nonatomicの役割

@property(nonatomic,retain)UItextFiled*userNameコンパイル時に自動的に生成されるコード

- (UITextField *) userName {
    return userName;
}

- (void) setUserName:(UITextField *)userName_ {
    [userName release];
    userName = [userName_ retain];
}

@property(retain)UItextFiled*userName自動生成コード

- (UITextField *) userName {
    UITextField *retval = nil;
    @synchronized(self) {
        retval = [[userName retain] autorelease];
    }
    return retval;
}

- (void) setUserName:(UITextField *)userName_ {
    @synchronized(self) {
        [userName release];
        userName = [userName_ retain];
    }
}

http://stackoverflow.com/questions/588866/objective-c-properties-atomic-vs-nonatomic