建立 Singleton 類並防止它使用 allocinit 擁有多個例項

我們可以建立 Singleton 類，使開發人員不得不使用共享例項（單例物件）而不是建立自己的例項。

@implementation MySingletonClass

+ (instancetype)sharedInstance
{
    static MySingletonClass *_sharedInstance = nil;
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        _sharedInstance = [[self alloc] initClass];
    });
    
    return _sharedInstance;
}

-(instancetype)initClass
{
    self = [super init];
    if(self)
    {
        //Do any additional initialization if required
    }
    return self;
}

- (instancetype)init
{
    @throw [NSException exceptionWithName:@"Not designated initializer"
                                   reason:@"Use [MySingletonClass sharedInstance]"
                                 userInfo:nil];
    return nil;
}
@end

/*Following line will throw an exception 
  with the Reason:"Use [MySingletonClass sharedInstance]" 
  when tried to alloc/init directly instead of using sharedInstance */
MySingletonClass *mySingletonClass = [[MySingletonClass alloc] init];