现在一般使用线程安全方式创建单例
+(DatabaseManager *)defaultManager {
<span style="white-space:pre"> </span>static DatabaseManager *defaultDatabaseManagerInstance = nil;
<span style="white-space:pre"> </span>static dispatch_once predicate;
<span style="white-space:pre"> </span>dispatch_once(&predicate,^{
<span style="white-space:pre"> </span>defaultDatabaseManagerInstance = [[self alloc]init];
<span style="white-space:pre"> </span>});
<span style="white-space:pre"> </span>return defaultDatabaseManagerInstance;
}