iOS 单例用法

+ (AccountManager *)sharedManager  
{  
        static AccountManager *sharedAccountManagerInstance = nil;  
        static dispatch_once_t predicate;  
        dispatch_once(&predicate, ^{  
                sharedAccountManagerInstance = [[self alloc] init];   
        });  
    return sharedAccountManagerInstance;  
}  

1.线程安全。
2.满足静态分析器的要求。
3.兼容了ARC