단일 모드:

단일 모드 사용 시 주의해야 할 문제

예를 들어 잘못된 단례 사용을 설명하다

         #import 
         @interface MessageCenterManager : NSObject

         @property (nonatomic ,assign) NSInteger messageType;
         + (instancetype)sharedInstance;
         @end
       
        .m 
         #import "MessageCenterManager.h"
         static MessageCenterManager *_sharedInstance;
         @implementation MessageCenterManager
         + (instancetype)sharedInstance{
              static dispatch_once_t onceToken;
             dispatch_once(&onceToken, ^{
                      _sharedInstance = [[MessageCenterManager alloc] init];
             });
             return _sharedInstance;
         }

         -(id)copyWithZone:(NSZone *)zone{
             return _sharedInstance;
         }

         +(id)allocWithZone:(struct _NSZone *)zone{
                static dispatch_once_t onceToken;
                        dispatch_once(&onceToken, ^{
                            _sharedInstance = [super allocWithZone:zone];
                         });
               return _sharedInstance;
         }
        @end

       #import "UserMessage.h"
       @implementation UserMessage
       - (void)someMethod{
      // ， 
        if([MessageCenterManager sharedInstance].messageType){
              //do something
        }else{
        }
       }
       @end

       # 
        @implementation BroadcastMessage
         - (void)someMethod{
                [[MessageCenterManager sharedInstance] setMessageType:0];
          }
        @end

중요한 일에 경고를 가하다