在numberOfRowsInSection中调用TableView BeginUpdates

Question

I have a tableview backed by an NSFectchedResultsController and I'm trying to show a custom cell when I have no results from the FRC. The problem I'm running into is that BeginUpdates recalls numberOfRowsInSection. I want to keep the tableview active (rather than just displaying an image in its place) so that the user can perform a pull to refresh.

The code:

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
if ([self.fetchedResultsController.fetchedObjects count] == 0) {
    if (!specialCellShowing) {
        specialCellShowing = TRUE;
        [self.tableView setSeparatorStyle:UITableViewCellSeparatorStyleNone];
        [self.tableView beginUpdates];
        [self.tableView insertRowsAtIndexPaths:@[[NSIndexPath indexPathForItem:0 inSection:0]] withRowAnimation:UITableViewRowAnimationFade];
        [self.tableView endUpdates];
    }
    return 1;
}
else {
    if (specialCellShowing) {
        specialCellShowing = FALSE;
        [self.tableView beginUpdates];
        [self.tableView deleteRowsAtIndexPaths:@[[NSIndexPath indexPathForItem:0 inSection:0]] withRowAnimation:UITableViewRowAnimationFade];
        [self.tableView endUpdates];
    }
    [self.tableView setSeparatorStyle:UITableViewCellSeparatorStyleSingleLine];
    return [self.fetchedResultsController.fetchedObjects count];
}

}

The problem is the return 1; statement. What happens is the first time numberOfRowsInSection is called it sets specialCellShowing = TRUE and hits begin updates, which recalls numberOfRowsInSection. The begin updates instance of the method sees that specialCellShowing is true and returns 1 and exits. Now the insert call is made and then a crash occurs on endUpdates because the tableview thinks that there is 1 cell in the table, 1 cell inserted, and 1 cell previously. The other problem is I need that return 1 where it is because in subsequent calls to numberOfRowsInSection I want it to not mess with the table and just return saying I have the one custom cell.

I guess what I'm wondering is if there is a better way to approach this?

Answer 1

You can't mutate the tableview while the tableview is updating its display. You don't need to call deleteRowsAtIndex paths unless you're mutating the tableview. You need to have a separate method: one that responds to an event and mutates the tableview's data (and perhaps calls begin/endUpdates and adds or removes rows). -numberOfRowsInSection should just inspect the backing data and return an answer. The tableview is doing a full update at this point, so adding and removing rows in the tableview is useless at this point anyhow.