Entity, Bag<String>> _groupsByEntity;

 GroupManager() : _entitiesByGroup = new Map<String, Bag<Entity>>(),
                  _groupsByEntity = new Map<Entity, Bag<String>>();

 void initialize() {}

 /**
  * Set the group of the entity.
  */
 void add(Entity e, String group) {
   Bag<Entity> entities = _entitiesByGroup[group];
   if (entities == null) {
     entities = new Bag<Entity>();
     _entitiesByGroup[group] = entities;
   }
   entities.add(e);

   Bag<String> groups = _groupsByEntity[e];
   if(groups == null) {
     groups = new Bag<String>();
     _groupsByEntity[e] = groups;
   }
   groups.add(group);
 }

 /**
  * Remove the entity from the specified group.
  */
 void remove(Entity e, String group) {
   Bag<Entity> entities = _entitiesByGroup[group];
   if(entities != null) {
     entities.remove(e);
   }

   Bag<String> groups = _groupsByEntity[e];
   if(groups != null) {
     groups.remove(group);
   }
 }

 void removeFromAllGroups(Entity e) {
   Bag<String> groups = _groupsByEntity[e];
   if(groups != null) {
     groups.forEach((group) {
       Bag<Entity> entities = _entitiesByGroup[group];
       if(entities != null) {
         entities.remove(e);
       }
     });
     groups.clear();
   }
 }

 /**
  * Get all entities that belong to the provided group.
  */
 ReadOnlyBag<Entity> getEntities(String group) {
   Bag<Entity> entities = _entitiesByGroup[group];
   if (entities == null) {
     entities = new Bag<Entity>();
     _entitiesByGroup[group] = entities;
   }
   return entities.readOnly;
 }

 /**
  * Returns the groups the entity belongs to, null if none.
  */
 ReadOnlyBag<String> getGroups(Entity e) {
   var result = _groupsByEntity[e];
   return result == null ? null : result.readOnly;
 }

 /**
  * Checks if the entity belongs to any group.
  */
 bool isInAnyGroup(Entity e) => getGroups(e) != null;

 /**
  * Check if the entity is in the supplied group.
  */
 bool isInGroup(Entity e, String group) {
   Bag<String> groups = _groupsByEntity[e];
   return (groups != null) && groups.contains(group);
 }

 void deleted(Entity e) => removeFromAllGroups(e);

}