iOS 创建单例的方法

方法一：

static AccountManager *DefaultManager = nil;  
+ (AccountManager *)defaultManager {  
    if (!DefaultManager) DefaultManager = [[self allocWithZone:NULL] init];  
    return DefaultManager;  
}

方法二：

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