iOS·枚举变量在 未赋值/赋值为空 的情况下，默认值为0（即第一个枚举类型）

typedef NS_ENUM(NSInteger, PopupType) {
    PopupTypeNormal = 0,
    PopupTypeBookInfo = 1
};

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    PopupType type1 = [[self.resource[indexPath.row] objectForKey:@"type"] integerValue];
    PopupType type2 = nil;
    type2;
    if (type1 == PopupTypeNormal) {
          //...

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    BOOL flag1 = [[[self.resource[indexPath.row] objectForKey:@"type"] class] isKindOfClass:[NSNull class]];
    BOOL flag2 = [[self.resource[indexPath.row] objectForKey:@"type"] class] == [NSNull class];
    BOOL flag3 = [[self.resource[indexPath.row] objectForKey:@"type"] isEqual:[NSNull null]];
    flag1;
    flag2;
    flag3;
//...