IOS创建单例的两种方法

1.0  苹果官方写法: 

 static AccountManager *DefaultManager = nil;  

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

2.0  在iOS4之后有了另外一种写法:

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