較嚴格的singleton寫法（封住init）

//利用dispatch_once來寫，會比@synchronized(self)快上三倍，但如果instance被消除，前言不會重新init一次，但外部本來就無法消除內部的instance，所以小心不要在內部消除即可

//自己的範例

#pragma mark - Initial with Singleton

+(instancetype) sharedInstance {

    static dispatch_once_t pred;

    static CBManager *instance = nil;

    dispatch_once(&pred, ^{instance = [[self alloc] initSingleton];});

    return instance;

}

- (id)init {

    return nil;

}

- (id)initSingleton {

    self = [super init];

    if ( self ) {

        //TODO: ...

    }

    return self;

}

//網路上的範例

+ (MYSingleton *)sharedSingleton {
     static dispatch_once_t pred;
     static MYSingleton *instance = nil;
     dispatch_once(&pred, ^{instance = [[self alloc] initSingleton];});
     return instance;
}
- (id)init {
    // Forbid calls to –init or +new
    NSAssert(NO, @”Cannot create instance of Singleton”);
    // You can return nil or [self initSingleton] here, 
    // depending on how you prefer to fail.
    return nil;
}
// Real (private) init method
- (id)initSingleton {
    self = [super init];
    if ((self = [super init])) {
    // Init code }
    return self;
}