public UnionCollection(IReadOnlyCollection <T1> source, Expander <T1, T2> expander)
{
Contract.IsNotNull(source);
Contract.IsNotNull(expander);
_expander = expander;
_src = source;
_addSource = new EventReceiver <ObjectEventArgs>((x, y) => this.OnAdd((T1)y.Arg));
_removeSource = new EventReceiver <ObjectEventArgs>((x, y) => OnRemove((T1)y.Arg));
_resetSource = new EventReceiver((x, y) => ReloadAll());
_addItem = new EventReceiver <ObjectEventArgs>((x, y) => this.OnAddItem((T2)y.Arg));
_removeItem = new EventReceiver <ObjectEventArgs>((x, y) => this.OnRemoveItem((T2)y.Arg));
_resetItems = new EventReceiver((x, y) => ResetSource((IReadOnlyCollection <T2>)x));
_src.Added.AddReceiver(_addSource);
_src.Removed.AddReceiver(_removeSource);
_src.Reset.AddReceiver(_resetSource);
_resetSource.Ping();
}
