苹果说ThereshouldtypicallybelittleneedtosubclassNSMutableDictionary.Ifyoudoneedtocustomizebehavior,itisoftenbettertoconsidercompositionratherthansubclassing.(参见https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSMutableDictionary_Class/)他们可能应该加强一点，并说风险自负。但是，在某