使用GCD创建单例

+ (KKTextHUB *)sharedTextHUB
{
    static KKTextHUB *sharedHub = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedHub = [[self alloc]initWithFrame:CGRectMake(0, 0, 0, 0)];
    });
    
    return sharedHub;
}