深拷贝

在做深拷贝的时候，需要注意自定义的对象，需要遵守NSCopying协议，同时需要实现copeWithZone协议方法，否则在执行copyItems:YES的时候，会发生断错误。

The NSCopying protocol declares a method for providing functional copies of an object. The exact meaning of “copy” can vary from class to class, but a copy must be a functionally independent object with values identical to the original at the time the copy was made. A copy produced with NSCopying is implicitly retained by the sender, who is responsible for releasing it.

NSCopying declares one method, copyWithZone:, but copying is commonly invoked with the convenience method copy. The copy method is defined for all objects inheriting from NSObject and simply invokescopyWithZone: with the default zone.